In vitro development and optimization of cell-laden injectable bioprinted gelatin methacryloyl (GelMA) microgels mineralized on the nanoscale

Bone defects may occur in different sizes and shapes due to trauma, infections, and cancer resection. Autografts are still considered the primary treatment choice for bone regeneration. However, they are hard to source and often create donor-site morbidity. Injectable microgels have attracted much attention in tissue engineering and regenerative medicine due to their ability to replace inert implants with a minimally invasive delivery. Here, we developed novel cell-laden bioprinted gelatin methacrylate (GelMA) injectable microgels, with controllable shapes and sizes that can be controllably mineralized on the nanoscale, while stimulating the response of cells embedded within the matrix. The injectable microgels were mineralized using a calcium and phosphate-rich medium that resulted in nanoscale crystalline hydroxyapatite deposition and increased stiffness within the crosslinked matrix of bioprinted GelMA microparticles. Next, we studied the effect of mineralization in osteocytes, a key bone homeostasis regulator. Viability stains showed that osteocytes were maintained at 98 % viability after mineralization with elevated expression of sclerostin in mineralized compared to non-mineralized microgels, showing that mineralization can effectively enhances osteocyte maturation. Based on our findings, bioprinted mineralized GelMA microgels appear to be an efficient material to approximate the bone microarchitecture and composition with desirable control of sample injectability and polymerization. These bone-like bioprinted mineralized biomaterials are exciting platforms for potential minimally invasive translational methods in bone regenerative therapies.

Poly(ethylene glycol)-Based Hydrogel Microcarriers Alter Secretory Activity of Genetically Modified Mesenchymal Stromal Cells

In order to scale up culture therapeutic cells, such as mesenchymal stromal cells (MSCs), culture in suspension bioreactors using microcarriers (μCs) is preferred. However, the impact of microcarrier type on the resulting MSC secretory activity has not been investigated. In this study, two poly(ethylene glycol) hydrogel formulations with different swelling ratios (named "stiffer" and "softer") were fabricated as μC substrates to culture MSCs and MSCs genetically modified to express the interleukin-1 receptor antagonist (IL-1Ra-MSCs). Changes in cell number, secretory and angiogenic activity, and changes in MAPK signaling were evaluated when cultured on hydrogel μCs, as well as on tissue culture plastic-based Synthemax μCs. We demonstrated that culture on stiffer μCs increased secretion of IL-1Ra compared to culture on Synthemax μCs by IL-1Ra-MSCs by 1.2- to 1.6-fold, as well as their in vitro angiogenic activity, compared to culture on Synthemax μCs, while culture on both stiffer and softer μCs altered the secretion of several other factors compared to culture on Synthemax μCs. Changes in angiogenic activity corresponded with increased gene expression and secretion of hepatocyte growth factor by MSCs cultured on softer μCs by 2.5- to 6-fold compared to MSCs cultured on Synthemax μCs. Quantification of phosphoprotein signaling with the MAPK pathway revealed broad reduction of pathway activation by IL-1Ra-MSCs cultured on both stiffer and softer μCs compared to Synthemax, where phosphorylated c-Jun, ATF2, and MEK1 were reduced specifically on softer μCs. Overall, this study showed that μC surfaces can influence the secretory activity of genetically modified MSCs and identified associated changes in MAPK pathway signaling, which is a known central regulator of cytokine secretion.

In vitro development and optimization of cell-laden injectable bioprinted gelatin methacryloyl (GelMA) microgels mineralized on the nanoscale

Bone defects may occur in different sizes and shapes due to trauma, infections, and cancer resection. Autografts are still considered the primary treatment choice for bone regeneration. However, they are hard to source and often create donor-site morbidity. Injectable microgels have attracted much attention in tissue engineering and regenerative medicine due to their ability to replace inert implants with a minimally invasive delivery. Here, we developed novel cell-laden bioprinted gelatin methacrylate (GelMA) injectable microgels, with controllable shapes and sizes that can be controllably mineralized on the nanoscale, while stimulating the response of cells embedded within the matrix. The injectable microgels were mineralized using a calcium and phosphate-rich medium that resulted in nanoscale crystalline hydroxyapatite deposition and increased stiffness within the crosslinked matrix of bioprinted GelMA microparticles. Next, we studied the effect of mineralization in osteocytes, a key bone homeostasis regulator. Viability stains showed that osteocytes were maintained at 98% viability after mineralization with elevated expression of sclerostin in mineralized compared to non-mineralized microgels, indicating that mineralization effectively enhances osteocyte maturation. Based on our findings, bioprinted mineralized GelMA microgels appear to be an efficient material to approximate the bone microarchitecture and composition with desirable control of sample injectability and polymerization. These bone-like bioprinted mineralized biomaterials are exciting platforms for potential minimally invasive translational methods in bone regenerative therapies.

Early Resistance Rehabilitation Improves Functional Regeneration Following Segmental Bone Defect Injury

Mechanical loading is integral to bone development and repair. The application of mechanical loads through rehabilitation are regularly prescribed as a clinical aide following severe bone injuries. However, current rehabilitation regimens typically involve long periods of non-loading and rely on subjective patient feedback, leading to muscle atrophy and soft tissue fibrosis. While many pre-clinical studies have focused on unloading, ambulatory loading, or direct mechanical compression, rehabilitation intensity and its impact on the local strain environment and subsequent bone healing have largely not been investigated. This study combines implantable strain sensors and subject-specific finite element models in a pre-clinical rodent model with a defect size on the cusp of critically-sized. Animals were enrolled in either high or low intensity rehabilitation one week post injury to investigate how rehabilitation intensity affects the local mechanical environment and subsequent functional bone regeneration. The high intensity rehabilitation animals were given free access to running wheels with resistance, which increased local strains within the regenerative niche by an average of 44% compared to the low intensity (no-resistance) group. Finite element modeling demonstrated that resistance rehabilitation significantly increased compressive strain by a factor of 2.0 at week 1 and 4.45 after 4 weeks of rehabilitation. The resistance rehabilitation group had significantly increased regenerated bone volume and higher bone bridging rates than its sedentary counterpart (bone volume: 22.00 mm3 ± 4.26 resistance rehabilitation vs 8.00 mm3 ± 2.27 sedentary; bridging rates: 90% resistance rehabilitation vs 50% sedentary). In addition, animals that underwent resistance running had femurs with improved mechanical properties compared to those left in sedentary conditions, with failure torque and torsional stiffness values matching their contralateral, intact femurs (stiffness: 0.036 Nm/deg ± 0.006 resistance rehabilitation vs 0.008 Nm/deg ± 0.006 sedentary). Running on a wheel with no resistance rehabilitation also increased bridging rates (100% no resistance rehabilitation vs 50% sedentary). Analysis of bone volume and von Frey suggest no-resistance rehabilitation may improve bone regeneration and hindlimb functionality. These results demonstrate the potential for early resistance rehabilitation as a rehabilitation regimen to improve bone regeneration and functional recovery.

Intra-articular delivery of micronized dehydrated human amnion/chorion membrane reduces degenerative changes after onset of post-traumatic osteoarthritis

Background: Micronized dehydrated human amnion/chorion membrane (mdHACM) has reduced short term post-traumatic osteoarthritis (PTOA) progression in rats when delivered 24 h after medial meniscal transection (MMT) and is being investigated for clinical use as a disease modifying therapy. Much remains to be assessed, including its potential for longer-term therapeutic benefit and treatment effects after onset of joint degeneration. Objectives: Characterize longer-term effects of acute treatment with mdHACM and determine whether treatment administered to joints with established PTOA could slow or reverse degeneration. Hypotheses: Acute treatment effects will be sustained for 6 weeks, and delivery of mdHACM after onset of joint degeneration will attenuate structural osteoarthritic changes. Methods: Rats underwent MMT or sham surgery (left leg). mdHACM was delivered intra-articularly 24 h or 3 weeks post-surgery (n = 5-7 per group). Six weeks post-surgery, animals were euthanized and left tibiae scanned using equilibrium partitioning of an ionic contrast agent microcomputed tomography (EPIC-µCT) to structurally quantify joint degeneration. Histology was performed to examine tibial plateau cartilage. Results: Quantitative 3D µCT showed that cartilage structural metrics (thickness, X-ray attenuation, surface roughness, exposed bone area) for delayed mdHACM treatment limbs were significantly improved over saline treatment and not significantly different from shams. Subchondral bone mineral density and thickness for the delayed treatment group were significantly improved over acute treated, and subchondral bone thickness was not significantly different from sham. Marginal osteophyte degenerative changes were decreased with delayed mdHACM treatment compared to saline. Acute treatment (24 h post-surgery) did not reduce longer-term joint tissue degeneration compared to saline. Histology supported µCT findings and further revealed that while delayed treatment reduced cartilage damage, chondrocytes displayed qualitatively different morphologies and density compared to sham. Conclusion: This study provides insight into effects of intra-articular delivery timing relative to PTOA progression and the duration of therapeutic benefit of mdHACM. Results suggest that mdHACM injection into already osteoarthritic joints can improve joint health, but a single, acute mdHACM injection post-injury does not prevent long term osteoarthritis associated with meniscal instability. Further work is needed to fully characterize the durability of therapeutic benefit in stable osteoarthritic joints and the effects of repeated injections.

Measuring translational research impact requires reaching beyond current metrics

Translational impact assessment is key to selecting those biomedical research discoveries most likely to be converted into viable new products to improve human health. However, metrics for translational success are variable, are not limited to commercial success, and may not be relevant to every case or institution. Societal impact is a top translational priority in a globalized society.

Patient-Specific iPSC-Derived Models Link Aberrant Endoplasmic Reticulum Stress Sensing and Response to Juvenile Osteochondritis Dissecans Etiology

Juvenile osteochondritis dissecans (JOCD) is a pediatric disease, which begins with an osteonecrotic lesion in the secondary ossification center which, over time, results in the separation of the necrotic fragment from the parent bone. JOCD predisposes to early-onset osteoarthritis. However, the knowledge gap in JOCD pathomechanisms severely limits current therapeutic strategies. To elucidate its etiology, we conducted a study with induced pluripotent stem cells (iPSCs) from JOCD and control patients. iPSCs from skin biopsies were differentiated to iMSCs (iPSC-derived mesenchymal stromal cells) and subjected to chondrogenic and endochondral ossification, and endoplasmic reticulum (ER)-stress induction assays. Our study, using 3 JOCD donors, showed that JOCD cells have lower chondrogenic capability and their endochondral ossification process differs from control cells; yet, JOCD- and control-cells accomplish osteogenesis of similar quality. Our findings show that endoplasmic reticulum stress sensing and response mechanisms in JOCD cells, which partially regulate chondrocyte and osteoblast differentiation, are related to these differences. We suggest that JOCD cells are more sensitive to ER stress than control cells, and in pathological microenvironments, such as microtrauma and micro-ischemia, JOCD pathogenesis pathways may be initiated. This study is the first, to the best of our knowledge, to realize the important role that resident cells and their differentiating counterparts play in JOCD and to put forth a novel etiological hypothesis that seeks to consolidate and explain previously postulated hypotheses. Furthermore, our results establish well-characterized JOCD-specific iPSC-derived in vitro models and identified potential targets which could be used to improve diagnostic tools and therapeutic strategies in JOCD.

Development and characterization of Factor Xa-responsive materials for applications in cell culture and biologics delivery

Stimuli-responsive biomaterials may be used to better control the release of bioactive molecules or cells for applications involving drug delivery and controlled cell release. In this study, we developed a Factor Xa (FXa)-responsive biomaterial capable of controlled release of pharmaceutical agents and cells from in vitro culture. FXa-cleavable substrates were formed as hydrogels that degraded in response to FXa enzyme over several hours. Hydrogels were shown to release both heparin and a model protein in response to FXa. Additionally, RGD-functionalized FXa-degradable hydrogels were used to culture mesenchymal stromal cells (MSCs), enabling FXa-mediated cell dissociation from hydrogels in a manner that preserved multicellular structures. Harvesting MSCs using FXa-mediated dissociation did not influence their differentiation capacity or indoleamine 2,3-dioxygenase (IDO) activity (a measure of immunomodulatory capacity). In all, this FXa-degradable hydrogel is a novel responsive biomaterial system that may be used for on-demand drug delivery, as well as for improving processes for in vitro culture of therapeutic cells.

Engineering of an Osteoinductive and Growth Factor-Free Injectable Bone-Like Microgel for Bone Regeneration

Bone autografts remain the gold standard for bone grafting surgeries despite having increased donor site morbidity and limited availability. Bone morphogenetic protein-loaded grafts represent another successful commercial alternative. However, the therapeutic use of recombinant growth factors has been associated with significant adverse clinical outcomes. This highlights the need to develop biomaterials that closely approximate the structure and composition of bone autografts, which are inherently osteoinductive and biologically active with embedded living cells, without the need for added supplements. Here, injectable growth factor-free bone-like tissue constructs are developed, that closely approximate the cellular, structural, and chemical composition of bone autografts. It is demonstrated that these micro-constructs are inherently osteogenic, and demonstrate the ability to stimulate mineralized tissue formation and regenerate bone in critical-sized defects in-vivo. Furthermore, the mechanisms that allow human mesenchymal stem cells (hMSCs) to be highly osteogenic in these constructs, despite the lack of osteoinductive supplements, are assessed, whereby Yes activated protein (YAP) nuclear localization and adenosine signaling appear to regulate osteogenic cell differentiation. The findings represent a step toward a new class of minimally invasive, injectable, and inherently osteoinductive scaffolds, which are regenerative by virtue of their ability to mimic the tissue cellular and extracellular microenvironment, thus showing promise for clinical applications in regenerative engineering.

In vitromagnetohydrodynamics system for modulating cell migration

Fluid shear stress (FSS) is an important parameter that regulates various cell functions such as proliferation and migration. While there are a number of techniques to generate FSSin vitro, many of them require physical deformation or movement of solid objects to generate the fluid shear, making it difficult to decouple the effects of FSS and mechanical strains. This work describes the development of a non-mechanical means to generate fluid flow and FSS in a 2Din vitrosetting. This was accomplished with a magnetohydrodynamic (MHD) pump, which creates liquid flow by generating a Lorentz force through the interaction between an electric field and an orthogonal magnetic field. The MHD pump system presented here consisted of trapezoidal prism-shaped magnets, a pair of platinum electrodes, and a modified petri dish. The system was validated and tested on anin vitrowound model, which is based on analyzing the migration of fibroblast cells through an artificially created scratch on a confluent cell culture surface. Experiments were performed to a control group, an electric field only group, and a group that was subject to fluid flow with the application of both electric field and magnetic field. Results show that fibroblast cells that experienced fluid shear have higher wound closure rate compared to the control group and the electric field only group. The data shows that the MHD pump can be a great tool to study FSSin vitro. Furthermore, due to its fluid flow generation without mechanical force, this system can be adapted and implemented to study the role of FSS and electric field on wound healingin vivo.

Magnetoelastic Monitoring System for Tracking Growth of Human Mesenchymal Stromal Cells

Magnetoelastic sensors, which undergo mechanical resonance when interrogated with magnetic fields, can be functionalized to measure various physical quantities and chemical/biological analytes by tracking their resonance behaviors. The unique wireless and functionalizable nature of these sensors makes them good candidates for biological sensing applications, from the detection of specific bacteria to tracking force loading inside the human body. In this study, we evaluate the viability of magnetoelastic sensors based on a commercially available magnetoelastic material (Metglas 2826 MB) for wirelessly monitoring the attachment and growth of human mesenchymal stromal cells (hMSCs) in 2D in vitro cell culture. The results indicate that the changes in sensor resonance are linearly correlated with cell quantity. Experiments using a custom-built monitoring system also demonstrated the ability of this technology to collect temporal profiles of cell growth, which could elucidate key stages of cell proliferation based on acute features in the profile. Additionally, there was no observed change in the morphology of cells after they were subjected to magnetic and mechanical stimuli from the monitoring system, indicating that this method for tracking cell growth may have minimal impact on cell quality and potency.

Amniotic membrane attenuates heterotopic ossification following high-dose bone morphogenetic protein-2 treatment of segmental bone defects

Treatment of large bone defects with supraphysiological doses of bone morphogenetic protein-2 (BMP-2) has been associated with complications including heterotopic ossification (HO), inflammation, and pain, presumably due to poor spatiotemporal control of BMP-2. We have previously recapitulated extensive HO in our rat femoral segmental defect model by treatment with high-dose BMP-2 (30 μg). Using this model and BMP-2 dose, our objective was to evaluate the utility of a clinically available human amniotic membrane (AM) around the defect space for guided bone regeneration and reduction of HO. We hypothesized that AM surrounding collagen sponge would attenuate heterotopic ossification compared with collagen sponge alone. In vitro, AM retained more BMP-2 than a synthetic poly(ε-caprolactone) membrane through 21 days. In vivo, as hypothesized, the collagen + AM resulted in significantly less heterotopic ossification and correspondingly, lower total bone volume (BV), compared with collagen sponge alone. Although bone formation within the defect was delayed with AM around the defect, by 12 weeks, defect BVs were equivalent. Torsional stiffness was significantly reduced with AM but was equivalent to that of intact bone. Collagen + AM resulted in the formation of dense fibrous tissue and mineralized tissue, while the collagen group contained primarily mineralized tissue surrounded by marrow-like structures. Especially in conjunction with high doses of growth factor delivered via collagen sponge, these findings suggest AM may be effective as an overlay adjacent to bone healing sites to spatially direct bone regeneration and minimize heterotopic ossification.

Translating musculoskeletal bioengineering into tissue regeneration therapies

Musculoskeletal injuries and disorders are the leading cause of physical disability worldwide and a considerable socioeconomic burden. The lack of effective therapies has driven the development of novel bioengineering approaches that have recently started to gain clinical approvals. In this review, we first discuss the self-repair capacity of the musculoskeletal tissues and describe causes of musculoskeletal dysfunction. We then review the development of novel biomaterial, immunomodulatory, cellular, and gene therapies to treat musculoskeletal disorders. Last, we consider the recent regulatory changes and future areas of technological progress that can accelerate translation of these therapies to clinical practice.

Implantable biosensors for musculoskeletal health

PURPOSE

A healthy musculoskeletal system requires complex functional integration of bone, muscle, cartilage, and connective tissues responsible for bodily support, motion, and the protection of vital organs. Conditions or injuries to musculoskeeltal tissues can devastate an individual's quality of life. Some conditions that are particularly disabling include severe bone and muscle injuries to the extremities and amputations resulting from unmanageable musculoskeletal conditions or injuries. Monitoring and managing musculoskeletal health is intricate because of the complex mechanobiology of these interconnected tissues.

METHODS

For this article, we reviewed literature on implantable biosensors related to clinical data of the musculoskeletal system, therapeutics for complex bone injuries, and osseointegrated prosthetics as example applications.

RESULTS

As a result, a brief summary of biosensors technologies is provided along with review of noteworthy biosensors and future developments needed to fully realize the translational benefit of biosensors for musculoskeletal health.

CONCLUSIONS

Novel implantable biosensors capable of tracking biophysical parameters in vivo are highly relevant to musculoskeletal health because of their ability to collect clinical data relevant to medical decisions, complex trauma treatment, and the performance of osseointegrated prostheses.

Systemic Immune Modulation Alters Local Bone Regeneration in a Delayed Treatment Composite Model of Non-Union Extremity Trauma

Bone non-unions resulting from severe traumatic injuries pose significant clinical challenges, and the biological factors that drive progression towards and healing from these injuries are still not well understood. Recently, a dysregulated systemic immune response following musculoskeletal trauma has been identified as a contributing factor for poor outcomes and complications such as infections. In particular, myeloid-derived suppressor cells (MDSCs), immunosuppressive myeloid-lineage cells that expand in response to traumatic injury, have been highlighted as a potential therapeutic target to restore systemic immune homeostasis and ultimately improve functional bone regeneration. Previously, we have developed a novel immunomodulatory therapeutic strategy to deplete MDSCs using Janus gold nanoparticles that mimic the structure and function of antibodies. Here, in a preclinical delayed treatment composite injury model of bone and muscle trauma, we investigate the effects of these nanoparticles on circulating MDSCs, systemic immune profiles, and functional bone regeneration. Unexpectedly, treatment with the nanoparticles resulted in depletion of the high side scatter subset of MDSCs and an increase in the low side scatter subset of MDSCs, resulting in an overall increase in total MDSCs. This overall increase correlated with a decrease in bone volume (P = 0.057) at 6 weeks post-treatment and a significant decrease in mechanical strength at 12 weeks post-treatment compared to untreated rats. Furthermore, MDSCs correlated negatively with endpoint bone healing at multiple timepoints. Single cell RNA sequencing of circulating immune cells revealed differing gene expression of the SNAb target molecule S100A8/A9 in MDSC sub-populations, highlighting a potential need for more targeted approaches to MDSC immunomodulatory treatment following trauma. These results provide further insights on the role of systemic immune dysregulation for severe trauma outcomes in the case of non-unions and composite injuries and suggest the need for additional studies on targeted immunomodulatory interventions to enhance healing.

Effects of osteogenic ambulatory mechanical stimulation on early stages of BMP-2 mediated bone repair

Purpose: Mechanical loading of bone defects through rehabilitation is a promising approach to stimulate repair and reduce nonunion risk; however, little is known about how therapeutic mechanical stimuli modulate early-stage repair before mineralized bone formation. The objective of this study was to investigate the early effects of osteogenic loading on cytokine expression and angiogenesis during the first 3 weeks of BMP-2 mediated segmental bone defect repair.Materials and Methods: A rat model of BMP-2 mediated bone defect repair was subjected to an osteogenic mechanical loading protocol using ambulatory rehabilitation and a compliant, load-sharing fixator with an integrated implantable strain sensor. The effect of fixator load-sharing on local tissue strain, angiogenesis, and cytokine expression was evaluated.Results: Using sensor readings for local measurements of boundary conditions, finite element simulations showed strain became amplified in remaining soft tissue regions between 1 and 3 weeks (Week 3: load-sharing: -1.89 ± 0.35% and load-shielded: -1.38 ± 0.35% vs. Week 1: load-sharing: -1.54 ± 0.17%; load-shielded: -0.76 ± 0.06%). Multivariate analysis of cytokine arrays revealed that load-sharing significantly altered expression profiles in the defect tissue at 2 weeks compared to load-shielded defects. Specifically, loading reduced VEGF (p = 0.052) and increased CXCL5 (LIX) levels. Subsequently, vascular volume in loaded defects was reduced relative to load-shielded defects but similar to intact bone at 3 weeks. Endochondral bone repair was also observed histologically in loaded defects at 3 weeks.Conclusions: Together, these results demonstrate that moderate ambulatory strains previously shown to stimulate bone regeneration significantly alter early angiogenic and cytokine signaling and may promote endochondral ossification.

Triple growth factor delivery promotes functional bone regeneration following composite musculoskeletal trauma

Successful bone healing in severe trauma depends on early revascularization to restore oxygen, nutrient, growth factor, and progenitor cell supply to the injury. Therapeutic angiogenesis strategies have therefore been investigated to promote revascularization following severe bone injuries; however, results have been inconsistent. This is the first study investigating the effects of dual angiogenic growth factors (VEGF and PDGF) with low-dose bone morphogenetic protein-2 (BMP-2; 2.5 µg) on bone healing in a clinically challenging composite bone-muscle injury model. Our hydrogel-based delivery systems demonstrated a more than 90% protein entrapment efficiency and a controlled simultaneous release of three growth factors over 28 days. Co-stimulation of microvascular fragment constructs with VEGF and PDGF promoted vascular network formation in vitro compared to VEGF or PDGF alone. In an in vivo model of segmental bone and volumetric muscle loss injury, combined VEGF (5 µg) and PDGF (7.5 µg or 15 µg) delivery with a low dose of BMP-2 significantly enhanced regeneration of vascularized bone compared to BMP-2 treatment alone. Notably, the regenerated bone mechanics reached ~60% of intact bone, a value that was previously only achieved by delivery of high-dose BMP-2 (10 µg) in this injury model. Overall, sustained delivery of VEGF, PDFG, and BMP-2 is a promising strategy to promote functional vascularized bone tissue regeneration following severe composite musculoskeletal injury. Although this study is conducted in a clinically relevant composite injury model in rats using a simultaneous release strategy, future studies are necessary to test the regenerative potential of spatiotemporally controlled delivery of triple growth factors on bone healing using large animal models. STATEMENT OF SIGNIFICANCE: Volumetric muscle loss combined with delayed union or non-union bone defect causes deleterious effects on bone regeneration even with the supplementation of bone morphogenetic protein-2 (BMP-2). In this study, the controlled delivery of dual angiogenic growth factors (vascular endothelial growth factor [VEGF] + Platelet-derived growth factor [PDGF]) increases vascular growth in vitro. Co-delivering VEGF+PDGF significantly increase the bone formation efficacy of low-dose BMP-2 and improves the mechanics of regenerated bone in a challenging composite bone-muscle injury model.

Magnetoelastic sensors for real-time tracking of cell growth

Magnetoelastic (ME) sensors, which can be remotely activated via magnetic fields, are an excellent choice for wireless monitoring of biological parameters due to their ability to be scaled into different sizes and have their surface functionalized for chemical or biological sensing. In this study, we present the application of a commercially available ME material (Metglas 2826 MB) to develop a sensor system that can monitor the attachment of anchorage-dependent mammalian cells in two-dimensional in vitro cell cultures. Results obtained with the developed sensors and detection system correlated with microscopic image analysis of cell quantification, which showed a linear relationship between the sensor response and attached fibroblast cells on the sensor surface. It was also revealed that the developed ME sensor system is capable of providing temporal profiles of cell growth corresponding to different stages of cell attachment and proliferation in real-time.

BMP-2 delivery strategy modulates local bone regeneration and systemic immune responses to complex extremity trauma

Bone nonunions arising from large bone defects and composite injuries remain compelling challenges for orthopedic surgeons. Biological changes associated with nonunions, such as systemic immune dysregulation, can contribute to an adverse healing environment. Bone morphogenetic protein 2 (BMP-2), an osteoinductive and potentially immunomodulatory growth factor, is a promising strategy; however, burst release from the clinical standard collagen sponge delivery vehicle can result in adverse side effects such as heterotopic ossification (HO) and irregular bone structure, especially when using supraphysiological BMP-2 doses for complex injuries at high risk for nonunion. To address this challenge, biomaterials that strongly bind BMP-2, such as heparin methacrylamide microparticles (HMPs), may be used to limit exposure and spatially constrain proteins within the injury site. Here, we investigate moderately high dose BMP-2 delivered in HMPs within an injectable hydrogel system in two challenging nonunion models exhibiting characteristics of systemic immune dysregulation. The HMP delivery system increased total bone volume and decreased peak HO compared to collagen sponge delivery of the same BMP-2 dose. Multivariate analyses of systemic immune markers showed the collagen sponge group correlated with markers that are hallmarks of systemic immune dysregulation, including immunosuppressive myeloid-derived suppressor cells, whereas the HMP groups were associated with immune effector cells, including T cells, and cytokines linked to robust bone regeneration. Overall, our results demonstrate that HMP delivery of moderately high doses of BMP-2 promotes repair of complex bone nonunion injuries and that local delivery strategies for potent growth factors like BMP-2 may positively affect the systemic immune response to traumatic injury.

Localized Sampling Enables Monitoring of Cell State via Inline Electrospray Ionization Mass Spectrometry

Nascent advanced therapies, including regenerative medicine and cell and gene therapies, rely on the production of cells in bioreactors that are highly heterogeneous in both space and time. Unfortunately, advanced therapies have failed to reach a wide patient population due to unreliable manufacturing processes that result in batch variability and cost prohibitive production. This can be attributed largely to a void in existing process analytical technologies (PATs) capable of characterizing the secreted critical quality attribute (CQA) biomolecules that correlate with the final product quality. The Dynamic Sampling Platform (DSP) is a PAT for cell bioreactor monitoring that can be coupled to a suite of sensor techniques to provide real-time feedback on spatial and temporal CQA content in situ. In this study, DSP is coupled with electrospray ionization mass spectrometry and direct-from-culture sampling to obtain measures of CQA content in bulk media and the cell microenvironment throughout the entire cell culture process (≈3 weeks). Post hoc analysis of this real-time data reveals that sampling from the microenvironment enables cell state monitoring (e.g., confluence, differentiation). These results demonstrate that an effective PAT should incorporate both spatial and temporal resolution to serve as an effective input for feedback control in biomanufacturing.

Early systemic immune biomarkers predict bone regeneration after trauma

Severe traumatic injuries are a widespread and challenging clinical problem, and yet the factors that drive successful healing and restoration of function are still not well understood. One recently identified risk factor for poor healing outcomes is a dysregulated immune response following injury. In a preclinical model of orthopedic trauma, we demonstrate that distinct systemic immune profiles are correlated with impaired bone regeneration. Most notably, elevated blood levels of myeloid-derived suppressor cells (MDSCs) and the immunosuppressive cytokine interleukin-10 (IL-10) are negatively correlated with functional bone regeneration as early as 1 wk posttreatment. Nonlinear multivariate regression also implicated these two factors as the most influential in predictive computational models. These results support a significant relationship between early systemic immune responses to trauma and subsequent local bone regeneration and indicate that elevated circulating levels of MDSCs and IL-10 may be predictive of poor functional healing outcomes and represent novel targets for immunotherapeutic intervention.

Functional repair of critically sized femoral defects treated with bioinspired titanium gyroid-sheet scaffolds

Despite the innate ability for bone to remodel and repair, its regeneration has a limit. In these cases of critically sized bone defects (CSBD), the bone deficit must be repaired using reconstructive techniques that support immediate load bearing and encourage bone bridging across the defect. High-strength porous titanium implants offer a solution for treatment of CSBD in which the scaffold can support physiological loads, provide a matrix to guide ingrowth, and carry graft materials and/or biologics. Fabrication of titanium meta-materials via additive manufacturing (AM) has unlocked the potential to modulate mechanical and biological performance to achieve a combination of properties previously unachievable. Meta-material scaffolds with topology based on triply periodic minimal surfaces (TPMS) have gained increasing interest for use in biomedical applications due to their bioinspired nature. Despite enthusiasm for TPMS-based titanium scaffolds due to their high strength to stiffness ratio, high permeability, and curvature similar to trabecular bone, there is little preclinical evidence to support their in vivo response in bone. The present study sought to evaluate the performance of gyroid-sheet titanium scaffolds produced via AM to repair a critically size femoral cortical bone defect in rats. Empty gyroid-sheet scaffolds were shown to repair segmental defects with up to 38% of torsional strength and 54% torsional stiffness of the intact femur (control) at 12-weeks. Gyroid-sheet scaffolds carrying recombinant bone morphogenic protein-2 demonstrated bridging bone growth across the length of the defect, with torsional strength and stiffness superior to that of the intact controls.

Development of systemic immune dysregulation in a rat trauma model of biomaterial-associated infection

Orthopedic biomaterial-associated infections remain a major clinical challenge, with Staphylococcus aureus being the most common pathogen. S. aureus biofilm formation enhances immune evasion and antibiotic resistance, resulting in a local, indolent infection that can persist long-term without symptoms before eventual hardware failure, bone non-union, or sepsis. Immune modulation is an emerging strategy to combat host immune evasion by S. aureus. However, most immune modulation strategies are focused on local immune responses at the site of infection, with little emphasis on understanding the infection-induced and orthopedic-related systemic immune responses of the host, and their role in local infection clearance and tissue regeneration. This study utilized a rat bone defect model to investigate how implant-associated infection affects the systemic immune response. Long-term systemic immune dysregulation was observed with a significant systemic decrease in T cells and a concomitant increase in immunosuppressive myeloid-derived suppressor cells (MDSCs) compared to non-infected controls. Further, the control group exhibited a regulated and coordinated systemic cytokine response, which was absent in the infection group. Multivariate analysis revealed high levels of MDSCs to be most correlated with the infection group, while high levels of T cells were most correlated with the control group. Locally, the infection group had attenuated macrophage infiltration and increased levels of MDSCs in the local soft tissue compared to non-infected controls. These data reveal the widespread impacts of an orthopedic infection on both the local and the systemic immune responses, uncovering promising targets for diagnostics and immunotherapies that could optimize treatment strategies and ultimately improve patient outcomes.

Effects of controlled dual growth factor delivery on bone regeneration following composite bone-muscle injury

The objective of this study was to investigate the controlled release of two growth factors (BMP-2 and VEGF) as a treatment strategy for bone healing in clinically challenging composite injuries, consisting of a femoral segmental bone defect and volumetric muscle loss. This is the first investigation of dual growth factor delivery in a composite injury model using an injectable delivery system consisting of heparin microparticles and alginate gel. The loading efficiency of growth factors into these biomaterials was found to be >90%, revealing a strong affinity of VEGF and BMP-2 to heparin and alginate. The system could achieve simultaneous or tunable release of VEGF and BMP-2 by varying the loading strategy. Single growth factor delivery (VEGF or BMP-2 alone) significantly enhanced vascular growth in vitro. However, no synergistic effect was observed for dual growth factor (BMP-2 + VEGF) delivery in vitro. Effective bone healing was achieved in all treatment groups (BMP-2, simultaneous or tunable delivery of BMP-2 and VEGF) in the composite injury model. The mechanics of the regenerated bone reached a maximum strength of ~52% of intact bone with tunable delivery of VEGF and BMP-2. Overall, simultaneous or tunable co-delivery of low-dose BMP-2 and VEGF failed to fully restore the mechanics of bone in this injury model. Given the severity of the composite injury, VEGF alone may not be sufficient to establish mature and stable blood vessels when compared with previous studies co-delivering BMP-2+VEGF enhanced bone tissue regeneration. Hence, future studies are warranted to develop an alternative treatment strategy focusing on better control over growth factor dose, spatiotemporal delivery, and additional growth factors to regenerate fully functional bone tissue. STATEMENT OF SIGNIFICANCE: We have developed an injectable delivery system consisting of heparin microparticles and an alginate hydrogel that is capable of delivering multiple growth factors in a tunable manner. We used this delivery system to deliver BMP-2 and VEGF in a rodent model of composite bone-muscle injury that mimics clinical type III open fractures. An advanced treatment strategy is necessary for these injuries in order to avoid the negative side effects of high doses of growth factors and because it has been shown that the addition of a muscle injury in this model attenuates the bone regenerative effect of BMP-2. This is the first study to test the effects of dual growth factor delivery (BMP-2/VEGF) on bone healing in a composite bone-muscle injury model and is expected to open up new directions in protein delivery for regenerative medicine.

Articular Cartilage- and Synoviocyte-Binding Poly(ethylene glycol) Nanocomposite Microgels as Intra-Articular Drug Delivery Vehicles for the Treatment of Osteoarthritis

Intra-articular (IA) injection is an attractive route of administration for the treatment of osteoarthritis (OA). However, free drugs injected into the joint space are rapidly cleared and many of them can induce adverse off-target effects on different IA tissues. To overcome these limitations, we designed nanocomposite 4-arm-poly(ethylene glycol)-maleimide (PEG-4MAL) microgels, presenting cartilage- or synoviocyte-binding peptides, containing poly(lactic-co-glycolic) acid (PLGA) nanoparticles (NPs) as an IA small molecule drug delivery system. Microgels containing rhodamine B (model drug)-loaded PLGA NPs were synthesized using microfluidics technology and exhibited a sustained, near zero-order release of the fluorophore over 16 days in vitro. PEG-4MAL microgels presenting synoviocyte- or cartilage-targeting peptides specifically bound to rabbit and human synoviocytes or to bovine articular cartilage in vitro, respectively. Finally, using a rat model of post-traumatic knee OA, PEG-4MAL microgels were shown to be retained in the joint space for at least 3 weeks without inducing any joint degenerative changes as measured by EPIC-μCT and histology. Additionally, all microgel formulations were found trapped in the synovial membrane and significantly increased the IA retention time of a model small molecule near-infrared (NIR) dye compared to that of the free dye. These results suggest that peptide-functionalized nanocomposite PEG-4MAL microgels represent a promising intra-articular vehicle for tissue-localized drug delivery and prolonged IA drug retention for the treatment of OA.

3D Printing of Microgel-Loaded Modular Microcages as Instructive Scaffolds for Tissue Engineering

Biomaterial scaffolds have served as the foundation of tissue engineering and regenerative medicine. However, scaffold systems are often difficult to scale in size or shape in order to fit defect-specific dimensions, and thus provide only limited spatiotemporal control of therapeutic delivery and host tissue responses. Here, a lithography-based 3D printing strategy is used to fabricate a novel miniaturized modular microcage scaffold system, which can be assembled and scaled manually with ease. Scalability is based on an intuitive concept of stacking modules, like conventional toy interlocking plastic blocks, allowing for literally thousands of potential geometric configurations, and without the need for specialized equipment. Moreover, the modular hollow-microcage design allows each unit to be loaded with biologic cargo of different compositions, thus enabling controllable and easy patterning of therapeutics within the material in 3D. In summary, the concept of miniaturized microcage designs with such straight-forward assembly and scalability, as well as controllable loading properties, is a flexible platform that can be extended to a wide range of materials for improved biological performance.

Biomaterial strategies for improved intra-articular drug delivery

Osteoarthritis (OA) is a joint degenerative disease that has become one of the leading causes of disability in the world. It is estimated that OA affects 50 million adults in the United States. Currently, there are no FDA-approved treatments that slow OA progression and its treatment is limited to pain management strategies and life style changes. Despite the discovery of several disease-modifying OA drugs (DMOADs) and promising results in preclinical studies, their clinical translation has been significantly limited because of poor intra-articular (IA) bioavailability and challenges in delivering these compounds to tissues of interest within the joint. Here, we review current OA treatments and their effectiveness at reducing joint pain, as well as novel targets for OA treatment and the challenges related to their clinical translation. Moreover, we discuss intra-articular (IA) drug delivery as a promising route of administration, describe its inherent challenges, and review recent advances in biomaterial-based IA drug delivery for OA treatment. Finally, we highlight the potential of tissue targeting in the development of effective IA drug delivery systems.

Extracellular matrix compression temporally regulates microvascular angiogenesis

Mechanical cues influence tissue regeneration, and although vasculature is known to be mechanically sensitive, little is known about the effects of bulk extracellular matrix deformation on the nascent vessel networks found in healing tissues. Previously, we found that dynamic matrix compression in vivo potently regulated revascularization during bone tissue regeneration; however, whether matrix deformations directly regulate angiogenesis remained unknown. Here, we demonstrated that load initiation time, magnitude, and mode all regulate microvascular growth, as well as upstream angiogenic and mechanotransduction signaling pathways. Immediate load initiation inhibited angiogenesis and expression of early sprout tip cell selection genes, while delayed loading enhanced microvascular network formation and upstream signaling pathways. This research provides foundational understanding of how extracellular matrix mechanics regulate angiogenesis and has critical implications for clinical translation of new regenerative medicine therapies and physical rehabilitation strategies designed to enhance revascularization during tissue regeneration.

Wireless sensor enables longitudinal monitoring of regenerative niche mechanics during rehabilitation that enhance bone repair

Mechanical loads exerted on the skeleton during activities such as walking are important regulators of bone repair, but dynamic biomechanical signals are difficult to measure inside the body. The inability to measure the mechanical environment in injured tissues is a significant barrier to developing integrative regenerative and rehabilitative strategies that can accelerate recovery from fracture, segmental bone loss, and spinal fusion. Here we engineered an implantable strain sensor platform and longitudinally measured strain across a bone defect in real-time throughout rehabilitation. The results showed that load-sharing permitted by a load-sharing fixator initially delivered a two-fold increase in deformation magnitude, subsequently increased mineralized bridging by nearly three-fold, and increased bone formation by over 60%. These data implicate a critical role for early mechanical cues on the long term healing response as strain cycle magnitude at 1 week (before appreciable healing occurred) had a significant positive correlation with the long-term bone regeneration outcomes. Furthermore, we found that sensor readings correlated with the status of healing, suggesting a role for strain sensing as an X-ray-free healing assessment platform. Therefore, non-invasive strain measurements may possess diagnostic potential to evaluate bone repair and reduce clinical reliance on current radiation-emitting imaging methods. Together, this study demonstrates a promising framework to quantitatively develop and exploit mechanical rehabilitation strategies that enhance bone repair.

American Society for Bone and Mineral Research-Orthopaedic Research Society Joint Task Force Report on Cell-Based Therapies - Secondary Publication

Cell-based therapies, defined here as the delivery of cells in vivo to treat disease, have recently gained increasing public attention as a potentially promising approach to restore structure and function to musculoskeletal tissues. Although cell-based therapy has the potential to improve the treatment of disorders of the musculoskeletal system, there is also the possibility of misuse and misrepresentation of the efficacy of such treatments. The medical literature contains anecdotal reports and research studies, along with web-based marketing and patient testimonials supporting cell-based therapy. Both the American Society for Bone and Mineral Research (ASBMR) and the Orthopaedic Research Society (ORS) are committed to ensuring that the potential of cell-based therapies is realized through rigorous, reproducible, and clinically meaningful scientific discovery. The two organizations convened a multidisciplinary and international Task Force composed of physicians, surgeons, and scientists who are recognized experts in the development and use of cell-based therapies. The Task Force was charged with defining the state-of-the art in cell-based therapies and identifying the gaps in knowledge and methodologies that should guide the research agenda. The efforts of this Task Force are designed to provide researchers and clinicians with a better understanding of the current state of the science and research needed to advance the study and use of cell-based therapies for skeletal tissues. The design and implementation of rigorous, thorough protocols will be critical to leveraging these innovative treatments and optimizing clinical and functional patient outcomes. In addition to providing specific recommendations and ethical considerations for preclinical and clinical investigations, this report concludes with an outline to address knowledge gaps in how to determine the cell autonomous and nonautonomous effects of a donor population used for bone regeneration. © 2020 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:485-502, 2020.

T1ρ, T2 mapping, and EPIC-µCT Imaging in a Canine Model of Knee Osteochondral Injury

The dog is the most commonly used large animal model for the study of osteoarthritis. Optimizing methods for assessing cartilage health would prove useful in reducing the number of dogs needed for a valid study of osteoarthritis and cartilage repair. Twelve beagles had critical-sized osteochondral defects created in the medial femoral condyle of both knees. Eight dogs had T1ρ and T2 magnetic resonance imaging (MRI) performed approximately 6 months after defect creation. Following MRI evaluations, all 12 dogs were humanely euthanatized and cartilage samples were obtained from the medial and lateral femoral condyles, medial and lateral tibial plateaus, trochlear groove, and patella for proteoglycan and collagen quantification. Equilibrium partitioning of an ionic contrast (EPIC)-µCT was then performed followed by the histologic assessment of the knees. Correlations between T1ρ, T2, EPIC-µCT and proteoglycan, collagen, and histology scores were assessed using a multivariate analysis accounting for correlations from samples within the same knee and in the same dog. Pearson's correlation coefficients were calculated to assess the strength of significant relationships. Correlations between µCT values and biochemical or histologic assessment were weak to moderately strong (0.09-0.41; p < 0.0001-0.66). There was a weak correlation between the T2 values and cartilage proteoglycan (-0.32; p = 0.04). The correlation between T1ρ values and cartilage proteoglycan were moderately strong (-0.38; p < 0.05) while the strongest correlation was between the T1ρ values and histological assessment of cartilage with a correlation coefficient of 0.58 (p < 0.0001). These data suggest that T1ρ shows promise for possible utility in the translational study of cartilage health and warrants further development in this species. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:368-377, 2020.

Heparin-mediated delivery of bone morphogenetic protein-2 improves spatial localization of bone regeneration

Supraphysiologic doses of bone morphogenetic protein-2 (BMP-2) are used clinically to promote bone formation in fracture nonunions, large bone defects, and spinal fusion. However, abnormal bone formation (i.e., heterotopic ossification) caused by rapid BMP-2 release from conventional collagen sponge scaffolds is a serious complication. We leveraged the strong affinity interactions between heparin microparticles (HMPs) and BMP-2 to improve protein delivery to bone defects. We first developed a computational model to investigate BMP-2-HMP interactions and demonstrated improved in vivo BMP-2 retention using HMPs. We then evaluated BMP-2-loaded HMPs as a treatment strategy for healing critically sized femoral defects in a rat model that displays heterotopic ossification with clinical BMP-2 doses (0.12 mg/kg body weight). HMPs increased BMP-2 retention in vivo, improving spatial localization of bone formation in large bone defects and reducing heterotopic ossification. Thus, HMPs provide a promising opportunity to improve the safety profile of scaffold-based BMP-2 delivery.

American Society for Bone and Mineral Research-Orthopaedic Research Society Joint Task Force Report on Cell-Based Therapies

Cell-based therapies, defined here as the delivery of cells in vivo to treat disease, have recently gained increasing public attention as a potentially promising approach to restore structure and function to musculoskeletal tissues. Although cell-based therapy has the potential to improve the treatment of disorders of the musculoskeletal system, there is also the possibility of misuse and misrepresentation of the efficacy of such treatments. The medical literature contains anecdotal reports and research studies, along with web-based marketing and patient testimonials supporting cell-based therapy. Both the American Society for Bone and Mineral Research (ASBMR) and the Orthopaedic Research Society (ORS) are committed to ensuring that the potential of cell-based therapies is realized through rigorous, reproducible, and clinically meaningful scientific discovery. The two organizations convened a multidisciplinary and international Task Force composed of physicians, surgeons, and scientists who are recognized experts in the development and use of cell-based therapies. The Task Force was charged with defining the state-of-the art in cell-based therapies and identifying the gaps in knowledge and methodologies that should guide the research agenda. The efforts of this Task Force are designed to provide researchers and clinicians with a better understanding of the current state of the science and research needed to advance the study and use of cell-based therapies for skeletal tissues. The design and implementation of rigorous, thorough protocols will be critical to leveraging these innovative treatments and optimizing clinical and functional patient outcomes. In addition to providing specific recommendations and ethical considerations for preclinical and clinical investigations, this report concludes with an outline to address knowledge gaps in how to determine the cell autonomous and nonautonomous effects of a donor population used for bone regeneration. © 2019 American Society for Bone and Mineral Research.

Reduced Size Profile of Amniotic Membrane Particles Decreases Osteoarthritis Therapeutic Efficacy

Osteoarthritis (OA) is a widespread disease that continues to lack approved and efficacious treatments that modify disease progression. Micronized dehydrated human amnion/chorion membrane (μ-dHACM) has been shown to be effective in reducing OA progression, but many of the engineering design parameters have not been explored. The objectives of this study were to characterize the particle size distributions of two μ-dHACM formulations and to investigate the influence of these distributions on the in vivo therapeutic efficacy of μ-dHACM. Male Lewis rats underwent medial meniscus transection (MMT) or sham surgery, and intra-articular injections of saline, μ-dHACM, or reduced particle size μ-dHACM (RPS μ-dHACM) were administered at 24 hours postsurgery (n = 9 per treatment group). After 3 weeks, the animals were euthanized, and left legs harvested for equilibrium partitioning of an ionic contrast agent microcomputed tomography and histological analysis. μ-dHACM and RPS μ-dHACM particles were fluorescently tagged and particle clearance was tracked in vivo for up to 42 days postsurgery. Protein elution from both formulations was quantified in vitro. Treatment with μ-HACM, but not RPS μ-dHACM, reduced lesion volume in the MMT model 3 weeks postsurgery. In contrast, RPS μ-dHACM increased cartilage surface roughness and osteophyte cartilage thickness and volume compared to saline treatment. There was no difference of in vivo fluorescently tagged particle clearance between the two μ-dHACM sizes. RPS μ-dHACM showed significantly greater protein elution in vitro over 21 days. Overall, delivery of RPS μ-dHACM did result in an increase of in vivo joint degeneration and in vitro protein elution compared to μ-dHACM, but did not result in differences in joint clearance in vivo. These results suggest that particle size and factor elution may be tailorable factors that are important to optimize for particulate amniotic membrane treatment to be an effective therapy for OA. Impact Statement Osteoarthritis (OA) is a widespread disease that continues to lack treatments that modify the progression of the disease. Micronized dehydrated human amnion/chorion membrane (μ-dHACM) has been shown to be effective in reducing OA progression, but many of the engineering design parameters have not been explored. This work investigates the effects of particle size profile of the μ-dHACM particles and lays out the methods used in these studies. The results of this work will guide engineers in designing μ-dHACM treatments specifically and disease-modifying OA therapeutics generally, and it demonstrates the utility of novel therapeutic evaluation methods such as contrast-enhanced microcomputed tomography.

Fatigue behavior of As-built selective laser melted titanium scaffolds with sheet-based gyroid microarchitecture for bone tissue engineering

Selective laser melting (SLM) has enabled the production of porous titanium structures with biological and mechanical properties that mimic bone for orthopedic applications. These porous structures have a reduced effective stiffness which leads to improved mechanotransduction between the implant and bone. Triply periodic minimal surfaces (TMPS), specifically the sheet-based gyroid structures, have improved compressive fatigue resistance due lack of stress concentrations. Sheet-based gyroid microarchitectures also have high surface area, permeability, and zero mean curvature. This study examines the effects of the gyroid microarchitectural design in parallel with SLM parameters on structure and function of as-built titanium alloy (Ti6Al4V ELI) scaffolds. Scaffold design was varied by varying unit cell size and wall thickness to produce scaffolds with porosity within the range of trabecular bone (50-90%). Manufacturer's default and refined laser parameters were used to examine the effect of input energy density on mechanical properties. Scaffolds exhibited a stretching-dominated deformation behavior under both compressive and tensile loading, and porosity dependent stiffness and strength. Internal void defects were observed within the walls of the gyroids structure, serving as sites for crack initiation leading to failure. Refinement of laser parameters resulted in increased compressive and tensile fatigue behavior, particularly for thicker walled gyroid microarchitectures, while thinner walls showed no significant change. The observed properties of as-built gyroid sheet microarchitectures indicates that these structures have potential for use in bone engineering applications. Furthermore, these results highlight the importance of parallel design and processing optimization for complex sheet-based porous structures produced via SLM. STATEMENT OF SIGNIFICANCE: Selective laser melting (SLM) is an additive manufacturing technology which produces complex porous scaffolds for orthopedic applications. Titanium alloy scaffolds with novel sheet-based gyroid microarchitectures were produced via SLM and evaluated for mechanical performance including fatigue behavior. Gyroid structures are function based topologies have been hypothesized to be promising for tissue engineering scaffolds due to the high surface area to volume ratio, zero mean curvature, and high permeability. This paper presents the effects of scaffold design and processing parameters in parallel, a novel study in the field on bone tissue scaffolds produced via additive manufacturing. Additionally, the comparison of compressive and tensile behavior of scaffolds presented is important in characterizing behavior and failure mechanisms of porous metals which undergo complex loading in orthopedic applications.

Decorin-supplemented collagen hydrogels for the co-delivery of bone morphogenetic protein-2 and microvascular fragments to a composite bone-muscle injury model with impaired vascularization

Traumatic musculoskeletal injuries that result in bone defects or fractures often affect both bone and the surrounding soft tissue. Clinically, these types of multi-tissue injuries have increased rates of complications and long-term disability. Vascular integrity is a key clinical indicator of injury severity, and revascularization of the injury site is a critical early step of the bone healing process. Our lab has previously established a pre-clinical model of composite bone-muscle injury that exhibits impaired bone healing; however, the vascularization response in this model had not yet been investigated. Here, the early revascularization of a bone defect following composite injury is shown to be impaired, and subsequently the therapeutic potential of combined vascularization and osteoinduction was investigated to overcome the impaired regeneration in composite injuries. A decorin (DCN)-supplemented collagen hydrogel was developed as a biomaterial delivery vehicle for the co-delivery microvascular fragments (MVF), which are multicellular segments of mature vasculature, and bone morphogenetic protein-2 (BMP-2), a potent osteoinductive growth factor. We hypothesized that collagen + DCN would increase BMP-2 retention over collagen alone due to DCN's ability to sequester TGF-ß growth factors. We further hypothesized that MVF would increase both early vascularization and subsequent BMP-2-mediated bone regeneration. Contrary to our hypothesis, BMP + MVF decreased the number of blood vessels relative to BMP alone and had no effect on bone healing. However, collagen + DCN was demonstrated to be a BMP-2 delivery vehicle capable of achieving bridging in the challenging composite defect model that is comparable to that achieved with a well-established alginate-based delivery system. STATEMENT OF SIGNIFICANCE: We have previously established a model of musculoskeletal trauma that exhibits impaired bone healing. For the first time, this work shows that the early revascularization response is also significantly, albeit modestly, impaired. A decorin-supplemented collagen hydrogel was used for the first time in vivo as a delivery vehicle for both a cell-based vascular therapeutic, MVF, and an osteoinductive growth factor, BMP-2. While MVF did not improve vascular volume or bone healing, collagen + DCN is a BMP-2 delivery vehicle capable of achieving bridging in the challenging composite defect model. Based on its support of robust angiogenesis in vitro, collagen + DCN may be extended for future use with other vascular therapeutics such as pre-formed vascular networks.

Effect of porous orthopaedic implant material and structure on load sharing with simulated bone ingrowth: A finite element analysis comparing titanium and PEEK

Osseointegration of load-bearing orthopaedic implants, including interbody fusion devices, is critical to long-term biomechanical functionality. Mechanical loads are a key regulator of bone tissue remodeling and maintenance, and stress-shielding due to metal orthopaedic implants being much stiffer than bone has been implicated in clinical observations of long-term bone loss in tissue adjacent to implants. Porous features that accommodate bone ingrowth have improved implant fixation in the short term, but long-term retrieval studies have sometimes demonstrated limited, superficial ingrowth into the pore layer of metal implants and aseptic loosening remains a problem for a subset of patients. Polyether-ether-ketone (PEEK) is a widely used orthopaedic material with an elastic modulus more similar to bone than metals, and a manufacturing process to form porous PEEK was recently developed to allow bone ingrowth while preserving strength for load-bearing applications. To investigate the biomechanical implications of porous PEEK compared to porous metals, we analyzed finite element (FE) models of the pore structure-bone interface using two clinically available implants with high (> 60%) porosity, one being constructed from PEEK and the other from electron beam 3D-printed titanium (Ti). The objective of this study was to investigate how porous PEEK and porous Ti mechanical properties affect load sharing with bone within the porous architectures over time. Porous PEEK substantially increased the load share transferred to ingrown bone compared to porous Ti under compression (i.e. at 4 weeks: PEEK = 66%; Ti = 13%), tension (PEEK = 71%; Ti = 12%), and shear (PEEK = 68%; Ti = 9%) at all time points of simulated bone ingrowth. Applying PEEK mechanical properties to the Ti implant geometry and vice versa demonstrated that the observed increases in load sharing with PEEK were primarily due to differences in intrinsic elastic modulus and not pore architecture (i.e. 4 weeks, compression: PEEK material/Ti geometry = 53%; Ti material/PEEK geometry = 12%). Additionally, local tissue energy effective strains on bone tissue adjacent to the implant under spinal load magnitudes were over two-fold higher with porous PEEK than porous Ti (i.e. 4 weeks, compression: PEEK = 784 ± 351 microstrain; Ti = 180 ± 300 microstrain; and 12 weeks, compression: PEEK = 298 ± 88 microstrain; Ti = 121 ± 49 microstrain). The higher local strains on bone tissue in the PEEK pore structure were below previously established thresholds for bone damage but in the range necessary for physiological bone maintenance and adaptation. Placing these strain magnitudes in the context of literature on bone adaptation to mechanical loads, this study suggests that porous PEEK structures may provide a more favorable mechanical environment for bone formation and maintenance under spinal load magnitudes than currently available porous 3D-printed Ti, regardless of the level of bone ingrowth.

PGC-1α overexpression partially rescues impaired oxidative and contractile pathophysiology following volumetric muscle loss injury

Volumetric muscle loss (VML) injury is characterized by a non-recoverable loss of muscle fibers due to ablative surgery or severe orthopaedic trauma, that results in chronic functional impairments of the soft tissue. Currently, the effects of VML on the oxidative capacity and adaptability of the remaining injured muscle are unclear. A better understanding of this pathophysiology could significantly shape how VML-injured patients and clinicians approach regenerative medicine and rehabilitation following injury. Herein, the data indicated that VML-injured muscle has diminished mitochondrial content and function (i.e., oxidative capacity), loss of mitochondrial network organization, and attenuated oxidative adaptations to exercise. However, forced PGC-1α over-expression rescued the deficits in oxidative capacity and muscle strength. This implicates physiological activation of PGC1-α as a limiting factor in VML-injured muscle's adaptive capacity to exercise and provides a mechanistic target for regenerative rehabilitation approaches to address the skeletal muscle dysfunction.

Effects of BMP-2 dose and delivery of microvascular fragments on healing of bone defects with concomitant volumetric muscle loss

Traumatic composite bone-muscle injuries, such as open fractures, often require multiple surgical interventions and still typically result in long-term disability. Clinically, a critical indicator of composite injury severity is vascular integrity; vascular damage alone is sufficient to assign an open fracture to the most severe category. Challenging bone injuries are often treated with bone morphogenetic protein 2 (BMP-2), an osteoinductive growth factor, delivered on collagen sponge. Previous studies in a composite defect model found that a minimally bridging dose in the segmental defect model was unable to overcome concomitant muscle damage, but the effect of BMP dose on composite injuries has not yet been studied. Here, we test the hypotheses that BMP-2-mediated functional regeneration of composite extremity injuries is dose dependent and can be further enhanced via co-delivery of adipose-derived microvascular fragments (MVF), which have been previously shown to increase tissue vascular volume. Although MVF did not improve healing outcomes, we observed a significant BMP-2 dose-dependent increase in regenerated bone volume and biomechanical properties. This is the first known report of an increased BMP-2 dose improving bone healing with concomitant muscle damage. While high dose BMP-2 delivery can induce heterotopic ossification (HO) and increased inflammation, the maximum 10 μg dose used in this study did not result in HO and was associated with a lower circulating inflammatory cytokine profile than the low dose (2.5 μg) group. These data support the potential benefits of an increased, though still moderate, BMP-2 dose for treatment of bone defects with concomitant muscle damage. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. J Orthop Res.

Impaired bone healing following treatment of established nonunion correlates with serum cytokine expression

Delayed union and nonunion are a significant concern in long bone fractures and spinal fusions. Treatment of nonunion often entails multiple revision surgeries that further increase the financial, physical, and emotional burden on patients. The optimal treatment strategy for nonunions remains unclear in many cases, and the risk of complications after revision procedures remains high. This is in part due to our limited understanding of the biological mechanisms that inhibit proper bone healing and lead to nonunion. And yet, few preclinical models directly investigate how healing is impacted after establishment of nonunion, with most instead primarily focusing on treatment immediately after a fresh bone injury. Here, we utilized a critical size femoral defect model in rats where treatment was delayed 8 weeks post-injury, at which time nonunion was established. In this study, acute and delayed treatments with bone morphogenetic protein-2 (BMP-2) were assessed. We found that delayed treatment resulted in decreased bone formation and reduced mechanical strength compared to acute treatment, even when BMP-2 dose was increased by 2.5 times the acute treatment dose. Interestingly, serum cytokine analysis at 12 weeks post-treatment revealed signs of chronic immune dysregulation after delayed treatment. In particular, non-responders (rats that did not exhibit defect bridging) demonstrated higher overall expression of inflammatory cytokines, including TNFα and IL-1β, compared to responders. These findings suggest that re-establishing long-term immune homeostasis may be critical for successful bone healing, particularly after nonunion. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:299-307, 2019.

Full-thickness rotator cuff tear in rat results in distinct temporal expression of multiple proteases in tendon, muscle, and cartilage

The etiology of joint tissue degeneration following rotator cuff tear remains unclear. Thus, the purpose of this study was to understand the timeline of protease activity in the soft tissues of the shoulder (tendon, muscle, and cartilage) that may lead to down-stream degeneration following rotator cuff tear. A well-established rat model involving suprascapular nerve denervation and supraspinatus/infraspinatus tendon transection was employed. Histological staining and/or micro-computed tomography (µCT) were used to observe structural damage in the supraspinatus tendon and muscle, humeral head cartilage, and subchondral bone. Multiplex gelatin zymography was utilized to assess protease activity in the supraspinatus tendon and muscle, and humeral head cartilage. Zymography analysis demonstrated that cathepsins were upregulated in the first week in all tissues, while MMP-2 maintained prolonged activity in supraspinatus tendon between 1 and 3 weeks and increased only at 3 weeks in supraspinatus muscle. In supraspinatus tendon, increased cathepsin L and MMP-2 activity in the first week was concurrent with matrix disorganization and infiltration of inflammatory cells. In contrast, significant upregulation of cathepsin L and K activity in supraspinatus muscle and humeral head cartilage did not correspond to any visible tissue damage at 1 week. However, focal defects developed in half of all animals' humeral head cartilage by 12 weeks (volume: 0.12 ± 0.09 mm³ ). This work provides a more comprehensive understanding of biochemical changes to joint tissue over time following rotator cuff tear. Overall, this provides insight into potential therapeutic targets and will better inform ideal intervention times and treatments for each tissue. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:490-502, 2019.

Chondroitin Sulfate Glycosaminoglycan Scaffolds for Cell and Recombinant Protein-Based Bone Regeneration

Bone morphogenetic protein 2 (BMP‐2)‐loaded collagen sponges remain the clinical standard for treatment of large bone defects when there is insufficient autograft, despite associated complications. Recent efforts to negate comorbidities have included biomaterials and gene therapy approaches to extend the duration of BMP‐2 release and activity. In this study, we compared the collagen sponge clinical standard to chondroitin sulfate glycosaminoglycan (CS‐GAG) scaffolds as a delivery vehicle for recombinant human BMP‐2 (rhBMP‐2) and rhBMP‐2 expression via human BMP‐2 gene inserted into mesenchymal stem cells (BMP‐2 MSC). We demonstrated extended release of rhBMP‐2 from CS‐GAG scaffolds compared to their collagen sponge counterparts, and further extended release from CS‐GAG gels seeded with BMP‐2 MSC. When used to treat a challenging critically sized femoral defect model in rats, both rhBMP‐2 and BMP‐2 MSC in CS‐GAG induced comparable bone formation to the rhBMP‐2 in collagen sponge, as measured by bone volume, strength, and stiffness. We conclude that CS‐GAG scaffolds are a promising delivery vehicle for controlling the release of rhBMP‐2 and to mediate the repair of critically sized segmental bone defects. stem cells translational medicine2019;8:575–585

Dynamic mass spectrometry probe for electrospray ionization mass spectrometry monitoring of bioreactors for therapeutic cell manufacturing

Large-scale manufacturing of therapeutic cells requires bioreactor technologies with online feedback control enabled by monitoring of secreted biomolecular critical quality attributes (CQAs). Electrospray ionization mass spectrometry (ESI-MS) is a highly sensitive label-free method to detect and identify biomolecules, but requires extensive sample preparation before analysis, making online application of ESI-MS challenging. We present a microfabricated, monolithically integrated device capable of continuous sample collection, treatment, and direct infusion for ESI-MS detection of biomolecules in high-salt solutions. The dynamic mass spectrometry probe (DMSP) uses a microfluidic mass exchanger to rapidly condition samples for online MS analysis by removing interfering salts, while concurrently introducing MS signal enhancers to the sample for sensitive biomolecular detection. Exploiting this active conditioning capability increases MS signal intensity and signal-to-noise ratio. As a result, sensitivity for low-concentration biomolecules is significantly improved, and multiple proteins can be detected from chemically complex samples. Thus, the DMSP has significant potential to serve as an enabling portion of a novel analytical tool for discovery and monitoring of CQAs relevant to therapeutic cell manufacturing.

Materials Science and Design Principles of Growth Factor Delivery Systems in Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are signaling molecules that direct cell development by providing biochemical cues for stem cell proliferation, migration, and differentiation. GFs play a key role in tissue regeneration, but one major limitation of GF-based therapies is dosage-related adverse effects. Additionally, the clinical applications and efficacy of GFs are significantly affected by the efficiency of delivery systems and other pharmacokinetic factors. Hence, it is crucial to design delivery systems that provide optimal activity, stability, and tunable delivery for GFs. Understanding the physicochemical properties of the GFs and the biomaterials utilized for the development of biomimetic GF delivery systems is critical for GF-based regeneration. Many different delivery systems have been developed to achieve tunable delivery kinetics for single or multiple GFs. The identification of ideal biomaterials with tunable properties for spatiotemporal delivery of GFs is still challenging. This review characterizes the types, properties, and functions of GFs, the materials science of widely used biomaterials, and various GF loading strategies to comprehensively summarize the current delivery systems for tunable spatiotemporal delivery of GFs aimed for tissue regeneration applications. This review concludes by discussing fundamental design principles for GF delivery vehicles based on the interactive physicochemical properties of the proteins and biomaterials.

The Effects of Age and Dose on Gene Expression and Segmental Bone Defect Repair After BMP-2 Delivery

Age is a well‐known influential factor in bone healing, with younger patients generally healing bone fractures more rapidly and suffering fewer complications compared with older patients. Yet the impact age has on the response to current bone healing treatments, such as delivery of bone morphogenetic protein 2 (BMP‐2), remains poorly characterized. It remains unclear how or if therapeutic dosing of BMP‐2 should be modified to account for age‐related differences in order to minimize potential adverse effects and consequently improve patient bone‐healing outcomes. For this study, we sought to address this issue by using a preclinical critically sized segmental bone defect model in rats to investigate age‐related differences in bone repair after delivery of BMP‐2 in a collagen sponge, the current clinical standard. Femoral defects were created in young (7‐week‐old) and adult (8‐month‐old) rats, and healing was assessed using gene expression analyses, longitudinal radiography, ex vivo micro‐computed tomography (µCT), as well as torsional testing. We found that young rats demonstrated elevated expression of genes related to osteogenesis, chondrogenesis, and matrix remodeling at the early 1‐week time point compared with adult rats. These early gene expression differences may have impacted long‐term healing as the regenerated bones of young rats exhibited higher bone mineral densities compared with those of adult rats after 12 weeks. Furthermore, the young rats demonstrated significantly more bone formation and increased mechanical strength when BMP‐2 dose was increased from 1 µg to 10 µg, a finding not observed in adult rats. Overall, these results indicate there are age‐related differences in BMP‐2‐mediated bone regeneration, including relative dose sensitivity, suggesting that age is an important consideration when implementing a BMP‐2 treatment strategy. © 2018 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Aggregate mesenchymal stem cell delivery ameliorates the regenerative niche for muscle repair

Duchenne muscular dystrophy is a severe muscle wasting disease due to the absence of the dystrophin protein from the muscle cell membrane, which renders the muscle susceptible to continuous damage. In Duchenne muscular dystrophy patients, muscle weakness, together with cycles of degeneration/regeneration and replacement with noncontractile tissue, limit mobility and lifespan. Because the loss of dystrophin results in loss of polarity and a reduction in the number of self-renewing satellite cells, it is postulated that these patients could achieve an improved quality of life if delivered cells could restore satellite cell function. In this study, we used both an established myotoxic injury model in wild-type (WT) mice and mdx mice alone (spontaneous muscle damage). Single (SC) and aggregated (AGG) mesenchymal stem cells (MSCs) were injected into the gastrocnemius muscles 4 hr after injury (WT mice). The recovery of peak isometric torque was longitudinally assessed over 5 weeks, with earlier takedowns for histological assessment of healing (fibre cross-section area and central nucleation) and MSC retention. AGG-treated WT mice had significantly greater torque recovery at Day 14 than SC or saline-treated mice and a greater CSA at Day 10, compared with SC/saline. AGG-treated mdx mice had a greater peak isometric torque compared with SC/saline. In vitro immunomodulatory factor secretion of AGG-MSCs was higher than SC-MSCs for all tested growth factors with the largest difference observed in hepatocyte growth factor. Future studies are necessary to pair immunomodulatory factor secretion with functional attributes, to better predict the potential therapeutic value of MSC treatment modalities.