Enhancement of bone regeneration by dual release of a macrophage recruitment agent and platelet-rich plasma from gelatin hydrogels

Stiffness modification of photopolymerizable gelatin-methacrylate hydrogels influences endothelial differentiation of human mesenchymal stem cells

For stem cell differentiation, the microenvironment can play an important role, and hydrogels can provide a three-dimensional microenvironment to allow native cell growth in vitro. A challenge is that the stem cell's differentiation can be influenced by the matrix stiffness. We demonstrate a low-toxicity method to create different stiffness matrices, by using a photopolymerizable gelatin methacrylate (GelMA) hydrogel cross-linked by blue light (440 nm). The stiffness and porosity of GelMA hydrogel is easily modified by altering its concentration. We used human bone marrow mesenchymal stem cells (MSCs) as a cell source and cultured the GelMA-encapsulated cells with EGM-2 medium to induce endothelial differentiation. In our GelMA blue light hydrogel system, we found that MSCs can be differentiated into both endothelial-like and osteogenic-like cells. The mRNA expressions of endothelial cell markers CD31, von Willebrand factor, vascular endothelial growth factor receptor-2, and CD34 were significantly increased in softer GelMA hydrogels (7.5% and 10%) compared with stiffer matrices (15% GelMA). On the other hand, the enhancements of osteogenic markers mRNA expressions (Alkaline phosphatase (ALP), Runx2, osteocalcin, and osteopontin) were highest in 10% GelMA. We also found that 10% GelMA hydrogel offered optimal conditions for MSCs to form capillary-like structures. These results suggest that the mechanical properties of the GelMA hydrogel can influence both endothelial and osteogenic differentiation of MSCs and sequent capillary-like formation.

A Magnetically Responsive Biomaterial System for Flexibly Regulating the Duration between Pro- and Anti-Inflammatory Cytokine Deliveries

While inflammation can be problematic, it is nonetheless necessary for proper tissue regeneration. However, it remains unclear how the magnitude and duration of the inflammatory response impacts regenerative outcome. This is partially due to the difficulty in temporally regulating macrophage phenotype at wound sites. Here, a magnetically responsive biomaterial system potentially capable of temporally regulating macrophage phenotypes through sequential, on-demand cytokine deliveries is presented. This material system is designed to (i) rapidly recruit proinflammatory macrophages (M1) through initial cytokine deliveries and (ii) subsequently transition macrophages toward anti-inflammatory phenotypes (M2s) through delayed, magnetically triggered cytokine release. Here, the ability of this system to initially deliver proinflammatory cytokines (i.e., monocyte chemoattractant protein-1 and interferon gamma), recruit, and harbor an expanding macrophage population, and delay deliveries of anti-inflammatory cytokines (i.e., IL-4 and IL-10) until the application of magnetic fields from simple hand-held magnets is demonstrated. Critically, the timing and rate of these delayed deliveries can be remotely/magnetically controlled. This biomaterial system can provide a powerful tool in (i) understanding the relationship between inflammation and regenerative outcome, (ii) developing optimized cytokine delivery strategies, and (iii) clinically implementing those optimized delivery strategies with the on-demand versatility needed to alter the course of therapies in real time.

Blood derivatives awaken in regenerative medicine strategies to modulate wound healing

Blood components play key roles in the modulation of the wound healing process and, together with the provisional fibrin matrix ability to selectively bind bioactive molecules and control its spatial-temporal presentation, define the complex microenvironment that characterize this biological process. As a biomimetic approach, the use of blood derivatives in regenerative strategies has awakened as a source of multiple therapeutic biomolecules. Nevertheless, and despite their clinical relevance, blood derivatives have been showing inconsistent therapeutic results due to several factors, including proper control over their delivery mechanisms. Herein, we highlight recent trends on the use biomaterials to protect, sequester and deliver these pools of biomolecules in tissue engineering and regenerative medicine approaches. Particular emphasis is given to strategies that enable to control their spatiotemporal delivery and improve the selectivity of presentation profiles of the biomolecules derived from blood derivatives rich in platelets. Finally, we discussed possible directions for biomaterials design to potentiate the aimed regenerative effects of blood derivatives and achieve efficient therapies.

Drug delivery and tissue engineering to promote wound healing in the immunocompromised host: Current challenges and future directions

As regenerative medicine matures as a field, more promising technologies are being translated from the benchtop to the clinic. However, many of these strategies are designed with otherwise healthy hosts in mind and validated in animal models without other co-morbidities. In reality, many of the patient populations benefiting from drug delivery and tissue engineering-based devices to enhance wound healing also have significant underlying immunodeficiency. Specifically, patients suffering from diabetes, malignancy, human immunodeficiency virus, post-organ transplantation, and other compromised states have significant pleotropic immune defects that affect wound healing. In this work, we review the role of different immune cells in the regenerative process, highlight the effect of several common immunocompromised states on wound healing, and discuss different drug delivery strategies for overcoming immunodeficiencies.

Harnessing macrophage-mediated degradation of gelatin microspheres for spatiotemporal control of BMP2 release

Biomaterials-based approaches to harnessing the immune and inflammatory responses to potentiate wound healing hold important promise. Bone fracture healing is characterized by an acute inflammatory phase, followed by a transition to a regenerative and repair phase. In this study, we developed genipin-crosslinked gelatin microspheres designed to be preferentially degraded by inflammatory (M1) macrophages. Highly crosslinked (>90%) microspheres allowed efficient incorporation of bioactive bone morphogenetic protein 2 (BMP2), a potent stimulator of osteogenesis in progenitor cells, via electrostatic interactions. Release of BMP2 was directly correlated with degradation of the gelatin matrix. Exposure of microspheres to polarized murine macrophages showed that degradation was significantly higher in the presence of M1 macrophages, relative to alternatively activated (M2) macrophages and unpolarized controls. Microsphere degradation in the presence of non-inflammatory cells resulted in very low degradation rates. The expression of matrix metalloproteinases (MMPs) and tissue inhibitors of MMP (TIMPs) by macrophages were consistent with the observed phenotype-dependent degradation rates. Indirect co-culture of BMP2-loaded microspheres and macrophages with isolated adipose-derived mesenchymal stem cells (MSC) showed that M1 macrophages produced the strongest osteogenic response, comparable to direct supplementation of the culture medium with BMP2. Controlled release systems that are synchronized with the inflammatory response have the potential to provide better spatiotemporal control of growth factor delivery and therefore may improve the outcomes of recalcitrant wounds.

Delivery of cellular factors to regulate bone healing

Bone tissue has a strong intrinsic regenerative capacity, thanks to a delicate and complex interplay of cellular and molecular processes, which tightly involve the immune system. Pathological settings of anatomical, biomechanical or inflammatory nature may lead to impaired bone healing. Innovative strategies to enhance bone repair, including the delivery of osteoprogenitor cells or of potent cytokines/morphogens, indicate the potential of 'orthobiologics', but are not fully satisfactory. Here, we review different approaches based on the delivery of regenerative cues produced by cells but in cell-free, possibly off-the-shelf configurations. Such strategies exploit the paracrine effect of the secretome of mesenchymal stem/stromal cells, presented in soluble form, shuttled through extracellular vesicles, or embedded within the network of extracellular matrix molecules. In addition to osteoinductive molecules, attention is given to factors targeting the resident immune cells, to reshape inflammatory and immunity processes from scarring to regenerative patterns.

Applications of Metals for Bone Regeneration

The regeneration of bone tissue is the main purpose of most therapies in dental medicine. For bone regeneration, calcium phosphate (CaP)-based substitute materials based on natural (allo- and xenografts) and synthetic origins (alloplastic materials) are applied for guiding the regeneration processes. The optimal bone substitute has to act as a substrate for bone ingrowth into a defect, as well as resorb in the time frame needed for complete regeneration up to the condition of restitution ad integrum. In this context, the modes of action of CaP-based substitute materials have been frequently investigated, where it has been shown that such materials strongly influence regenerative processes such as osteoblast growth or differentiation and also osteoclastic resorption due to different physicochemical properties of the materials. However, the material characteristics needed for the required ratio between new bone tissue formation and material degradation has not been found, until now. The addition of different substances such as collagen or growth factors and also of different cell types has already been tested but did not allow for sufficient or prompt application. Moreover, metals or metal ions are used differently as a basis or as supplement for different materials in the field of bone regeneration. Moreover, it has already been shown that different metal ions are integral components of bone tissue, playing functional roles in the physiological cellular environment as well as in the course of bone healing. The present review focuses on frequently used metals as integral parts of materials designed for bone regeneration, with the aim to provide an overview of currently existing knowledge about the effects of metals in the field of bone regeneration.

3D printed microchannel networks to direct vascularisation during endochondral bone repair

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.

Core-shell poly(lactide-co-ε-caprolactone)-gelatin fiber scaffolds as pH-sensitive drug delivery systems

Dual-drug-loaded pH-responsive fiber scaffolds were successfully prepared by coaxial electrospinning. These were designed with the aim of being sutured into the resection site after tumor removal, to aid recovery and prevent cancer recurrence. The shell was made up of a mixture of gelatin and sodium bicarbonate (added to provide pH-sensitivity), and was loaded with the anti-inflammatory drug ciprofloxacin; the core comprised poly(lactide-co-ε-caprolactone) with the chemotherapeutic doxorubicin hydrochloride. Scanning electron microscopy revealed most fibers were smooth and homogeneous. Transmission electron microscopy demonstrated the presence of a clear core/shell structure. The fiber scaffolds were further characterized using infrared spectroscopy and X-ray diffraction, which proved that both drugs were present in the fibers in the amorphous form. The gelatin shells were cross-linked with glutaraldehyde to enhance their stability, and water contact angle measurements used to confirm they remained hydrophilic after this process, with angles between 10 and 35°. This is important for onward applications, since a hydrophilic surface is known to encourage cell proliferation. During in vitro drug release studies, a rapid and acid-responsive release of ciprofloxacin was seen, accompanied by sustained and long-term doxorubicin release. Both the release profiles and the mechanical strength of the fibers can effectively be tuned through the sodium bicarbonate content of the fibers: for instance, the break stress varies from 2.00 MPa to 2.57 MPa with an increase in sodium bicarbonate content. The pH values of aqueous media exposed to the scaffolds decrease only slightly, by less than 0.5 pH units, over the two-month timescale, suggesting that only minimal fiber degradation occurs during this time. The fiber scaffolds also have good biocompatibility, as revealed by in vitro cytotoxicity experiments. Overall, our results demonstrate that the novel scaffolds reported here are promising pH-sensitive drug delivery systems, and may be candidates for use after tumor resection surgery.

Antioxidant N-Acetylcysteine and Glutathione Increase the Viability and Proliferation of MG63 Cells Encapsulated in the Gelatin Methacrylate/VA-086/Blue Light Hydrogel System

Photoencapsulation of cells inside a hydrogel system can provide a suitable path to establish a gel in situ for soft tissue regeneration applications. However, the presence of photoinitiators and blue or UV light irradiation can result in cell damage and an increase of reactive oxygen species. We here evaluate the benefits of an antioxidant pretreatment on the photoencapsulated cells. We study this by evaluating proliferation and viability of MG63 cells, which we combined with a gelatin methacrylate (GelMA) hydrogel system, using the photoinitiator, VA-086, cured with 440 nm blue light. We found that blue light irradiation as well as the presence of 1% VA-086 reduced MG63 cell proliferation rates. Adding a short pretreatment step to the MG63 cells, consisting of the antioxidant molecules N-acetylcysteine (NAC) and reduced glutathione (GSH), and optimizing the GelMA encapsulation steps, we found that both NAC and GSH pretreatments of MG63 cells significantly increased both proliferation and viability of the cells, when using a 15% GelMA hydrogel, 1% VA-086, and 1-min blue light exposure. These findings suggest that the use of antioxidant pretreatment can counteract the negative presence of the photoinitiators and blue light exposure and result in a suitable environment for photoencapsulating cells in situ for tissue engineering and soft tissue applications.

Biomaterial Property Effects on Platelets and Macrophages: An in Vitro Study

The purpose of this study was to evaluate the effects of surface properties of bone implants coated with hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) on platelets and macrophages upon implant installation and compare them to grit-blasted Ti and Thermanox used as a control. Surface properties were characterized using scanning electron microscopy, profilometry, crystallography, Fourier transform infrared spectroscopy, and coating stability. For platelets, platelet adherence and morphology were assessed. For macrophages, morphology, proliferation, and polarization were evaluated. Surface characterization showed similar roughness of ∼2.5 μm for grit-blasted Ti discs, both with and without coating. Coating stability assessment showed substantial dissolution of HA and β-TCP coatings. Platelet adherence was significantly higher for grit-blasted Ti, Ti-HA, and Ti-β-TCP coatings compared to that of cell culture control Thermanox. Macrophage cultures revealed a decreased proliferation on both HA and β-TCP coated discs compared to both Thermanox and grit-blasted Ti. In contrast, secretion of pro-inflammatory cytokine TNF-α and anti-inflammatory cytokine TGF-β were marginal for grit-blasted Ti and Thermanox, while a coating-dependent increased secretion of pro- and anti-inflammatory cytokines was observed for HA and β-TCP coatings. The results demonstrated a significantly upregulated pro-inflammatory and anti-inflammatory cytokine secretion and marker gene expression of macrophages on HA and β-TCP coatings. Furthermore, HA induced an earlier M1 macrophage polarization but more M2 phenotype potency than β-TCP. In conclusion, our data showed that material surface affects the behaviors of first cell types attached to implants. Due to the demonstrated crucial roles of platelets and macrophages in bone healing and implant integration, this information will greatly aid the design of metallic implants for a higher rate of success in patients.

An in situ photocrosslinkable platelet rich plasma - Complexed hydrogel glue with growth factor controlled release ability to promote cartilage defect repair

The repair of articular cartilage injury is a great clinical challenge. Platelet-rich plasma (PRP) has attracted much attention for the repair of articular cartilage injury, because it contains various growth factors that are beneficial for wound repair. However, current administration methods of PRP have many shortcomings, such as unstable biological fixation and burst release of growth factors, all of which complicate its application in the repair of articular cartilage and compromise its therapeutic efficacy. In this study, based on our previously reported photoinduced imine crosslinking (PIC) reaction, we developed an in situ photocrosslinkable PRP hydrogel glue (HNPRP) through adding a photoresponsive hyaluronic acid (HA-NB) which could generate aldehyde groups upon light irradiation and subsequently react with amino groups, into autologous PRP. Our study showed that HNPRP hydrogel glue was cytocompatible and could be conveniently and rapidly prepared in situ, forming a robust hydrogel scaffold. In addition, our results demonstrated that HNPRP hydrogel not only achieved controlled release of growth factors, but also showed strong tissue adhesive ability. Therefore, HNPRP hydrogel was quite suitable for cartilage defect regeneration. Our further in vitro experiment showed that HNPRP hydrogel could promote the proliferation and migration of chondrocytes and bone marrow stem cells (BMSCs). In vivo testing using a rabbit full-thickness cartilage defect model demonstrated that HNPRP hydrogel could achieve integrative hyaline cartilage regeneration and its therapeutic efficacy was better than thrombin activated PRP gel.

STATEMENT OF SIGNIFICANCE

In this study, we have developed a photocrosslinkable platelet rich plasma (PRP) - complexed hydrogel glue (HNPRP) for cartilage regeneration. The in situ formed HNPRP hydrogel glue showed not only the controlled release ability of growth factors, but also strong tissue adhesiveness, which could resolve the current problems in clinical application of PRP. Furthermore, HNPRP hydrogel glue could promote integrative hyaline cartilage regeneration, and its reparative efficacy for cartilage defect was better than thrombin activated PRP gel. This study provided not only an effective repair material for cartilage regeneration, but also developed an advanced method for PRP application.

Sphingosine 1-phosphate (S1P) signalling: Role in bone biology and potential therapeutic target for bone repair

The lipid mediator sphingosine 1-phosphate (S1P) affects cellular functions in most systems. Interest in its therapeutic potential has increased following the discovery of its G protein-coupled receptors and the recent availability of agents that can be safely administered in humans. Although the role of S1P in bone biology has been the focus of much less research than its role in the nervous, cardiovascular and immune systems, it is becoming clear that this lipid influences many of the functions, pathways and cell types that play a key role in bone maintenance and repair. Indeed, S1P is implicated in many osteogenesis-related processes including stem cell recruitment and subsequent differentiation, differentiation and survival of osteoblasts, and coupling of the latter cell type with osteoclasts. In addition, S1P's role in promoting angiogenesis is well-established. The pleiotropic effects of S1P on bone and blood vessels have significant potential therapeutic implications, as current therapeutic approaches for critical bone defects show significant limitations. Because of the complex effects of S1P on bone, the pharmacology of S1P-like agents and their physico-chemical properties, it is likely that therapeutic delivery of S1P agents will offer significant advantages compared to larger molecular weight factors. Hence, it is important to explore novel methods of utilizing S1P agents therapeutically, and improve our understanding of how S1P and its receptors modulate bone physiology and repair.

Optimization of pure platelet-rich plasma preparation: A comparative study of pure platelet-rich plasma obtained using different centrifugal conditions in a single-donor model

While it has been proved that centrifugal conditions for pure platelet-rich plasma (P-PRP) preparation influence the cellular composition of P-PRP obtained, the optimal centrifugal conditions to prepare P-PRP have not yet been identified. In the present study, platelet-containing plasma (PCP) was prepared with the first-spin of different double-spin methods and P-PRP was prepared with different double-spin methods. Whole-blood analysis was performed to evaluate the cellular composition of PCP and P-PRP. The basal and ADP-induced CD62P expression rates of platelets were assessed by flow cytometry to evaluate the function of platelets in PCP and P-PRP. Enzyme-linked immune sorbent assay was performed to quantify interleukin-1β, tumor necrosis factor-α, platelet-derived growth factor AB and transforming growth factor β1 concentrations of PCP and P-PRP. Correlations between the cellular characteristics and cytokine concentrations of P-PRP were analyzed by Pearson correlation analysis. Effects of P-PRP on the proliferation, survival and migration of human bone marrow-derived mesenchymal stem cells and human articular chondrocytes were evaluated by a Cell Counting Kit-8 assay, live/dead staining and Transwell assay, respectively. The results showed that centrifugation at 160 × g for 10 min and 250 × g for 15 min successively captured and concentrated platelets and growth factors significantly more efficiently with preservation of platelet function compared with other conditions (P<0.05). The correlation analysis showed that the similar leukocyte concentrations and leukocyte-reducing efficiencies resulted in similar pro-inflammatory cytokine concentrations in P-PRP (P>0.05) and the maximization of platelet concentration, platelet enrichment factor, platelet capture efficiency and platelet function resulted in the maximization of growth factor concentrations in P-PRP obtained using the optimal conditions (P<0.05). Compared with P-PRP obtained under other conditions, P-PRP obtained under the optimal conditions significantly promoted the proliferation and migration of cells (P<0.05) and did not alter cell survival (P>0.05). Therefore, centrifugation at 160 × g for 10 min and 250 × g for 15 min successively with removal of the buffy coat as a crucial step may provide an optimal preparation system of P-PRP for clinical application.

The Role of the Chemokine System in Tissue Response to Prosthetic By-products Leading to Periprosthetic Osteolysis and Aseptic Loosening

Millions of total joint replacements are performed annually worldwide, and the number is increasing every year. The overall proportion of patients achieving a successful outcome is about 80–90% in a 10–20-years time horizon postoperatively, periprosthetic osteolysis (PPOL) and aseptic loosening (AL) being the most frequent reasons for knee and hip implant failure and reoperations. The chemokine system (chemokine receptors and chemokines) is crucially involved in the inflammatory and osteolytic processes leading to PPOL/AL. Thus, the modulation of the interactions within the chemokine system may influence the extent of PPOL. Indeed, recent studies in murine models reported that (i) blocking the CCR2–CCL2 or CXCR2–CXCL2 axis or (ii) activation of the CXCR4–CXCL12 axis attenuate the osteolysis of artificial joints. Importantly, chemokines, inhibitory mutant chemokines, antagonists of chemokine receptors, or neutralizing antibodies to the chemokine system attached to or incorporated into the implant surface may influence the tissue responses and mitigate PPOL, thus increasing prosthesis longevity. This review summarizes the current state of the art of the knowledge of the chemokine system in human PPOL/AL. Furthermore, the potential for attenuating cell trafficking to the bone–implant interface and influencing tissue responses through modulation of the chemokine system is delineated. Additionally, the prospects of using immunoregenerative biomaterials (including chemokines) for the prevention of failed implants are discussed. Finally, this review highlights the need for a more sophisticated understanding of implant debris-induced changes in the chemokine system to mitigate this response effectively.

Osteomacs and Bone Regeneration

PURPOSE OF REVIEW

Mounting evidence supporting the critical contribution of macrophages, in particular osteal macrophages, to bone regeneration is reviewed. We specifically examine the potential role of macrophages in the basic multicellular units coordinating lifelong bone regeneration via remodelling and bone regeneration in response to injury. We review and discuss the distinctions between macrophage and osteoclast contributions to bone homeostasis, particularly the dichotomous role of the colony-stimulating factor 1-colony-stimulating factor 1 receptor axis.

RECENT FINDINGS

The impact of inflammation associated with aging and other hallmarks of aging, including senescence, on macrophage function is addressed in the context of osteoporosis and delayed fracture repair. Resident macrophages versus recruited macrophage contributions to fracture healing are also discussed. We identify some of the remaining knowledge gaps that will need to be closed in order to maximise benefits from therapeutically modulating or mimicking the function of macrophages to improve bone health and regeneration over a lifetime.

Harnessing macrophage plasticity for tissue regeneration

Macrophages are versatile and plastic effector cells of the immune system, and contribute to diverse immune functions including pathogen or apoptotic cell removal, inflammatory activation and resolution, and tissue healing. Macrophages function as signaling regulators and amplifiers, and influencing their activity is a powerful approach for controlling inflammation or inducing a wound-healing response in regenerative medicine. This review discusses biomaterials-based approaches for altering macrophage activity, approaches for targeting drugs to macrophages, and approaches for delivering macrophages themselves as a therapeutic intervention.

Promoting tissue regeneration by modulating the immune system

The immune system plays a central role in tissue repair and regeneration. Indeed, the immune response to tissue injury is crucial in determining the speed and the outcome of the healing process, including the extent of scarring and the restoration of organ function. Therefore, controlling immune components via biomaterials and drug delivery systems is becoming an attractive approach in regenerative medicine, since therapies based on stem cells and growth factors have not yet proven to be broadly effective in the clinic. To integrate the immune system into regenerative strategies, one of the first challenges is to understand the precise functions of the different immune components during the tissue healing process. While remarkable progress has been made, the immune mechanisms involved are still elusive, and there is indication for both negative and positive roles depending on the tissue type or organ and life stage. It is well recognized that the innate immune response comprising danger signals, neutrophils and macrophages modulates tissue healing. In addition, it is becoming evident that the adaptive immune response, in particular T cell subset activities, plays a critical role. In this review, we first present an overview of the basic immune mechanisms involved in tissue repair and regeneration. Then, we highlight various approaches based on biomaterials and drug delivery systems that aim at modulating these mechanisms to limit fibrosis and promote regeneration. We propose that the next generation of regenerative therapies may evolve from typical biomaterial-, stem cell-, or growth factor-centric approaches to an immune-centric approach.

STATEMENT OF SIGNIFICANCE

Most regenerative strategies have not yet proven to be safe or reasonably efficient in the clinic. In addition to stem cells and growth factors, the immune system plays a crucial role in the tissue healing process. Here, we propose that controlling the immune-mediated mechanisms of tissue repair and regeneration may support existing regenerative strategies or could be an alternative to using stem cells and growth factors. The first part of this review we highlight key immune mechanisms involved in the tissue healing process and marks them as potential target for designing regenerative strategies. In the second part, we discuss various approaches using biomaterials and drug delivery systems that aim at modulating the components of the immune system to promote tissue regeneration.

Biomaterials and Bioactive Factor Delivery Systems for the Control of Macrophage Activation in Regenerative Medicine

Macrophages play an important role in tissue repair, regeneration, and the ability of biomaterials to mediate these processes. Macrophages are highly plastic cells that exhibit altered behavior in response to changes in the microenvironment. With the growing knowledge of the roles that different macrophage phenotypes play in specific pathologies and/or injuries, researchers are now focusing on designing biomaterials to actively control macrophage behavior and promote healing outcomes. In this review, we highlight a variety of biomaterial strategies for controlling macrophage phenotype in chronic wounds, tissue defects, and inflammatory conditions, although these strategies can be applied to many other applications. In particular, we highlight the different situations in which biomaterials should inhibit or promote M1 or M2 activation, or both, for therapeutic outcomes.

Biomimetic Approaches for Bone Tissue Engineering

Although autologous bone grafts are considered a gold standard for the treatment of bone defects, they are limited by donor site morbidities and geometric requirements. We propose that tissue engineering technology can overcome such limitations by recreating fully viable and biological bone grafts. Specifically, we will discuss the use of bone scaffolds and autologous cells with bioreactor culture systems as a tissue engineering paradigm to grow bone in vitro. We will also discuss emergent vascularization strategies to promote graft survival in vivo, as well as the role of inflammation during bone repair. Finally, we will highlight some recent advances and discuss new solutions to bone repair inspired by endochondral ossification.

Exosomes derived from human platelet-rich plasma prevent apoptosis induced by glucocorticoid-associated endoplasmic reticulum stress in rat osteonecrosis of the femoral head via the Akt/Bad/Bcl-2 signal pathway

An excess of glucocorticoids (GCs) is reported to be one of the most common causes of osteonecrosis of the femoral head (ONFH). In addition, GCs can induce bone cell apoptosis through modulating endoplasmic reticulum (ER) stress. Among the three main signal pathways in ER stress, the PERK (protein kinase RNA-like ER kinase)/CHOP (CCAAT-enhancer-binding protein homologous protein) pathway has been considered to be closely associated with apoptosis. Platelet-rich plasma (PRP) has been referred to as a concentration of growth factors and the exosomes derived from PRP (PRP-Exos) have a similar effect to their parent material. The enriched growth factors can be encapsulated into PRP-Exos and activate Akt and Erk pathways to promote angiogenesis. Activation of the Akt pathway may promote the expression of anti-apoptotic proteins like Bcl-2, while CHOP can inhibit B-cell lymphoma 2 (Bcl-2) expression to increase the level of cleaved caspase-3 and lead to cell death. Consequently, we hypothesized that PRP-Exos prevent apoptosis induced by glucocorticoid-associated ER stress in rat ONFH via the Akt/Bad/Bcl-2 signal pathway. To verify this hypothesis, a dexamethasone (DEX)-treated in vitro cell model and methylprednisolone (MPS)-treated in vivo rat model were adopted. Characterization of PRP-Exos, and effects of PRP-Exos on proliferation, apoptosis, angiogenesis, and osteogenesis of cells treated with GCs in vitro and in vivo were examined. Furthermore, the mechanism by which PRP-Exos rescue the GC-induced apoptosis through the Akt/Bad/Bcl-2 pathway was also investigated. The results indicate that PRP-Exos have the capability to prevent GC-induced apoptosis in a rat model of ONFH by promoting Bcl-2 expression via the Akt/Bad/Bcl-2 signal pathway under ER stress.

Biomaterial-mediated strategies targeting vascularization for bone repair

Repair of non-healing bone defects through tissue engineering strategies remains a challenging feat in the clinic due to the aversive microenvironment surrounding the injured tissue. The vascular damage that occurs following a bone injury causes extreme ischemia and a loss of circulating cells that contribute to regeneration. Tissue-engineered constructs aimed at regenerating the injured bone suffer from complications based on the slow progression of endogenous vascular repair and often fail at bridging the bone defect. To that end, various strategies have been explored to increase blood vessel regeneration within defects to facilitate both tissue-engineered and natural repair processes. Developments that induce robust vascularization will need to consolidate various parameters including optimization of embedded therapeutics, scaffold characteristics, and successful integration between the construct and the biological tissue. This review provides an overview of current strategies as well as new developments in engineering biomaterials to induce reparation of a functional vascular supply in the context of bone repair.

Platelet-rich plasma for the treatment of bone defects: from pre-clinical rational to evidence in the clinical practice. A systematic review

PURPOSE

The treatment of large bone defects represents a significant challenge for orthopaedic surgeons. In recent years, biologic agents have also been used to further improve bone healing. Among these, platelet-rich plasma (PRP) is the most exploited strategy. The aim of the present study was to systematically review the available literature to identify: 1) preclinical in-vivo results supporting the rational of PRP use for bone healing; 2) evidence from the clinical practice on the actual clinical benefit of PRP for the treatment of fractures and complications such as delayed unions and non-unions.

METHODS

A systematic review of the literature was performed on the application of PRP in bone healing, using the following inclusion criteria: pre-clinical and clinical reports of any level of evidence, written in English language, published in the last 20 years (1996-2016), on the use of PRP to stimulate long-bone defect treatment, with focus on fracture and delayed/non-unions healing.

RESULTS

The search in the Pubmed database identified 64 articles eligible for inclusion: 45 were preclinical in-vivo studies and 19 were clinical studies. Despite the fact that the overall pre-clinical results seem to support the benefit of PRP in 91.1 % of the studies, a more in depth analysis underlined a lower success rate, with a positive outcome of 84.4 % in terms of histological analysis, and even lower values considering radiological and biomechanical results (75.0 % and 72.7 % positive outcome respectively). This was also mirrored in the clinical literature, where the real benefit of PRP use to treat fractures and non-unions is still under debate.

CONCLUSION

Overall, the available literature presents major limitations in terms of low quality and extreme heterogeneity, which hamper the possibility to optimize PRP treatment and translate it into a real clinical benefit despite positive preclinical findings on its biological potential to favour bone healing.

Graphene oxide scaffold accelerates cellular proliferative response and alveolar bone healing of tooth extraction socket

Graphene oxide (GO) consisting of a carbon monolayer has been widely investigated for tissue engineering platforms because of its unique properties. For this study, we fabricated a GO-applied scaffold and assessed the cellular and tissue behaviors in the scaffold. A preclinical test was conducted to ascertain whether the GO scaffold promoted bone induction in dog tooth extraction sockets. For this study, GO scaffolds were prepared by coating the surface of a collagen sponge scaffold with 0.1 and 1 µg/mL GO dispersion. Scaffolds were characterized using scanning electron microscopy (SEM), physical testing, cell seeding, and rat subcutaneous implant testing. Then a GO scaffold was implanted into a dog tooth extraction socket. Histological observations were made at 2 weeks postsurgery. SEM observations show that GO attached to the surface of collagen scaffold struts. The GO scaffold exhibited an interconnected structure resembling that of control subjects. GO application improved the physical strength, enzyme resistance, and adsorption of calcium and proteins. Cytocompatibility tests showed that GO application significantly increased osteoblastic MC3T3-E1 cell proliferation. In addition, an assessment of rat subcutaneous tissue response revealed that implantation of 1 µg/mL GO scaffold stimulated cellular ingrowth behavior, suggesting that the GO scaffold exhibited good biocompatibility. The tissue ingrowth area and DNA contents of 1 µg/mL GO scaffold were, respectively, approximately 2.5-fold and 1.4-fold greater than those of the control. Particularly, the infiltration of ED2-positive (M2) macrophages and blood vessels were prominent in the GO scaffold. Dog bone-formation tests showed that 1 µg/mL GO scaffold implantation enhanced bone formation. New bone formation following GO scaffold implantation was enhanced fivefold compared to that in control subjects. These results suggest that GO was biocompatible and had high bone-formation capability for the scaffold. The GO scaffold is expected to be beneficial for bone tissue engineering therapy.

Hydrogel as a bioactive material to regulate stem cell fate

The encapsulation of stem cells in a hydrogel substrate provides a promising future in biomedical applications. However, communications between hydrogels and stem cells is complicated; various factors such as porosity, different polymer types, stiffness, compatibility and degradation will lead to stem cell survival or death. Hydrogels mimic the three-dimensional extracellular matrix to provide a friendly environment for stem cells. On the other hand, stem cells can sense the surroundings to make the next progression, stretching out, proliferating or just to remain. As such, understanding the correlation between stem cells and hydrogels is crucial. In this Review, we first discuss the varying types of the hydrogels and stem cells, which are most commonly used in the biomedical fields and further investigate how hydrogels interact with stem cells from the perspective of their biomedical application, while providing insights into the design and development of hydrogels for drug delivery, tissue engineering and regenerative medicine purpose. In addition, we compare the results such as stiffness, degradation time and pore size as well as peptide types of hydrogels from respected journals. We also discussed most recently magnificent materials and their effects to regulate stem cell fate.

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Hydrogels as Extracellular Matrix (ECM) mimics stem cells proliferation and differentiation.

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Discuss how hydrogels interact with stem cells from the perspective of their biomedical applications.

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Recent magnificent materials and their effects to regulate stem cells fate.

Drug delivery strategies to control macrophages for tissue repair and regeneration

Tissue repair and regeneration is a complex process. Our bodies have an excellent capacity to regenerate damaged tissues in many situations. However, tissue healing is impaired in injuries that exceed a critical size or are exacerbated by chronic inflammatory diseases like diabetes. In these instances, biomaterials and drug delivery strategies are often required to facilitate tissue regeneration by providing physical and biochemical cues. Inflammation is the body's response to injury. It is critical for wound healing and biomaterial integration and vascularization, as long as the timing is well controlled. For example, chronic inflammation is well known to impair healing in chronic wounds. In this review, we highlight the importance of a well-controlled inflammatory response, primarily mediated by macrophages in tissue repair and regeneration and discuss various strategies designed to promote regeneration by controlling macrophage behavior. These strategies include temporally controlled delivery of anti-inflammatory drugs, delivery of macrophages as cellular therapy, controlled release of cytokines that modulate macrophage phenotype, and the design of nanoparticles that exploit the inherent phagocytic behavior of macrophages. A clear outcome of this review is that a deeper understanding of the role and timing of complex macrophage phenotypes or activation states is required to fully harness their abilities with drug delivery strategies.

Platelet-Rich Blood Derivatives for Stem Cell-Based Tissue Engineering and Regeneration

Platelet rich blood derivatives have been widely used in different fields of medicine and stem cell based tissue engineering. They represent natural cocktails of autologous growth factor, which could provide an alternative for recombinant protein based approaches. Platelet rich blood derivatives, such as platelet rich plasma, have consistently shown to potentiate stem cell proliferation, migration, and differentiation. Here, we review the spectrum of platelet rich blood derivatives, discuss their current applications in tissue engineering and regenerative medicine, reflect on their effect on stem cells, and highlight current translational challenges.

Modulation of Inflammatory Response and Induction of Bone Formation Based on Combinatorial Effects of Resveratrol

The success of bone tissue engineering strategies critically depends on the rapid formation of a mature vascular network in the scaffolds after implantation. Conventional methods to accelerate the infiltration of host vasculature into the scaffolds need to consider the role of host response in regulation of bone tissue ingrowth and extent of vascularization. The long term goal of this study was to harness the potential of inflammatory response to enhance angiogenesis and bone formation in three dimensional (3D) scaffolds. Towards this goal, we explored the use of resveratrol, a natural compound commonly used in complementary medicine, to enable the concurrently (i) mediate M1 to M2 macrophage plasticity, (ii) impart natural release of angiogenic factors by macrophages and (iii) enhance osteogenic differentiation of human mesenchymal stem cells (hMSCs). We mapped the time-dependent response of macrophage gene expression as well as hMSC osteogenic differentiation to varying doses of resveratrol. The utility of this approach was evaluated in 3D poly (lactide-co-glycolide) (PLGA) sintered microsphere scaffolds for bone tissue engineering applications. Our results altogether delineate the potential to synergistically accelerate angiogenic factor release and upregulate osteogenic signaling pathways by "dialing" the appropriate degree of resveratrol release.

Delivery strategies to control inflammatory response: Modulating M1-M2 polarization in tissue engineering applications

Macrophages are key players in many physiological scenarios including tissue homeostasis. In response to injury, typically the balance between macrophage sub-populations shifts from an M1 phenotype (pro-inflammatory) to an M2 phenotype (anti-inflammatory). In tissue engineering scenarios, after implantation of any device, it is desirable to exercise control on this M1-M2 progression and to ensure a timely and smooth transition from the inflammatory to the healing stage. In this review, we briefly introduce the current state of knowledge regarding macrophage function and nomenclature. Next, we discuss the use of controlled release strategies to tune the balance between the M1 and M2 phenotypes in the context of tissue engineering applications. We discuss recent literature related to the release of anti-inflammatory molecules (including nucleic acids) and the sequential release of cytokines to promote a timely M1-M2 shift. In addition, we describe the use of macrophages as controlled release agents upon stimulation by physical and/or mechanical cues provided by scaffolds. Moreover, we discuss current and future applications of "smart" implantable scaffolds capable of controlling the cascade of biochemical events related to healing and vascularization. Finally, we provide our opinion on the current challenges and the future research directions to improve our understanding of the M1-M2 macrophage balance and properly exploit it in tissue engineering and regenerative medicine applications.

Extracellular matrix-inspired growth factor delivery systems for bone regeneration

Growth factors are very promising molecules to enhance bone regeneration. However, their translation to clinical use has been seriously limited, facing issues related to safety and cost-effectiveness. These problems derive from the vastly supra-physiological doses of growth factor used without optimized delivery systems. Therefore, these issues have motivated the development of new delivery systems allowing better control of the spatiotemporal release and signaling of growth factors. Because the extracellular matrix (ECM) naturally plays a fundamental role in coordinating growth factor activity in vivo, a number of novel delivery systems have been inspired by the growth factor regulatory function of the ECM. After introducing the role of growth factors during the bone regeneration process, this review exposes different issues that growth factor-based therapies have encountered in the clinic and highlights recent delivery approaches based on the natural interaction between growth factor and the ECM.

Dual-controlled release system of drugs for bone regeneration

Controlled release systems have been noted to allow drugs to enhance their ability for bone regeneration. To this end, various biomaterials have been used as the release carriers of drugs, such as low-molecular-weight drugs, growth factors, and others. The drugs are released from the release carriers in a controlled fashion to maintain their actions for a long time period. Most research has been focused on the controlled release of single drugs to demonstrate the therapeutic feasibility. Controlled release of two combined drugs, so-called dual release systems, are promising and important for tissue regeneration. This is because the tissue regeneration process of bone formation is generally achieved by multiple bioactive molecules, which are produced from cells by other molecules. If two types of bioactive molecules, (i.e., drugs), are supplied in an appropriate fashion, the regeneration process of living bodies will be efficiently promoted. This review focuses on the bone regeneration induced by dual-controlled release of drugs. In this paper, various dual-controlled release systems of drugs aiming at bone regeneration are overviewed explaining the type of drugs and their release materials.

Periodontal tissue regeneration with PRP incorporated gelatin hydrogel sponges

Gelatin hydrogels have been designed and prepared for the controlled release of the transforming growth factor (TGF-b1) and the platelet-derived growth factor (PDGF-BB). PRP (Platelet rich plasma) contains many growth factors including the PDGF and TGF-b1. The objective of this study was to evaluate the regeneration of periodontal tissue following the controlled release of growth factors in PRP. For the periodontal ligament cells and osteoblast, PRP of different concentrations was added. The assessment of DNA, mitochondrial activity and ALP activity were measured. To evaluate the TGF-β1 release from PRP incorporated gelatin sponge, amounts of TGF-β1 in each supernatant sample were determined by the ELISA. Transplantation experiments to prepare a bone defect in a rat alveolar bone were an implanted gelatin sponge incorporated with different concentration PRP. In DNA assay and MTT assay, after the addition of PRP to the periodontal ligament cells and osteoblast, the cell count and mitochondrial activity had increased the most in the group with the addition of 5  ×  PRP. In the ALP assay, after the addition of PRP to the periodontal ligament cells, the cell activity had increased the most in the group with the addition of 3  ×  PRP. In the transplantation, the size of the bone regenerated in the defect with 3  ×  PRP incorporated gelatin sponge was larger than that of the other group.

Platelet-Rich Plasma: A Study of the Variables that May Influence Its Effect on Bone Regeneration

BACKGROUND

Currently, the use of platelet-rich plasma in bone regeneration is a real option, although more than one opinion has alerted us to the absence of clinical benefits.

PURPOSE

Analysis of the factors able to modify the characteristics of the platelet preparation obtained by Curasan, Plasma Rich in Growth Factors (PRGF), Platelet Concentrate Collection System (PCCS) and SmartPrep systems, relating them to the type of clinical application and the final bone regeneration achieved.

MATERIALS AND METHODS

A search was conducted in PubMed using the keywords "platelet-rich plasma," "PRP," "platelet rich growth factors," and "oral bone regeneration." Four widely accepted protocols for the obtention of PRP (above) were analyzed. Any clinical studies with controls, using the four preparation protocols and with a 4 to 6 weeks follow-up period were compared. The protocols were also grouped according to the type of PRP application: PRP-alone, with bone, or with bone substitutes.

RESULTS

Bone regeneration was not achieved in any of the cases using PRP obtained by Curasan and PCCS systems, whereas PRP obtained by SmartPrep achieved it only in one in three published cases and PRGF in one in six.

CONCLUSION

Based on the poor results observed in current literature, the use of PRP in oral surgery cannot be recommended.

Towards the design of 3D multiscale instructive tissue engineering constructs: Current approaches and trends

The design of 3D constructs with adequate properties to instruct and guide cells both in vitro and in vivo is one of the major focuses of tissue engineering. Successful tissue regeneration depends on the favorable crosstalk between the supporting structure, the cells and the host tissue so that a balanced matrix production and degradation are achieved. Herein, the major occurring events and players in normal and regenerative tissue are overviewed. These have been inspiring the selection or synthesis of instructive cues to include into the 3D constructs. We further highlight the importance of a multiscale perception of the range of features that can be included on the biomimetic structures. Lastly, we focus on the current and developing tissue-engineering approaches for the preparation of such 3D constructs: top-down, bottom-up and integrative. Bottom-up and integrative approaches present a higher potential for the design of tissue engineering devices with multiscale features and higher biochemical control than top-down strategies, and are the main focus of this review.

Biomaterial-mediated modification of the local inflammatory environment

Inflammation plays a major role in the rejection of biomaterial implants. In addition, despite playing an important role in the early stages of wound healing, dysregulated inflammation has a negative impact on the wound healing processes. Thus, strategies to modulate excessive inflammation are needed. Through the use of biomaterials to control the release of anti-inflammatory therapeutics, increased control over inflammation is possible in a range of pathological conditions. However, the choice of biomaterial (natural or synthetic), and the form it takes (solid, hydrogel, or micro/nanoparticle) is dependent on both the cause and tissue location of inflammation. These considerations also influence the nature of the anti-inflammatory therapeutic that is incorporated into the biomaterial to be delivered. In this report, the range of biomaterials and anti-inflammatory therapeutics that have been combined will be discussed, as well as the functional benefit observed. Furthermore, we point toward future strategies in the field that will bring more efficacious anti-inflammatory therapeutics closer to realization.

New and emerging strategies in platelet-rich plasma application in musculoskeletal regenerative procedures: general overview on still open questions and outlook

Despite its pervasive use, the clinical efficacy of platelet-rich plasma (PRP) therapy and the different mechanisms of action have yet to be established. This overview of the literature is focused on the role of PRP in bone, tendon, cartilage, and ligament tissue regeneration considering basic science literature deriving from in vitro and in vivo studies. Although this work provides evidence that numerous preclinical studies published within the last 10 years showed promising results concerning the application of PRP, many key questions remain unanswered and controversial results have arisen. Additional preclinical studies are needed to define the dosing, timing, and frequency of PRP injections, different techniques for delivery and location of delivery, optimal physiologic conditions for injections, and the concomitant use of recombinant proteins, cytokines, additional growth factors, biological scaffolds, and stems cells to develop optimal treatment protocols that can effectively treat various musculoskeletal conditions.

Regulation of the Stem Cell-Host Immune System Interplay Using Hydrogel Coencapsulation System with an Anti-Inflammatory Drug

The host immune system is known to influence mesenchymal stem cell (MSC)-mediated bone tissue regeneration. However, the therapeutic capacity of hydrogel biomaterial to modulate the interplay between MSCs and T-lymphocytes is unknown. Here it is shown that encapsulating hydrogel affects this interplay when used to encapsulate MSCs for implantation by hindering the penetration of pro-inflammatory cells and/or cytokines, leading to improved viability of the encapsulated MSCs. This combats the effects of the host pro-inflammatory T-lymphocyte-induced nuclear factor kappaB pathway, which can reduce MSC viability through the CASPASE-3 and CAS-PASE-8 associated proapoptotic cascade, resulting in the apoptosis of MSCs. To corroborate rescue of engrafted MSCs from the insult of the host immune system, the incorporation of the anti-inflammatory drug indomethacin into the encapsulating alginate hydrogel further regulates the local microenvironment and prevents pro-inflammatory cytokine-induced apoptosis. These findings suggest that the encapsulating hydrogel can regulate the MSC-host immune cell interplay and direct the fate of the implanted MSCs, leading to enhanced tissue regeneration.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Osteoarthritis year in review 2014: highlighting innovations in basic research and clinical applications in regenerative medicine

Regenerative medicine is an emerging area that will influence the treatment of joint diseases in the future. It involves the use of biomaterials, cell therapy, and bioactive factors such as growth factors, drugs and small molecules, to regenerate damaged tissues. This "year in review" highlights a personal selection of promising studies published between March 2013 and March 2014 that inform on the direction in which this field is moving. This multidisciplinary field has been very active, with rapid development of new technologies that emerge from basic sciences such as the possibility to generate pluripotent stem cells without genetic modification and genetic engineering of growth factors to enhance their capacity to induce tissue repair. The increasing knowledge of the interaction between all tissues in the joint, such as the effect of bone remodeling and synovial inflammation on cartilage repair, will eventually make tissue regeneration in a compromised joint environment possible.