An In Vitro Microfluidic Alveolus Model to Study Lung Biomechanics

The gas exchange units of the lung, the alveoli, are mechanically active and undergo cyclic deformation during breathing. The epithelial cells that line the alveoli contribute to lung function by reducing surface tension via surfactant secretion, which is highly influenced by the breathing-associated mechanical cues. These spatially heterogeneous mechanical cues have been linked to several physiological and pathophysiological states. Here, we describe the development of a microfluidically assisted lung cell culture model that incorporates heterogeneous cyclic stretching to mimic alveolar respiratory motions. Employing this device, we have examined the effects of respiratory biomechanics (associated with breathing-like movements) and strain heterogeneity on alveolar epithelial cell functions. Furthermore, we have assessed the potential application of this platform to model altered matrix compliance associated with lung pathogenesis and ventilator-induced lung injury. Lung microphysiological platforms incorporating human cells and dynamic biomechanics could serve as an important tool to delineate the role of alveolar micromechanics in physiological and pathological outcomes in the lung.

Molecularly Tailored Interface for Long-Term Xenogeneic Cell Transplantation

Encapsulation of therapeutic cells in a semipermeable device can mitigate the need for systemic immune suppression following cell transplantation by providing local immunoprotection while being permeable to nutrients, oxygen, and different cell-secreted biomolecules. However, fibrotic tissue deposition around the device has been shown to compromise the long-term function of the transplanted cells. Herein, a macroencapsulation device design that improves long-term survival and function of the transplanted cells is reported. The device is comprised of a semipermeable chitosan pouch with a tunable reservoir and molecularly engineered interface. The chitosan pouch interface decorated with 1,12-dodecanedioic acid (DDA), limits the cell adhesion and vigorous foreign body response while maintaining the barrier properties amenable to cell encapsulation. The device provides long-term protection to the encapsulated human primary hepatocytes in the subcutaneous space of immunocompetent mice. The device supports the encapsulated cells for up to 6 months as evident from cell viability and presence of human specific albumin in circulation. Solutions that integrate biomaterials and interfacial engineering such as the one described here may advance development of easy-to manufacture and retrievable devices for the transplantation of therapeutic cells in the absence of immunosuppression.

Human adipose-derived stem cell spheroids incorporating platelet-derived growth factor (PDGF) and bio-minerals for vascularized bone tissue engineering

Stem cells with mineralized materials have been used for bone regeneration; however, engineering the complex vascularized structure of the natural bone remains a challenge. Here, we developed platelet-derived growth factor (PDGF) and bio-mineral coated fibers which were then assembled with human adipose-derived stem cells (hADSCs) to form spheroids as building blocks for vascularized bone regeneration. The PDGF incorporated within the spheroid increased the proliferation of hADSCs, which was characterized by Ki-67 staining and DNA contents. Furthermore, the PDGF enhanced not only osteogenic differentiation, but also endothelial differentiation of hADSCs; the cells within the spheroids showed significantly greater gene expression by 2.46 ± 0.14 fold for osteocalcin (OCN) and by 12.85 ± 3.36 fold for von Willebrand factor (vWF) than those without PDGF. Finally, at two months following transplantation of PDGF-incorporated spheroids onto in vivo mouse calvarial defect, the regenerated bone area (42.48 ± 10.84%) was significantly enhanced and the greatest number of capillaries and arterioles with indication of transplanted hADSCs were observed. Moreover, millimeter-scale in vitro tissue prepared by fused assembly of the spheroids exhibited greater mRNA expression-associated to endothelial lineage. Taken together, these findings indicate that stem cell spheroids incorporating PDGF and bio-minerals could be used as a module for successful vascularized bone regeneration.

Polydopamine-assisted one-step modification of nanofiber surfaces with adenosine to tune the osteogenic differentiation of mesenchymal stem cells and the maturation of osteoclasts

Adenosine and its receptors have emerged as alternative targets to control cellular functions for bone healing. However, the soluble delivery of adenosine has not proven effective because of its fast degradation in vivo. We therefore designed a stable coating of adenosine for biomaterial surfaces through polydopamine chemistry to control osteogenesis and osteoclastogenesis via A2bR signaling. First, we prepared electrospun poly (ι-lactic acid) (PLLA) nanofiber sheets, which were modified through a one-step adenosine polydopamine coating process. Scanning electron microscopy (SEM) revealed deposition of particles on the adenosine polydopamine-coated PLLA (AP-PL) sheets compared to the polydopamine-only sheets. Moreover, X-ray photoelectron spectroscopy analysis confirmed an increase in nitrogen signals due to adenosine. Furthermore, adenosine loading efficiency and retention were significantly enhanced in AP-PL sheets compared to polydopamine-only sheets. Human adipose-derived stem cells (hADSCs) cultured on AP-PL expressed A2bR (1.30 ± 0.19 fold) at significantly higher levels than those cultured on polydopamine-only sheets. This in turn significantly elevated the expression of Runx2 (16.94 ± 1.68 and 51.69 ± 0.07 fold), OPN (1.63 ± 0.16 and 30.56 ± 0.25 fold), OCN (1.16 ± 0.13 and 5.23 ± 0.16 fold), and OSX (10.01 ± 0.81 and 62.48 ± 0.25 fold) in cells grown in growth media on days 14 and 21, respectively. Similarly, mineral deposition was enhanced to a greater extent in the AP-PL group than the polydopamine group, while blocking of A2bR significantly downregulated osteogenesis. Finally, osteoclast differentiation of RAW 264.7 cells was significantly inhibited by growth on AP-PL sheets. However, osteoclast differentiation was significantly stimulated after A2bR was blocked. Taken together, we propose that polydopamine-assisted one-step coating of adenosine is a viable method for surface modification of biomaterials to control osteogenic differentiation of stem cells and bone healing.

Surface engineering of titanium alloy using metal-polyphenol network coating with magnesium ions for improved osseointegration

Although titanium-based implants are widely used in orthopedic and dental clinics, improved osseointegration at the bone–implant interface is still required.

Current Advances in Immunomodulatory Biomaterials for Bone Regeneration

Biomaterials with suitable surface modification strategies are contributing significantly to the rapid development of the field of bone tissue engineering. Despite these encouraging results, utilization of biomaterials is poorly translated to human clinical trials potentially due to lack of knowledge about the interaction between biomaterials and the body defense mechanism, the "immune system". The highly complex immune system involves the coordinated action of many immune cells that can produce various inflammatory and anti-inflammatory cytokines. Besides, bone fracture healing initiates with acute inflammation and may later transform to a regenerative or degenerative phase mainly due to the cross-talk between immune cells and other cells in the bone regeneration process. Among various immune cells, macrophages possess a significant role in the immune defense, where their polarization state plays a key role in the wound healing process. Growing evidence shows that the macrophage polarization state is highly sensitive to the biomaterial's physiochemical properties, and advances in biomaterial research now allow well controlled surface properties. This review provides an overview of biomaterial-mediated modulation of the immune response for regulating key bone regeneration events, such as osteogenesis, osteoclastogenesis, and inflammation, and it discusses how these strategies can be utilized for future bone tissue engineering applications.

Harnessing biochemical and structural cues for tenogenic differentiation of adipose derived stem cells (ADSCs) and development of an in vitro tissue interface mimicking tendon-bone insertion graft

Tendon-bone interface tissue is extremely challenging to engineer because it exhibits complex gradients of structure, composition, biologics, and cellular phenotypes. As a step toward engineering these transitional zones, we initially analyzed how different (topographical or biological) cues affect tenogenic differentiation of adipose-derived stem cells (ADSCs). We immobilized platelet-derived growth factor - BB (PDGF-BB) using polydopamine (PD) chemistry on random and aligned nanofibers and investigated ADSC proliferation and tenogenic differentiation. Immobilized PDGF greatly enhanced the proliferation and tenogenic differentiation of ADSCs; however, nanofiber alignment had no effect. Interestingly, the PDGF immobilized aligned nanofiber group showed a synergistic effect with maximum expression of tenogenic markers for 14 days. We also generated a nanofiber surface with spatially controlled presentation of immobilized PDGF on an aligned architecture, mimicking native tendon tissue. A gradient of immobilized PDGF was able to control the phenotypic differentiation of ADSCs into tenocytes in a spatially controlled manner, as confirmed by analysis of the expression of tenogenic markers and immunofluorescence staining. We further explored the gradient formation strategy by generation of a symmetrical gradient on the nanofiber surface for the generation of a structure mimicking bone-patellar-tendon-bone with provision for gradient immobilization of PDGF and controlled mineralization. Our study reveals that, together with biochemical cues, favorable topographical cues are important for tenogenic differentiation of ADSCs, and gradient presentation of PDGF can be used as a tool for engineering stem cell-based bone-tendon interface tissues.

Osteoinductive superparamagnetic Fe nanocrystal/calcium phosphate heterostructured microspheres

Functional magnetic and biocompatible particles are of great interest because of their potential use in various bioapplications such as hyperthermia for cancer treatment, magnetic resonance imaging (MRI) contrast agents and drug delivery. Herein, we introduce a facile method for synthesizing magnetic Fe nanocrystal/Fe-substituted calcium phosphate (Fe/FeCaP) heterostructured microspheres using a two-step procedure: (1) one-pot hydrothermal synthesis to prepare uniform-sized FeCaP microspheres and (2) post-reduction annealing at 600 °C for Fe extraction from FeCaP. This approach results in the fabrication of Fe/FeCaP heterostructured microspheres that exhibit superparamagnetism with a saturation magnetization of 10.77 emu g^-1. The Fe/FeCaP particles annealed at 600 °C show a much higher magnetic moment compared with the non-annealed FeCaP particles. Moreover, T2-weighted MRI phantom images reveal that the Fe/FeCaP heterostructured microspheres possess higher relaxivity than paramagnetic FeCaP, demonstrating their potential as superior and biocompatible MRI contrast agents. Moreover, the enhancement in osteoconductivity for Fe/FeCaP microspheres without any evidence of cytotoxicity was verified. Our results demonstrate the great potential of multi-functional Fe/FeCaP microspheres for use as biocompatible bone regeneration agents as well as MRI contrast agents.

Oxygen-dependent generation of a graded polydopamine coating on nanofibrous materials for controlling stem cell functions

A nanofiber with gradient in polydopamine coating was generated by controlling oxidative polymerization of dopamine for tuning stem cell responses.

Effects of Immobilized BMP-2 and Nanofiber Morphology on In Vitro Osteogenic Differentiation of hMSCs and In Vivo Collagen Assembly of Regenerated Bone

Engineering bone tissue is particularly challenging because of the distinctive structural features of bone within a complex biochemical environment. In the present study, we fabricated poly(L-lactic acid) (PLLA) electrospun nanofibers with random and aligned morphology immobilized with bone morphogenic protein-2 (BMP-2) and investigated how these signals modulate (1) in vitro osteogenic differentiation of human mesenchymal stem cells (hMSCs) and (2) in vivo bone growth rate, mechanical properties, and collagen assembly of newly formed bone. The orientation of adherent cells followed the underlying nanofiber morphology; however, nanofiber alignment did not show any difference in alkaline phosphate activity or in calcium mineralization of hMSCs after 14 days of in vitro culture in osteogenic differentiation media. In vivo bone regeneration was significantly higher in the nanofiber implanted groups (approximately 65-79%) as compared to the defect-only group (11.8 ± 0.2%), while no significant difference in bone regeneration was observed between random and aligned groups. However, nanoindentation studies of regenerated bone revealed Young's modulus and contact hardness with anisotropic feature for aligned group as compared to random group. More importantly, structural analysis of collagen at de novo bone showed the ability of nanofiber morphology to guide collagen deposition. SEM and TEM images revealed regular, highly ordered collagen assemblies on aligned nanofibers as compared to random fibers, which showed irregular, randomly organized collagen deposition. Taken together, we conclude that nanofibers in the presence of osteoinductive signals are a potent tool for bone regeneration, and nanofiber alignment can be used for engineering bone tissues with structurally assembled collagen fibers with defined direction.

Effective immobilization of BMP-2 mediated by polydopamine coating on biodegradable nanofibers for enhanced in vivo bone formation

Although bone morphogenic proteins (BMPs) have been widely used for bone regeneration, the ideal delivery system with optimized dose and minimized side effects is still active area of research. In this study, we developed bone morphogenetic protein-2(BMP-2) immobilized poly(l-lactide) (PLLA) nanofibers inspired by polydopamine, which could be ultimately used as membranes for guided bone regeneration, and investigated their effect on guidance of in vitro cell behavior and in vivo bone formation. Surface chemical analysis of the nanofibers confirmed successful immobilization of BMP-2 mediated by polydopamine, and about 90% of BMP-2 was stably retained on the nanofiber surface for at least 28 days. The alkaline phosphatase activity and calcium mineralization of human mesenchymal stem cells (hMSCs) after 14 days of in vitro culture was significantly enhanced on nanofibers immobilized with BMP-2. More importantly, BMP-2 at a relatively small dose was highly active following implantation to the critical-sized defect in the cranium of mice; radiographic analysis demonstrated that 77.8 ± 11.7% of newly formed bone was filled within the defect for a BMP-2-immobilized groups at the concentration of 124 ± 9 ng/cm(2), as compared to 5.9 ± 1.0 and 34.1 ± 5.5% recovery, for a defect-only and a polydopamine-only group, respectively. Scanning and transmission electron microscopy of samples from the BMP-2 immobilized group showed fibroblasts and osteoblasts with nanofiber strands in the middle of regenerated bone tissue, revealing the importance of interaction between implanted nanofibers and the neighboring extracellular environment. Taken together, our data support that the presentation of BMP-2 on the surface of nanofibers as immobilized by utilizing polydopamine chemistry may be an effective method to direct bone growth at relatively low local concentration.