Tendon cell contraction of collagen-GAG matrices in vitro: effect of cross-linking

Exposure to oxLDL impairs TGF-β activity in human tendon cells

BACKGROUND

Previous studies have shown that patients with hypercholesterolemia experience elevated levels of oxidized LDL (oxLDL), a molecule which triggers inflammation and collagenase activity. In this study we discovered novel mechanistic effects of oxLDL on tendon cells and the mediators regulating matrix remodeling by analyzing the expression and activity of related proteins and enzymes. These effects may contribute to tendon damage in patients with high cholesterol.

METHODS

Isolated human tendon cells (male and female donors age 28 ± 1.4 age 37 ± 5.7, respectively) were incubated in the presence or absence of oxLDL. The influence of oxLDL on the expression level of key mRNA and proteins was examined using real time quantitative PCR, ELISA and Western blots. The activities of enzymes relevant to collagen synthesis and breakdown (lysyl oxidase and matrix metalloproteinases) were quantified using fluorometry. Finally, the isolated human tendon cells in a 3D construct were exposed to combinations of oxLDL and TGF-β to examine their interacting effects on collagen matrix remodeling.

RESULTS

The one-way ANOVA of gene expression indicates that key mRNAs including TGFB, COL1A1, DCN, and LOX were significantly reduced in human tendon cells by oxLDL while MMPs were increased. The oxLDL reduced the activity of LOX at 50 µg/ml, whereas conversely MMP activities were induced at 25 µg/ml (P ≤ 0.01). COL1A1 synthesis and TGF-β secretion were also inhibited (P ≤ 0.05). Adding recombinant TGF-β reversed the effects of oxLDL on the expression of collagens and LOX. OxLDL also impaired collagen matrix remodeling (P ≤ 0.01), and adding TGF-β restored the native phenotype.

CONCLUSION

Exposure to oxLDL in patients with hypercholesterolemia may adversely affect the mechanical and structural properties of tendon tissue through a direct action of oxLDL on tendon cells, including impairment of TGF-β expression. This impairment leads to disturbed matrix remodeling and synthesis, thereby potentially leading to increased risk of acute or chronic tendon injury. Our discovery may provide an opportunity for developing effective treatments for tendon injury in hypercholesterolemia patients by targeting the TGF-β pathway.

Advanced strategies for constructing interfacial tissues of bone and tendon/ligament

Enthesis, the interfacial tissue between a tendon/ligament and bone, exhibits a complex histological transition from soft to hard tissue, which significantly complicates its repair and regeneration after injury. Because traditional surgical treatments for enthesis injury are not satisfactory, tissue engineering has emerged as a strategy for improving treatment success. Rapid advances in enthesis tissue engineering have led to the development of several strategies for promoting enthesis tissue regeneration, including biological scaffolds, cells, growth factors, and biophysical modulation. In this review, we discuss recent advances in enthesis tissue engineering, particularly the use of biological scaffolds, as well as perspectives on the future directions in enthesis tissue engineering.

Personalized Scaffolds for Diabetic Foot Ulcer Healing Using Extracellular Matrix from Induced Pluripotent Stem-Reprogrammed Patient Cells

Diabetic foot ulcers (DFU) are chronic wounds sustained by pathological fibroblasts and aberrant extracellular matrix (ECM). Porous collagen-based scaffolds (CS) have shown clinical promise for treating DFUs but may benefit from functional enhancements. Our previous work showed fibroblasts differentiated from induced pluripotent stem cells are an effective source of new ECM mimicking fetal matrix, which notably promotes scar-free healing. Likewise, functionalizing CS with this rejuvenated ECM showed potential for DFU healing. Here, we demonstrate for the first time an approach to DFU healing using biopsied cells from DFU patients, reprogramming those cells, and functionalizing CS with patient-specific ECM as a personalized acellular tissue engineered scaffold. We took a two-pronged approach: 1) direct ECM blending into scaffold fabrication; and 2) seeding scaffolds with reprogrammed fibroblasts for ECM deposition followed by decellularization. The decellularization approach reduced cell number requirements and maintained naturally deposited ECM proteins. Both approaches showed enhanced ECM deposition from DFU fibroblasts. Decellularized scaffolds additionally enhanced glycosaminoglycan deposition and subsequent vascularization. Finally, reprogrammed ECM scaffolds from patient-matched DFU fibroblasts outperformed those from healthy fibroblasts in several metrics, suggesting ECM is in fact able to redirect resident pathological fibroblasts in DFUs towards healing, and a patient-specific ECM signature may be beneficial.

Quantification of cell contractile behavior based on non-destructive macroscopic measurement of tension forces on bioprinted hydrogel

Contraction assay based on surface measurement have been widely used to evaluate cell contractility in 3D models. This method is straightforward and requires no specific equipment, but it does not provide quantitative data about contraction forces generated by cells. We expanded this method with a new biomechanical model, based on the work-energy theorem, to provide non-destructive longitudinal monitoring of contraction forces generated by cells in 3D. We applied this method on hydrogels seeded with either fibroblasts or osteoblasts. Hydrogel mechanical characteristics were modulated to enhance (condition HCA_High: hydrogel contraction assay high contraction) or limit (condition HCA_Low: hydrogel contraction assay low contraction) cell contractile behaviors. Macroscopic measures were further correlated with cell contractile behavior and descriptive analysis of their physiology in response to different mechanical environments. Fibroblasts and osteoblasts contracted their matrix up to 47% and 77% respectively. Contraction stress peaked at day 5 with 1.1 10^-14 Pa for fibroblasts and 3.5 10^-14 Pa for osteoblasts, which correlated with cell attachment and spreading. Negligible contraction was seen in HCA_Low. Both fibroblasts and osteoblasts expressed α-SMA contractile fibers in HCA_High and HCA_Low. Failure to contract HCA_Low was attributed to increased cross-linking and resistance to proteolytic degradation of the hydrogel.

Tendon tissue engineering: Cells, growth factors, scaffolds and production techniques

Tendon injuries are a global health problem that affects millions of people annually. The properties of tendons make their natural rehabilitation a very complex and long-lasting process. Thanks to the development of the fields of biomaterials, bioengineering and cell biology, a new discipline has emerged, tissue engineering. Within this discipline, diverse approaches have been proposed. The obtained results turn out to be promising, as increasingly more complex and natural tendon-like structures are obtained. In this review, the nature of the tendon and the conventional treatments that have been applied so far are underlined. Then, a comparison between the different tendon tissue engineering approaches that have been proposed to date is made, focusing on each of the elements necessary to obtain the structures that allow adequate regeneration of the tendon: growth factors, cells, scaffolds and techniques for scaffold development. The analysis of all these aspects allows understanding, in a global way, the effect that each element used in the regeneration of the tendon has and, thus, clarify the possible future approaches by making new combinations of materials, designs, cells and bioactive molecules to achieve a personalized regeneration of a functional tendon.

Anterior cruciate ligament reconstruction in a rabbit model using a silk-collagen scaffold modified by hydroxyapatite at both ends: a histological and biomechanical study

Background

To investigate osteointegration at the graft-bone interface and the prevention of osteoarthritis after anterior cruciate ligament (ACL) reconstruction using a silk-collagen scaffold with both ends modified by hydroxyapatite (HA) in a rabbit model.

Methods

The HA/silk-collagen scaffold was fabricated using a degummed, knitted silk scaffold, collagen I matrix, and simulated body fluid (SBF). The HA/silk-collagen scaffold was rolled up to make a graft for replacing the native ACL in the experimental group (HA group), and the silk-collagen scaffold was used in the control (S group). All specimens were harvested at 16 weeks postoperatively to evaluate graft-bone healing and osteoarthritis prevention.

Results

Histological staining revealed the massive formation of more mature bone at the tendon-bone interface, and immunohistochemistry staining revealed more collagen I and osteocalcin deposition in the HA group than in the S group. Higher signals indicating more bone mineral formation were detected in the HA group than in the S group, which was consistent with the results of biomechanical testing. Better osteoarthritis prevention was also observed in the HA group, indicating a more stable knee joint in the HA group than in the S group.

Conclusion

The HA/silk-collagen scaffold promotes osteointegration at the tendon-bone interface after ACL reconstruction and has great potential for clinical applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-021-02281-0.

Effects and Mechanism of Berberine on the Dexamethasone-Induced Injury of Human Tendon Cells

Objective

To investigate the effects of berberine (Berb) on dexamethasone- (Dex-) induced injury of human tendon cells and its potential mechanism.

Methods

CCK-8 assay was used to explore the appropriate concentration of Dex-induced injury of tendon cells and the doses of Berb attenuates Dex cytotoxicity; cell wound healing assay was used to detect the effects (P < 0.05) of Berb and Dex on the migration ability of tendon cells; flow cytometry was used to measure cell apoptosis; DCF DA fluorescent probe was used to measure the ROS activity of cells. Western blotting was used to detect the expression of phenotype related factors including smooth muscle actin α (SMA-α), type I collagen (Col I), col III, apoptosis-related factors, caspase-3, cleaved caspase-3, caspase-9, cleaved caspase-9, and PI3K/AKT.

Results

CCK-8 assay showed that 1-100 μM Dex significantly inhibited the proliferation of tendon cells in a concentration-dependent manner (P < 0.05), where the inhibitory effect of 100 μM Dex was most significant (P < 0.005), and the pretreatment of 150, 200 μM Berb could reverse those inhibitions (all P < 0.05). Compared with the control group, Dex significantly inhibited cell migration (P < 0.05), while Berb pretreatment could enhance cell migration (P < 0.05). Flow cytometry and ROS assay showed that Dex could induce apoptosis and oxidative stress response of tendon cells (all P < 0.05), while Berb could reverse those responses (P < 0.05). Western blot showed that Dex could inhibit the expression of the col I and III as well as α-SMA (all P < 0.05) and enhance the expression of apoptosis-related factors including cleaved caspase-3 and cleaved caspase-9 (all P < 0.05). Besides, Dex could also inhibit the activation of the PI3K/AKT signaling pathway (all P < 0.05), thus affecting cell function, while Berb treatment significantly reversed the expression of those above proteins (all P < 0.05).

Conclusion

Berb attenuated DEX induced reduction of proliferation and migration, oxidative stress, and apoptosis of tendon cells by activating the PI3K/AKT signaling pathway and regulated the expression of phenotype related biomarkers in tendon cells. However, further studies are still needed to clarify the protective effects of Berb in vivo.

Evaluation of 1-Ethyl-3-(3-Dimethylaminopropyl) Carbodiimide Cross-Linked Collagen Membranes for Guided Bone Regeneration in Beagle Dogs

The purpose of this study was to evaluate the bone regeneration efficacy of an 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)-cross-linked collagen membrane for guided bone regeneration (GBR). A non-cross-linked collagen membrane (Control group), and an EDC-cross-linked collagen membrane (Test group) were used in this study. In vitro, mechanical, and degradation testing and cell studies were performed. In the animal study, 36 artificial bone defects were formed in the mandibles of six beagles. Implants were inserted at the time of bone grafting, and membranes were assigned randomly. Eight weeks later, animals were sacrificed, micro-computed tomography was performed, and hematoxylin-eosin stained specimens were prepared. Physical properties (tensile strength and enzymatic degradation rate) were better in the Test group than in the Control group. No inflammation or membrane collapse was observed in either group, and bone volumes (%) in defects around implants were similar in the two groups (p > 0.05). The results of new bone areas (%) analysis also showed similar values in the two groups (p > 0.05). Therefore, it can be concluded that cross-linking the collagen membranes with EDC is the method of enhancing the physical properties (tensile strength and enzymatic degradation) of the collagen membranes without risk of toxicity.

Scaffolds Functionalized with Matrix from Induced Pluripotent Stem Cell Fibroblasts for Diabetic Wound Healing

Diabetic foot ulcers (DFUs) are chronic wounds, with 20% of cases resulting in amputation, despite intervention. A recently approved tissue engineering product-a cell-free collagen-glycosaminoglycan (GAG) scaffold-demonstrates 50% success, motivating its functionalization with extracellular matrix (ECM). Induced pluripotent stem cell (iPSC) technology reprograms somatic cells into an embryonic-like state. Recent findings describe how iPSCs-derived fibroblasts ("post-iPSF") are proangiogenic, produce more ECM than their somatic precursors ("pre-iPSF"), and their ECM has characteristics of foetal ECM (a wound regeneration advantage, as fetuses heal scar-free). ECM production is 45% higher from post-iPSF and has favorable components (e.g., Collagen I and III, and fibronectin). Herein, a freeze-dried scaffold using ECM grown by post-iPSF cells (Post-iPSF Coll) is developed and tested vs precursors ECM-activated scaffolds (Pre-iPSF Coll). When seeded with healthy or DFU fibroblasts, both ECM-derived scaffolds have more diverse ECM and more robust immune responses to cues. Post-iPSF-Coll had higher GAG, higher cell content, higher Vascular Endothelial Growth Factor (VEGF) in DFUs, and higher Interleukin-1-receptor antagonist (IL-1ra) vs. pre-iPSF Coll. This work constitutes the first step in exploiting ECM from iPSF for tissue engineering scaffolds.

Multi-differentiation potential is necessary for optimal tenogenesis of tendon stem cells

Background

Tendon injury is a significant clinical problem due to poor healing and a high reinjury rate; successful treatment is limited by our poor understanding of endogenous tendon stem cells. Recent evidence suggests that adult stem cells are phenotypically diverse, even when comparing stem cells isolated from the same tissue from the same individual, and may in fact exist on a spectrum of proliferation and differentiation capacities. Additionally, the relationships between and clinical relevance of this phenotypic variation are poorly understood. In particular, tenogenic capacity has not been studied in comparison to tenogenic differentiation and cell proliferation. Toward this end, we performed a comprehensive assessment of cell proliferation and differentiation capacity toward four connective tissue lineages (tendon, cartilage, bone, and adipose) using tendon stem cell lines derived from single cells released directly from tendon tissue to (1) evaluate the differences, if any, in tenogenic potential, and (2) identify the relationships between differentiation phenotypes and proliferation capacity.

Methods

Tendon stem cells were derived from the endotenon of superficial digital flexor tendon from 3 horses. The cell suspension from each horse was separately plated simultaneously (1) at moderate density to generate a heterogenous population of cells—parent tendon cell line—and (2) at low density to separate single cells from each other to allow isolation of colonies that derive from single mother cells—clonal tendon stem cell lines.

Thirty clonal tendon stem cell lines—10 from each horse—and each parent tendon cell line were assessed for tenogenesis, tri-lineage differentiation, and cell proliferation. Differentiation was confirmed by lineage-specific cell staining and quantified by the relative gene expression of lineage-specific markers. Statistical significance was determined using analysis of variance and post hoc Tukey’s tests.

Results

Three distinct differentiation phenotypes—differentiation potency toward all 4 tissue lineages and two tri-lineage differentiation potencies—were identified in tendon clonal stem cell lines. These phenotypes were differentiation toward (1) tendon, cartilage, bone, and adipose (TCOA); (2) tendon, cartilage, and bone (TCO); and (3) tendon, cartilage, and adipose (TCA). Further, clonal cell lines that differentiated toward all four lineages had the highest expression of scleraxis and mohawk upon tenogenesis. Moreover, cell proliferation was significantly different between phenotypic groups, as evidenced by increased numbers of cumulative cell population doublings in clonal cell lines that did not differentiate toward adipose.

Conclusions

Our study provides evidence of the heterogenous character of adult stem cells and identifies key differences in tendon stem cell differentiation and proliferative potentials from the same individual and from the same tendon. Isolation of tendon stem cell lines with the capacity to differentiate into all four connective tissue lineages may yield improved therapeutic benefits in clinical models of repair and promote a native, regenerative phenotype in engineered tendons. Future studies may be targeted to understanding the functional contributions of each tendon stem cell phenotype in vivo and identifying additional cell phenotypes.

Meniscus-Derived Matrix Bioscaffolds: Effects of Concentration and Cross-Linking on Meniscus Cellular Responses and Tissue Repair

Meniscal injuries, particularly in the avascular zone, have a low propensity for healing and are associated with the development of osteoarthritis. Current meniscal repair techniques are limited to specific tear types and have significant risk for failure. In previous work, we demonstrated the ability of meniscus-derived matrix (MDM) scaffolds to augment the integration and repair of an in vitro meniscus defect. The objective of this study was to determine the effects of percent composition and dehydrothermal (DHT) or genipin cross-linking of MDM bioscaffolds on primary meniscus cellular responses and integrative meniscus repair. In all scaffolds, the porous microenvironment allowed for exogenous cell infiltration and proliferation, as well as endogenous meniscus cell migration. The genipin cross-linked scaffolds promoted extracellular matrix (ECM) deposition and/or retention. The shear strength of integrative meniscus repair was improved with increasing percentages of MDM and genipin cross-linking. Overall, the 16% genipin cross-linked scaffolds were most effective at enhancing integrative meniscus repair. The ability of the genipin cross-linked scaffolds to attract endogenous meniscus cells, promote glycosaminoglycan and collagen deposition, and enhance integrative meniscus repair reveals that these MDM scaffolds are promising tools to augment meniscus healing.

Ultraviolet light (365 nm) transmission properties of articular cartilage as a function of depth, extracellular matrix, and swelling

Current tissue engineering approaches for treatment of injured or diseased articular cartilage use ultraviolet light (UV) for in situ photopolymerization of biomaterials to fill chondral and osteochondral defects as well as resurfacing, stiffening and bonding the extracellular matrix and tissue interfaces. The most commonly used UV light wavelength is UVA 365 nm, the least cytotoxic and deepest penetrating. However, little information is available on the transmission of UVA 365 nm light through the cartilage matrix. In the present study, 365 nm UV light transmission was measured as a function of depth through 100 μm thick slices of healthy articular cartilage removed from mature bovine knees. Transmission properties were measured in normal (Native) cartilage and after swelling equilibration in phosphate-buffered saline (Swollen). Single-factor and multiple linear regression analyses were performed to determine depth-dependencies between the effective attenuation coefficients and proteoglycan, collagen and water contents. For both cartilages, a significant depth-dependency was found for the effective attenuation coefficients, being highest at the articular surface (superficial zone) and decreasing with depth. The effective attenuation coefficients for full-thickness cartilages were approximately a third lower than the total attenuation coefficients calculated from the individual slices. Analysis of absorption and scattering effects due to the ECM and chondrocytes found that UV light scatter coefficients were ∼10 times greater than absorption coefficients. The greater transmittance of UV light through the thicker cartilage was attributed to the collagen within the ECM causing significant backscatter forward reflectance.

A collagen-coated sponge silk scaffold for functional meniscus regeneration

Tissue engineering is a promising solution for meniscal regeneration after meniscectomy. However, in situ reconstruction still poses a formidable challenge due to multifunctional roles of the meniscus in the knee. In this study, we fabricate a silk sponge from 9% (w/v) silk fibroin solution through freeze drying and then coat its internal space and external surface with collagen sponge. Subsequently, various characteristics of the silk-collagen scaffold are evaluated, and cytocompatibility of the construct is assessed in vitro and subcutaneously. The efficacy of this composite scaffold for meniscal regeneration is evaluated through meniscus reconstruction in a rabbit meniscectomy model. It is found that the internally coated collagen sponge enhances the cytocompatibility of the silk sponge, and the external layer of collagen sponge significantly improves the initial frictional property. Additionally, the silk-collagen composite group shows more tissue ingrowth and less cartilage wear than the pure silk sponge group at 3 months postimplantation in situ. These findings thus demonstrate that the composite scaffold had less damage to the joint surface than the silk alone through promoting functional meniscal regeneration after meniscectomy, which indicates its clinical potential in meniscus reconstruction.

Production and Characterization of Chemically Cross-Linked Collagen Scaffolds

Chemical cross-linking of collagen-based devices is used as a means of increasing the mechanical stability and control the degradation rate upon implantation. Herein, we describe techniques to produce cross-linked with glutaraldehyde (GTA; amine terminal cross-linker), 4-arm polyethylene glycol succinimidyl glutarate (4SP; amine terminal cross-linker), diphenyl phosphoryl azide (DPPA; carboxyl terminal cross-linker), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC; carboxyl terminal cross-linker) collagen films. In addition, we provide protocols to characterize the biophysical (swelling), biomechanical (tensile), and biological (metabolic activity, proliferation and viability using human dermal fibroblasts and THP-1 macrophages) properties of the cross-linked collagen scaffolds.

Microstructure, local viscoelasticity and cell culture suitability of 3D hybrid HA/collagen scaffolds

As mechanical properties of cell culture substrates matter, methods for mechanical characterization of scaffolds on a relevant length scale are required. We used multiple particle tracking microrheology to close the gap between elasticity determined from bulk measurements and elastic properties sensed by cells. Structure and elasticity of macroporous, three-dimensional cryogel scaffolds from mixtures of hyaluronic acid (HA) and collagen (Coll) were characterized. Both one-component gels formed homogeneous networks, whereas hybrid gels were heterogeneous in terms of elasticity. Most strikingly, local elastic moduli were significantly lower than bulk moduli presumably due to non-equilibrium chain conformations between crosslinks. This was more pronounced in Coll and hybrid gels than in pure HA gels. Local elastic moduli were similar for all gels, irrespective of their different swelling ratio and bulk moduli. Fibroblast cell culture proved the biocompatibility of all investigated compositions. Coll containing gels enabled cell migration, adhesion and proliferation inside the gels.

Incorporation of the Amniotic Membrane as an Immunomodulatory Design Element in Collagen Scaffolds for Tendon Repair

Tendon injuries often require surgical intervention and even then result in poor outcomes due to scar formation and repeated failure. Biomaterial implants offer the potential to address multiple underlying concerns preventing improved tendon repair. Here, we describe modifications to the composition of an anisotropic collagen-glycosaminoglycan (CG) scaffold biomaterial, incorporating amniotic membrane (AM)-derived matrix to alter the inflammatory response and establish conditions for improved regenerative repair. We explored two methods of AM matrix incorporation to address multiple concerns associated with tendon repair. Amniotic membrane-derived matrix was incorporated directly into the scaffold microstructure during fabrication to form a C/AM composite. Alternatively, decellularized amniotic matrix was wrapped around the traditional collagen-chondroitin sulfate (C/CS) scaffold to form a core-shell composite (C/CS plus AM wrap) in a manner similar to current collagen membrane wraps used in rotator cuff and Achilles tendon surgeries to improve the mechanical strength of the repair. Human mesenchymal stem cells (MSCs) cultured within these materials were evaluated for metabolic health and immunomodulatory gene expression in response to inflammatory media challenge of interleukin 1 β and tumor necrosis factor α. The scaffolds were able to maintain MSC metabolic activity in all media conditions over the course of a 7 day culture. Expression of genes encoding for pro-inflammatory cytokines were down-regulated in AM containing scaffolds, suggesting the potential to employ AM-modified CG scaffolds for tendon-repair applications.

Minimal modulation of the host immune response to SIS matrix implants by mesenchymal stem cells from the amniotic fluid

PURPOSE

Surgical restoration of soft tissue defects often requires implantable devices. The clinical outcome of the surgery is determined by the properties inherent to the used matrix. Mesenchymal stem cells (MSC) modulate the immune processes after in vivo transplantation and their addition to matrices is associated with constructive remodeling. Herein we evaluate the potential of MSC derived from the amniotic fluid (AF-MSC), an interesting MSC source for cell therapeutic applications in the perinatal period, for immune modulation when added to a biomaterial.

METHODS

We implant cell free small intestinal submucosa (SIS) or SIS seeded with AF-MSC at a density of 1 × 10⁵/cm² subcutaneously at the abdominal wall in immune competent rats. The host immune response is evaluated at 3, 7 and 14 days postoperatively.

RESULTS

The matrix-specific or cellular characteristics are not altered after 24 h of in vitro co-culture of SIS with AF-MSC. The host immune response was not different between animals implanted with cell free or AF-MSC-seeded SIS in terms of cellular infiltration, vascularity, macrophage polarization or scaffold replacement. Profiling the mRNA expression level of inflammatory cytokines at the matrix interface shows a significant reduction in the expression of the pro-inflammatory marker Tnf-α and a trend towards lower iNos expression upon AF-MSC-seeding of the SIS matrix. Anti-inflammatory marker expression does not alter upon cell seeding of matrix implants.

CONCLUSION

We conclude that SIS is a suitable substrate for in vitro culture of AF-MSC and fibroblasts. AF-MSC addition to SIS does not significantly modulate the host immune response after subcutaneous implantation in rats.

The influence of pore size and stiffness on tenocyte bioactivity and transcriptomic stability in collagen-GAG scaffolds

Orthopedic injuries, particularly those involving tendons and ligaments, are some of the most commonly treated musculoskeletal ailments, but are associated with high costs and poor outcomes. A significant barrier in the design of biomaterials for tendon tissue engineering is the rapid de-differentiation observed for primary tenocytes once removed from the tendon body. Herein, we evaluate the use of an anisotropic collagen-glycosaminoglycan (CG) scaffold as a tendon regeneration platform. We report the effects of structural properties of the scaffold (pore size, collagen fiber crosslinking density) on resultant tenocyte bioactivity, viability, and gene expression. In doing so we address a standing hypothesis that scaffold anisotropy and strut flexural rigidity (stiffness) co-regulate long-term maintenance of a tenocyte phenotype. We report changes in equine tenocyte specific gene expression profiles and bioactivity across a homologous series of anisotropic collagen scaffolds with defined changes in pore size and crosslinking density. Anisotropic scaffolds with higher crosslinking densities and smaller pore sizes were more able to resist cell-mediated contraction forces, promote increased tenocyte metabolic activity, and maintain and increase expression of tenogenic gene expression profiles. These results suggest that control over scaffold strut flexural rigidity via crosslinking and porosity provides an ideal framework to resolve structure-function maps relating the influence of scaffold anisotropy, stiffness, and nutrient biotransport on tenocyte-mediated scaffold remodeling and long-term phenotype maintenance.

Controlling Biostability of Collagen Films for Fibroblast Cytocompatibility

The influence of cross-linking reagent, glutaraldehyde, on the biostability of collagen films was examined. With greater cross-linking, water absorption ability, swelling degree and degradation rate were all decreased. Based on scanning electron microscopy observations, the collagen films changed from a compact structure to a fiber-dominated morphology after cross-linking. Cross-linking provides a more integrated fiber-like structure that impeded degradation. To assess the influence of the cross-linking on cytocompatibility, in vitro cell culture was performed on the cross-linked collagen films using human fibroblasts. Good cytocompatibility of the initial collagen was largely retained after cross-linking, while biostability was greatly improved.

Triethyl orthoformate covalently cross-linked chitosan-(poly vinyl) alcohol based biodegradable scaffolds with heparin-binding ability for promoting neovascularisation

There is a need to develop pro-angiogenic biomaterials to promote wound healing and to assist in regenerative medicine. To this end, various growth factors have been exploited which have the potential to promote angiogenesis. However, these are generally expensive and labile which limits their effectiveness. An alternative approach is to immobilize heparin onto biocompatible degradable hydrogels. The heparin in turn will then bind endogenous proangiogenic growth factors to induce formation of new blood vessels.

In this study, we continue our development of hydrogels for wound healing purposes by exploring covalently cross-linking chitosan and polyvinyl alcohol hydrogels using triethyl orthoformate. Two concentrations of triethyl orthoformate (4 and 16%) were compared for their effects on the structure of hydrogels – their swelling, pore size, and rate of degradation and for their ability to support the growth of cells and for their heparin-binding capacity and their effects on angiogenesis in a chick chorioallantoic membrane assay.

Hydrogels formed with 4 or 16% both triethyl orthoformate cross-linker were equally cyto-compatible. Hydrogels formed with 4% triethyl orthoformate absorbed slightly more water than those made with 16% triethyl orthoformate and broke down slightly faster than non-cross-linked hydrogels. When soaked in heparin the hydrogel formed with 16% triethyl orthoformate showed more blood vessel formation in the CAM assay than that formed with 4% triethyl orthoformate.

Naturally derived biomaterials for addressing inflammation in tissue regeneration

Tissue regeneration strategies have traditionally relied on designing biomaterials that closely mimic features of the native extracellular matrix (ECM) as a means to potentially promote site-specific cellular behaviors. However, inflammation, while a necessary component of wound healing, can alter processes associated with successful tissue regeneration following an initial injury. These processes can be further magnified by the implantation of a biomaterial within the wound site. In addition to designing biomaterials to satisfy biocompatibility concerns as well as to replicate elements of the composition, structure, and mechanics of native tissue, we propose that ECM analogs should also include features that modulate the inflammatory response. Indeed, strategies that enhance, reduce, or even change the temporal phenotype of inflammatory processes have unique potential as future pro-regenerative analogs. Here, we review derivatives of three natural materials with intrinsic anti-inflammatory properties and discuss their potential to address the challenges of inflammation in tissue engineering and chronic wounds.

Fabrication of anatomically-shaped cartilage constructs using decellularized cartilage-derived matrix scaffolds

The native extracellular matrix of cartilage contains entrapped growth factors as well as tissue-specific epitopes for cell-matrix interactions, which make it a potentially attractive biomaterial for cartilage tissue engineering. A limitation to this approach is that the native cartilage extracellular matrix possesses a pore size of only a few nanometers, which inhibits cellular infiltration. Efforts to increase the pore size of cartilage-derived matrix (CDM) scaffolds dramatically attenuate their mechanical properties, which makes them susceptible to cell-mediated contraction. In previous studies, we have demonstrated that collagen crosslinking techniques are capable of preventing cell-mediated contraction in CDM disks. In the current study, we investigated the effects of CDM concentration and pore architecture on the ability of CDM scaffolds to resist cell-mediated contraction. Increasing CDM concentration significantly increased scaffold mechanical properties, which played an important role in preventing contraction, and only the highest CDM concentration (11% w/w) was able to retain the original scaffold dimensions. However, the increase in CDM concentration led to a concomitant decrease in porosity and pore size. Generating a temperature gradient during the freezing process resulted in unidirectional freezing, which aligned the formation of ice crystals during the freezing process and in turn produced aligned pores in CDM scaffolds. These aligned pores increased the pore size of CDM scaffolds at all CDM concentrations, and greatly facilitated infiltration by mesenchymal stem cells (MSCs). These methods were used to fabricate of anatomically-relevant CDM hemispheres. CDM hemispheres with aligned pores supported uniform MSC infiltration and matrix deposition. Furthermore, these CDM hemispheres retained their original architecture and did not contract, warp, curl, or splay throughout the entire 28-day culture period. These findings demonstrate that given the appropriate fabrication parameters, CDM scaffolds are capable of maintaining complex structures that support MSC chondrogenesis.

Immunomodulatory effects of amniotic membrane matrix incorporated into collagen scaffolds

Adult tendon wound repair is characterized by the formation of disorganized collagen matrix which leads to decreases in mechanical properties and scar formation. Studies have linked this scar formation to the inflammatory phase of wound healing. Instructive biomaterials designed for tendon regeneration are often designed to provide both structural and cellular support. In order to facilitate regeneration, success may be found by tempering the body's inflammatory response. This work combines collagen-glycosaminoglycan scaffolds, previously developed for tissue regeneration, with matrix materials (hyaluronic acid and amniotic membrane) that have been shown to promote healing and decreased scar formation in skin studies. The results presented show that scaffolds containing amniotic membrane matrix have significantly increased mechanical properties and that tendon cells within these scaffolds have increased metabolic activity even when the media is supplemented with the pro-inflammatory cytokine interleukin-1 beta. Collagen scaffolds containing hyaluronic acid or amniotic membrane also temper the expression of genes associated with the inflammatory response in normal tendon healing (TNF-α, COLI, MMP-3). These results suggest that alterations to scaffold composition, to include matrix known to decrease scar formation in vivo, can modify the inflammatory response in tenocytes. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 1332-1342, 2016.

Bioinspired Collagen/Glycosaminoglycan-Based Cellular Microenvironments for Tuning Osteoclastogenesis

Replicating the biocomplexity of native extracellular matrices (ECM) is critical for a deeper understanding of biochemical signals influencing bone homeostasis. This will foster the development of bioinspired biomaterials with adjustable bone-inducing properties. Collagen-based coatings containing single HA derivatives have previously been reported to promote osteogenic differentiation and modulate osteoclastogenesis and resorption depending on their sulfation degree. However, the potential impact of different GAG concentrations as well as the interplay of multiple GAGs in these coatings is not characterized in detail to date. These aspects were addressed in the current study by integrating HA and different sulfate-modified HA derivatives (sHA) during collagen in vitro fibrillogenesis. Besides cellular microenvironments with systematically altered single-GAG concentrations, matrices containing both low and high sHA (sHA1, sHA4) were characterized by biochemical analysis such as agarose gel electrophoresis, performed for the first time with sHA derivatives. The morphology and composition of the collagen coatings were altered in a GAG sulfation- and concentration-dependent manner. In multi-GAG microenvironments, atomic force microscopy revealed intermediate collagen fibril structures with thin fibrils and microfibrils. GAG sulfation altered the surface charge of the coatings as demonstrated by ζ-potential measurements revealed for the first time as well. This highlights the prospect of GAG-containing matrices to adjust defined surface charge properties. The sHA4- and the multi-GAG coatings alike significantly enhanced the viability of murine osteoclast-precursor-like RAW264.7 cells. Although in single-GAG matrices there was no dose-dependent effect on cell viability, osteoclastogenesis was significantly suppressed only on sHA4-coatings in a dose-dependent fashion. The multi-GAG coatings led to an antiosteoclastogenic effect in-between those with single-GAGs which cannot simply be attributed to the overall content of sulfate groups. These data suggest that the interplay of sGAGs influences bone cell behavior. Whether these findings translate into favorable biomaterial properties needs to be validated in vivo.

Chondrogenic potential of physically treated bovine cartilage matrix derived porous scaffolds on human dermal fibroblast cells

Extracellular matrices have drawn attention in tissue engineering as potential biomaterials for scaffold fabrication because of their bioactive components. Noninvasive techniques of scaffold fabrication and cross-linking treatments are believed to maintain the integrity of bioactive molecules while providing proper architectural and mechanical properties. Cartilage matrix derived scaffolds are designed to support the maintenance of chondrocytes and provide proper signals for differentiation of chondroinducible cells. Chondroinductive potential of bovine articular cartilage matrix derived porous scaffolds on human dermal fibroblasts and the effect of scaffold shrinkage on chondrogenesis were investigated. An increase in sulfated glycosaminoglycans production along with upregulation of chondrogenic genes confirmed that physically treated cartilage matrix derived scaffolds have chondrogenic potential on human dermal fibroblasts.

Anterior cruciate ligament reconstruction in a rabbit model using silk-collagen scaffold and comparison with autograft

The objective of the present study was to perform an in vivo assessment of a novel silk-collagen scaffold for anterior cruciate ligament (ACL) reconstruction. First, a silk-collagen scaffold was fabricated by combining sericin-extracted knitted silk fibroin mesh and type I collagen to mimic the components of the ligament. Scaffolds were electron-beam sterilized and rolled up to replace the ACL in 20 rabbits in the scaffold group, and autologous semitendinosus tendons were used to reconstruct the ACL in the autograft control group. At 4 and 16 weeks after surgery, grafts were retrieved and analyzed for neoligament regeneration and tendon-bone healing. To evaluate neoligament regeneration, H&E and immunohistochemical staining was performed, and to assess tendon-bone healing, micro-CT, biomechanical test, H&E and Russell-Movat pentachrome staining were performed. Cell infiltration increased over time in the scaffold group, and abundant fibroblast-like cells were found in the core of the scaffold graft at 16 weeks postoperatively. Tenascin-C was strongly positive in newly regenerated tissue at 4 and 16 weeks postoperatively in the scaffold group, similar to observations in the autograft group. Compared with the autograft group, tendon-bone healing was better in the scaffold group with trabecular bone growth into the scaffold. The results indicate that the silk-collagen scaffold has considerable potential for clinical application.

Collagen/chitosan based two-compartment and bi-functional dermal scaffolds for skin regeneration

Inspired from the sophisticated bilayer structures of natural dermis, here, we reported collagen/chitosan based two-compartment and bi-functional dermal scaffolds. Two functions refer to mediating rapid angiogenesis based on recombinant human vascular endothelial growth factor (rhVEGF) and antibacterial from gentamicin, which were encapsulated in PLGA microspheres. The gentamicin and rhVEGF encapsulated PLGA microspheres were further combined with collagen/chitosan mixtures in low (lower layer) and high (upper layer) concentrations, and molded to generate the two-compartment and bi-functional scaffolds. Based on morphology and pore structure analyses, it was found that the scaffold has a distinct double layered porous and connective structure with PLGA microspheres encapsulated. Statistical analysis indicated that the pores in the upper layer and in the lower layer have great variations in diameter, indicative of a two-compartment structure. The release profiles of gentamicin and rhVEGF exceeded 28 and 49 days, respectively. In vitro culture of mouse fibroblasts showed that the scaffold can facilitate cell adhesion and proliferation. Moreover, the scaffold can obviously inhibit proliferation of Staphylococcus aureus and Serratia marcescens, exhibiting its unique antibacterial effect. The two-compartment and bi-functional dermal scaffolds can be a promising candidate for skin regeneration.

Critical seeding density improves the properties and translatability of self-assembling anatomically shaped knee menisci

A recent development in the field of tissue engineering is the rise of all-biologic, scaffold-free engineered tissues. Since these biomaterials rely primarily upon cells, investigation of initial seeding densities constitutes a particularly relevant aim for tissue engineers. In this study, a scaffold-free method was used to create fibrocartilage in the shape of the rabbit knee meniscus. The objectives of this study were to: (i) determine the minimum seeding density, normalized by an area of 44 mm(2), necessary for the self-assembling process of fibrocartilage to occur; (ii) examine relevant biomechanical properties of engineered fibrocartilage, such as tensile and compressive stiffness and strength, and their relationship to seeding density; and (iii) identify a reduced, or optimal, number of cells needed to produce this biomaterial. It was found that a decreased initial seeding density, normalized by the area of the construct, produced superior mechanical and biochemical properties. Collagen per wet weight, glycosaminoglycans per wet weight, tensile properties and compressive properties were all significantly greater in the 5 million cells per construct group as compared to the historical 20 million cells per construct group. Scanning electron microscopy demonstrated that a lower seeding density results in a denser tissue. Additionally, the translational potential of the self-assembling process for tissue engineering was improved though this investigation, as fewer cells may be used in the future. The results of this study underscore the potential for critical seeding densities to be investigated when researching scaffold-free engineered tissues.

Neural stem cell transplantation in a double-layer collagen membrane with unequal pore sizes for spinal cord injury repair

A novel double-layer collagen membrane with unequal pore sizes in each layer was designed and tested in this study. The inner, loose layer has about 100-μm-diameter pores, while the outer, compact layer has about 10-μm-diameter pores. In a rat model of incomplete spinal cord injury, a large number of neural stem cells were seeded into the loose layer, which was then adhered to the injured side, and the compact layer was placed against the lateral side. The results showed that the transplantation of neural stem cells in a double-layer collagen membrane with unequal pore sizes promoted the differentiation of neural stem cells, attenuated the pathological lesion, and significantly improved the motor function of the rats with incomplete spinal cord injuries. These experimental findings suggest that the transplantation of neural stem cells in a double-layer collagen membrane with unequal pore sizes is an effective therapeutic strategy to repair an injured spinal cord.

Structural and biochemical modification of a collagen scaffold to selectively enhance MSC tenogenic, chondrogenic, and osteogenic differentiation

Biomaterial approaches for engineering orthopedic interfaces such as the tendon-bone junction (TBJ) are limited by a lack of understanding of how insoluble (microstructure, composition) and soluble regulators of stem cell fate work in concert to promote bioactivity and differentiation. One strategy for regenerating the interface is to design biomaterials containing spatially graded structural properties sufficient to induce divergent mesenchymal stem cell (MSC) differentiation into multiple interface-specific phenotypes. This work explores the hypothesis that selective structural modification to a 3D collagen-glycosaminoglycan (CG) scaffold combined with biochemical supplementation can drive human bone-marrow-derived MSC differentiation down tenogenic, osteogenic, and chondrogenic lineages. Tenogenic differentiation is enhanced in geometrically anisotropic scaffolds versus a standard isotropic control. Notably, blebbistatin treatment abrogates this microstructurally driven effect. Further, enhanced osteogenic differentiation and new mineral synthesis are achieved by incorporation of a calcium phosphate mineral phase within the CG scaffold along with the use of osteogenic induction media. Finally, chondrogenic differentiation is optimally driven by combining chondrogenic induction media with a reduced density scaffold that promotes increased cellular condensation, significantly higher expression of chondrogenic genes, and increased GAG deposition. Together these data provide critical insight regarding design rules for elements of an integrated biomaterial platform for orthopedic interface regeneration.

Collagen-GAG scaffold biophysical properties bias MSC lineage choice in the presence of mixed soluble signals

Biomaterial strategies for regenerating multitissue structures require unique approaches. One strategy is to design scaffolds so that their local biophysical properties can enhance site-specific effects of an otherwise heterogeneous biomolecular environment. This investigation examined the role of biomaterial physical properties (relative density, mineral content) on the human mesenchymal stem cell phenotype in the presence of mixed soluble signals to drive osteogenesis or chondrogenesis. We tested a series of three-dimensional collagen-glycosaminoglycan scaffolds with properties inspired by extracellular matrix characteristics across the osteotendinous interface (tendon, cartilage, and bone). We found that selective scaffold mineralization induced a depressed chondrogenic response compared with nonmineralized groups as demonstrated by gene expression and histological analyses. Interestingly, the greatest chondrogenic response was found in a higher density, nonmineralized scaffold variant despite increased contraction and cellular condensation in lower density nonmineralized scaffolds. In fact, the lower density scaffolds demonstrated a significantly higher expression of osteogenic transcripts as well as ample mineralization after 21 days of culture. This effect may be due to local stiffening of the scaffold microenvironment as the scaffold contracts, leading to increased cell density, accelerated differentiation, and possible endochondral ossification as evidenced by a transition from a glycosaminoglycan (GAG)-rich milieu to higher mineralization at later culture times. These findings will help shape the design rules for graded biomaterials to regenerate distinct fibrillar, fibrocartilagenous, and mineralized regions of orthopedic interfaces.

Synthesis of aliphatic polyester hydrogel for cardiac tissue engineering

Despite clinical advances, ischemic heart disease continues to be a major cause of morbidity and mortality worldwide. Prolonged cardiac ischemia and loss of cardiomyocytes frequently result in progressive pathological remodeling of the myocardium. If the heart is unable to adapt, patients may succumb to terminal heart failure. Cardiac tissue regeneration combining biodegradable biomaterials and stem cells has emerged as a new approach to restore heart function. Biomaterials, including injectable hydrogels and spongy scaffolds, can facilitate stem cell engraftment and survival and prevent adverse ventricular remodeling. Promising early results with injectable, biodegradable hydrogels for cardiac repair have provided new opportunities for designing innovative therapies to treat injured hearts. Hydrogels can be made from natural or synthetic polymers and have a water content, flexibility, and other physiochemical characteristics similar to those of living tissue, which makes them excellent candidates for tissue repair. In addition, hydrogels can be used as a vehicle to deliver cytokines or cells to the heart and can be employed to encapsulate biological macromolecules or cells and release them into the surrounding tissues during degradation. Hydrogels undergo physicochemical modifications in response to changes in temperature or pH, depending upon their polymer composition, converting from a liquid to a gel. The gel form retains cytokine molecules, allows their prolonged, controlled release, and preserves their bioactivity for extended periods. Polyethylene glycol is a water-soluble, biocompatible polymer that has negligible immunogenicity and can produce efficient conjugation of hydrogels to growth factors. In this chapter, we provide insight into the composition, polymerization, and use of a temperature-sensitive, biodegradable, aliphatic polyester hydrogel that transforms to a gel at physiological temperatures and is a potential candidate for cardiac tissue regeneration.

The use of bioinspired alterations in the glycosaminoglycan content of collagen-GAG scaffolds to regulate cell activity

The design of biomaterials for regenerative medicine can require biomolecular cues such as growth factors to induce a desired cell activity. Signal molecules are often incorporated into the biomaterial in either freely-diffusible or covalently-bound forms. However, biomolecular environments in vivo are often complex and dynamic. Notably, glycosaminoglycans (GAGs), linear polysaccharides found in the extracellular matrix, are involved in transient sequestration of growth factors via charge interactions. Biomaterials mimicking this phenomenon may offer the potential to amplify local biomolecular signals, both endogenously produced and exogenously added. GAGs of increasing sulfation (hyaluronic acid, chondroitin sulfate, heparin) were incorporated into a collagen-GAG (CG) scaffold under development for tendon tissue engineering. Manipulating the degree of GAG sulfation significantly impacts sequestration of growth factors from the media. Increasing GAG sulfation improved equine tenocyte metabolic activity in normal serum (10% FBS), low serum (1% FBS), and IGF-1 supplemented media conditions. Notably, previously reported dose-dependent changes in tenocyte bioactivity to soluble IGF-1 within the CG scaffold were replicated by using a single dose of soluble IGF-1 in scaffolds containing increasingly sulfated GAGs. Collectively, these results suggest that CG scaffold GAG content can be systematically manipulated to regulate the sequestration and resultant enhanced bioactivity of growth factor signals on cell behavior within the matrix.

The effects of crosslinking of scaffolds engineered from cartilage ECM on the chondrogenic differentiation of MSCs

Scaffolds fabricated from cartilage extracellular matrix provide a chondroinductive environment that stimulates cartilaginous matrix synthesis in a variety of cell types. A limitation of these cartilage-derived matrix (CDM) scaffolds is that they contract during in vitro culture, which unpredictably alters their shape. The current study examined the hypothesis that collagen crosslinking techniques could inhibit cell-mediated contraction of CDM scaffolds. We analyzed the effects of dehydrothermal (DHT) treatment, ultraviolet light irradiation (UV), and the chemical crosslinker carbodiimide (CAR) on scaffold contraction and chondrogenic differentiation of adult human bone marrow-derived stem cells (MSCs). Both physical and chemical crosslinking treatments retained the original scaffold dimensions. DHT and UV treatments produced significantly higher glycosaminoglycan and collagen contents than CAR crosslinked and non-crosslinked constructs. Crosslinking treatments influenced the composition of newly synthesized matrix, and DHT treatment best matched the composition of native cartilage. DHT, UV, and non-crosslinked CDM films supported cell attachment, while CAR crosslinking inhibited cell adhesion. These results affirm that collagen crosslinking treatments can prevent cell-mediated contraction of CDM scaffolds. Interestingly, crosslinking treatments influence chondrogenic differentiation. These effects seem to be mediated by modifications to cell-matrix interactions between MSCs and the CDM; however, further work is necessary to elucidate the specific mechanisms involved in this process.

MECHANICAL BEHAVIOR UNDER UNCONFINED COMPRESSION LOADINGS OF DENSE FIBRILLAR COLLAGEN MATRICES MIMETIC OF LIVING TISSUES

Bio-artificial tissues are being developed as replacements for damaged biologic tissues and their mechanical properties are critical for load-bearing applications. Reconstituted dense three-dimensional (3D) fibrillar collagen matrices are promising materials for tissue engineering, at the light of their interaction with fibroblasts.1,2 The mechanical properties of these fibrillar collagen matrices are now being characterized under unconfined compression loading for various strain rates and collagen concentrations. The data were compared to those obtained in the same conditions with a biological tissue, the rat dermis. The results show a very sensitive behavior to both the displacement rate, typical of biological soft tissues, and the collagen concentration varying between 5 and 40 mg/ml. The link between the mechanical properties and the microscopic structure of the collagen scaffolds show an increasing viscoelastic modulus with respect to the fibril density. It is found that the matrices at 5 mg/ml and the dorsal rat skin (DRS) exhibit similar stress–strain response when submitted to the same external unconfined compression load. Such results highlight the interest of these matrices as potential tissue substitutes.

All-trans-retinoid acid induces the differentiation of encapsulated mouse embryonic stem cells into GABAergic neurons

Embryonic stem (ES) cells are pluripotent cells that can differentiate into all three main germ layers: endoderm, mesoderm, and ectoderm. Although a number of methods have been developed to differentiate ES cells into neuronal phenotypes such as sensory and motor neurons, the efficient generation of GABAergic interneurons from ES cells still presents an ongoing challenge. Because the main output of inhibitory GABAergic interneurons is the gamma-aminobutyric-acid (GABA), a neurotransmitter whose controlled homeostasis is required for normal brain function, the efficient generation in culture of functional interneurons may have future implications on the treatment of neurological disorders such as epilepsy, autism, and schizophrenia. The goal of this work was to examine the generation of GABAergic neurons from mouse ES cells by comparing an embryoid body-based methodology versus a hydrogel-based encapsulation protocol that involves the use of all-trans-retinoid acid (RA). We observed that (1) there was a 2-fold increase in neuronal differentiation in encapsulated versus non-encapsulated cells and (2) there was an increase in the specificity for interneuronal differentiation in encapsulated cells, as assessed by mRNA expression and electrophysiology approaches. Furthermore, our results indicate that most of the neurons obtained from encapsulated mouse ES cells are GABA-positive (∼87%). Thus, these results suggest that combining encapsulation of ES cells and RA treatment provide a more efficient and scalable differentiation strategy for the generation in culture of functional GABAergic interneurons. This technology may have implications for future cell replacement therapies and the treatment of CNS disorders.

Tenocyte proliferation on collagen scaffolds protects against degradation and improves scaffold properties

Tissue engineering scaffolds encourage cell proliferation whilst degrading to facilitate tissue regeneration. Their mechanical properties therefore change, decreasing due to scaffold degradation and increasing due to extracellular matrix deposition. This work compares the changing properties of collagen scaffolds incubated in culture medium, with and without human tenocytes, in order to investigate the relationship between degradation and tenocyte proliferation. The material properties of scaffolds are compared over 26 days using mechanical testing, differential scanning calorimetry, infra-red spectroscopy, and histology and biochemical assays. For medium-only scaffolds, the mechanical properties decrease rapidly, while culture medium sulfhydryl content increases significantly, with no significant changes in the denaturation temperature of scaffold collagen content. Conversely, the mechanical properties and collagen content of tenocyte-seeded scaffolds increase significantly while culture medium sulfhydryl content decreases and denaturation temperature remains the same. These results indicate that tenocytes proliferation both reduces the degradation of collagen scaffolds incubated in culture medium and produces scaffolds with improved properties.

The influence of collagen-glycosaminoglycan scaffold relative density and microstructural anisotropy on tenocyte bioactivity and transcriptomic stability

Biomaterials for orthopedic tissue engineering must balance mechanical and bioactivity concerns. This work describes the fabrication of a homologous series of anisotropic collagen-GAG (CG) scaffolds with aligned tracks of ellipsoidal pores but increasing relative densities (ρ(∗)/ρ(s)), and we report the role scaffold relative density plays in directing tenocyte bioactivity. Scaffold permeability and mechanical properties, both in tension and compression, were significantly influenced by relative density in a manner predicted by cellular solids models. Equine tenocytes showed greater levels of attachment, metabolic activity, soluble collagen synthesis, and alignment as well as less cell-mediated scaffold contraction in anisotropic CG scaffolds of increasing relative density. Notably, the lowest density scaffolds experienced significant cell-mediated contraction with associated decreases in tenocyte number as well as loss of microstructural integrity, aligned contact guidance cues, and preferential tenocyte orientation over a 14 day culture period. Gene expression analyses suggested tenocyte de-differentiation in the lowest density scaffold while indicating that the highest density scaffold supported significant increases in COMP (4-fold), tenascin-C (3-fold), and scleraxis (15-fold) expression as well as significant decreases in MMP-1 (9-fold) and MMP-13 (13-fold) expression on day 14. These results suggest that anisotropic scaffold relative density can help to modulate the maintenance of a more tendon-like microenvironment and aid long-term tenocyte transcriptomic stability. Overall, this work demonstrates that relative density is a critical scaffold parameter, not only for insuring mechanical competence, but also for directing cell transcriptomic stability and behavior.

Evaluation of the ability of collagen-glycosaminoglycan scaffolds with or without mesenchymal stem cells to heal bone defects in Wistar rats

PURPOSE

The aim of this experiment was to examine the capacity of collagen-glycosaminoglycan scaffolds, with or without mesenchymal stem cells, to satisfactorily repair a 5-mm rat calvarial defect.

METHODS

Fifty-five Wistar rats were used in the study. The defects were either left empty to serve as controls (n = 7) or filled with cell-free scaffolds (n = 11), cell-seeded scaffolds that were pre-cultured in standard culture medium (n = 13), cell-seeded scaffolds that were pre-cultured in osteoinductive factor-supplemented medium (n = 12) or particulate autogenous bone (n = 12). The animals were sacrificed at 12 weeks after surgery, and specimens were prepared for histomorphometric analysis. The linear bone healing and the bone area within the defect were measured.

RESULTS

Comparable results were obtained using cell-free collagen-glycosaminoglycan scaffolds and autogenous bone both in terms of linear bone healing (P < 0.986) and area of new bone (P < 0.846). While the test groups showed significantly more bone formation compared to the empty defect control group, the linear bone healing and area of new bone within the defect were significantly lower in the cell-seeded scaffolds than in the cell-free scaffolds. The results have demonstrated that a cell-free collagen-glycosaminoglycan scaffold is capable of repairing a 5-mm rat calvarial defect as effectively as autogenous bone and that seeding the scaffold with pre-cultured mesenchymal stem cells prior to implantation offered no beneficial effect and resulted in incomplete healing of the defect.

CONCLUSIONS

The results thus suggest that the scaffold has immense potential for tissue repair showing favorable osteoconductive properties, biocompatibility and degradability.

The development of collagen-GAG scaffold-membrane composites for tendon tissue engineering

Current tissue engineering approaches for tendon defects require improved biomaterials to balance microstructural and mechanical design criteria. Collagen-glycosaminoglycan (CG) scaffolds have shown considerable success as in vivo regenerative templates and in vitro constructs to study cell behavior. While these scaffolds possess many advantageous qualities, their mechanical properties are typically orders of magnitude lower than orthopedic tissues such as tendon. Taking inspiration from mechanically efficient core-shell composites in nature such as plant stems and porcupine quills, we have created core-shell CG composites that display high bioactivity and improved mechanical integrity. These composites feature integration of a low density, anisotropic CG scaffold core with a high density, CG membrane shell. CG membranes were fabricated via an evaporative process that allowed separate tuning of membrane thickness and elastic moduli and were found to be isotropic in-plane. The membranes were then integrated with an anisotropic CG scaffold core via freeze-drying and subsequent crosslinking. Increasing the relative thickness of the CG membrane shell was shown to increase composite tensile elastic modulus by as much as a factor of 36 in a manner consistent with predictions from layered composites theory. CG scaffold-membrane composites were found to support tendon cell viability, proliferation, and metabolic activity in vitro, suggesting they maintain sufficient permeability while demonstrating improved mechanical strength. This work suggests an effective, biomimetic approach for balancing strength and bioactivity requirements of porous scaffolds for tissue engineering.

The effect of anisotropic collagen-GAG scaffolds and growth factor supplementation on tendon cell recruitment, alignment, and metabolic activity

Current surgical and tissue engineering approaches for treating tendon injuries have shown limited success, suggesting the need for new biomaterial strategies. Here we describe the development of an anisotropic collagen-glycosaminoglycan (CG) scaffold and use of growth factor supplementation strategies to create a 3D platform for tendon tissue engineering. We fabricated cylindrical CG scaffolds with aligned tracks of ellipsoidal pores that mimic the native physiology of tendon by incorporating a directional solidification step into a conventional lyophilization strategy. By modifying the freezing temperature, we created a homologous series of aligned CG scaffolds with constant relative density and degree of anisotropy but a range of pore sizes (55-243 μm). Equine tendon cells showed greater levels of attachment, metabolic activity, and alignment as well as less cell-mediated scaffold contraction, when cultured in anisotropic scaffolds compared to an isotropic CG scaffold control. The anisotropic CG scaffolds also provided critical contact guidance cues for cell alignment. While tendon cells were randomly oriented in the isotropic control scaffold and the transverse (unaligned) plane of the anisotropic scaffolds, significant cell alignment was observed in the direction of the contact guidance cues in the longitudinal plane of the anisotropic scaffolds. Scaffold pore size was found to significantly influence tendon cell viability, proliferation, penetration into the scaffold, and metabolic activity in a manner predicted by cellular solids arguments. Finally, the addition of the growth factors PDGF-BB and IGF-1 to aligned CG scaffolds was found to enhance tendon cell motility, viability, and metabolic activity in dose-dependent manners. This work suggests a composite strategy for developing bioactive, 3D material systems for tendon tissue engineering.

Applications of knitted mesh fabrication techniques to scaffolds for tissue engineering and regenerative medicine

Knitting is an ancient and yet, a fresh technique. It has a history of no less than 1,000 years. The development of tissue engineering and regenerative medicine provides a new role for knitting. Several meshes knitted from synthetic or biological materials have been designed and applied, either alone, to strengthen materials for the patching of soft tissues, or in combination with other kinds of biomaterials, such as collagen and fibroin, to repair or replace damaged tissues/organs. In the latter case, studies have demonstrated that knitted mesh scaffolds (KMSs) possess excellent mechanical properties and can promote more effective tissue repair, ligament/tendon/cartilage regeneration, pipe-like-organ reconstruction, etc. In the process of tissue regeneration induced by scaffolds, an important synergic relationship emerges between the three-dimensional microstructure and the mechanical properties of scaffolds. This paper presents a comprehensive overview of the status and future prospects of knitted meshes and its KMSs for tissue engineering and regenerative medicine.

Engineering orthopedic tissue interfaces

While a wide variety of approaches to engineering orthopedic tissues have been proposed, less attention has been paid to the interfaces, the specialized areas that connect two tissues of different biochemical and mechanical properties. The interface tissue plays an important role in transitioning mechanical load between disparate tissues. Thus, the relatively new field of interfacial tissue engineering presents new challenges--to not only consider the regeneration of individual orthopedic tissues, but also to design the biochemical and cellular composition of the linking tissue. Approaches to interfacial tissue engineering may be distinguished based on if the goal is to recreate the interface itself, or generate an entire integrated tissue unit (such as an osteochondral plug). As background for future efforts in engineering orthopedic interfaces, a brief review of the biology and mechanics of each interface (cartilage-bone, ligament-bone, meniscus-bone, and muscle-tendon) is presented, followed by an overview of the state-of-the-art in engineering each tissue, including advances and challenges specific to regenerating the interfaces.