Tailoring hydrogel surface properties to modulate cellular response to shear loading.

Biological tissues at articulating surfaces, such as articular cartilage, typically have remarkable low-friction properties that limit tissue shear during movement. However, these frictional properties change with trauma, aging, and disease, resulting in an altered mechanical state within the tissues. Yet, it remains unclear how these surface changes affect the behaviour of embedded cells when the tissue is mechanically loaded. Here, we developed a cytocompatible, bilayered hydrogel system that permits control of surface frictional properties without affecting other bulk physicochemical characteristics such as compressive modulus, mass swelling ratio, and water content. This hydrogel system was applied to investigate the effect of variations in surface friction on the biological response of human articular chondrocytes to shear loading. Shear strain in these hydrogels during dynamic shear loading was significantly higher in high-friction hydrogels than in low-friction hydrogels. Chondrogenesis was promoted following dynamic shear stimulation in chondrocyte-encapsulated low-friction hydrogel constructs, whereas matrix synthesis was impaired in high-friction constructs, which instead exhibited increased catabolism. Our findings demonstrate that the surface friction of tissue-engineered cartilage may act as a potent regulator of cellular homeostasis by governing the magnitude of shear deformation during mechanical loading, suggesting a similar relationship may also exist for native articular cartilage. STATEMENT OF SIGNIFICANCE: Excessive mechanical loading is believed to be a major risk factor inducing pathogenesis of articular cartilage and other load-bearing tissues. Yet, the mechanisms leading to increased transmission of mechanical stimuli to cells embedded in the tissue remain largely unexplored. Here, we demonstrate that the tribological properties of loadbearing tissues regulate cellular behaviour by governing the magnitude of mechanical deformation arising from physiological tissue function. Based on these findings, we propose that changes to articular surface friction as they occur with trauma, aging, or disease, may initiate tissue pathology by increasing the magnitude of mechanical stress on embedded cells beyond a physiological level.

Formation of blood clot on biomaterial implants influences bone healing.

The first step in bone healing is forming a blood clot at injured bones. During bone implantation, biomaterials unavoidably come into direct contact with blood, leading to a blood clot formation on its surface prior to bone regeneration. Despite both situations being similar in forming a blood clot at the defect site, most research in bone tissue engineering virtually ignores the important role of a blood clot in supporting healing. Dental implantology has long demonstrated that the fibrin structure and cellular content of a peri-implant clot can greatly affect osteoconduction and de novo bone formation on implant surfaces. This article reviews the formation of a blood clot during bone healing in relation to the use of platelet-rich plasma (PRP) gels. It is implicated that PRP gels are dramatically altered from a normal clot in healing, resulting in conflicting effect on bone regeneration. These results indicate that the effect of clots on bone regeneration depends on how the clots are formed. Factors that influence blood clot structure and properties in relation to bone healing are also highlighted. Such knowledge is essential for developing strategies to optimally control blood clot formation, which ultimately alter the healing microenvironment of bone. Of particular interest are modification of surface chemistry of biomaterials, which displays functional groups at varied composition for the purpose of tailoring blood coagulation activation, resultant clot fibrin architecture, rigidity, susceptibility to lysis, and growth factor release. This opens new scope of in situ blood clot modification as a promising approach in accelerating and controlling bone regeneration.

Controlling whole blood activation and resultant clot properties by carboxyl and alkyl functional groups on material surfaces: a possible therapeutic approach for enhancing bone healing.

Most research virtually ignores the important role of a blood clot in supporting bone healing. In this study, we investigated the effects of surface functional groups carboxyl and alkyl on whole blood coagulation, complement activation and blood clot formation. We synthesised and tested a series of materials with different ratios of carboxyl (-COOH) and alkyl (-CH3, -CH2CH3 and -(CH2)3CH3) groups. We found that surfaces with -COOH/-(CH2)3CH3 induced a faster coagulation activation than those with -COOH/-CH3 and -CH2CH3, regardless of the -COOH ratios. An increase in -COOH ratios on -COOH/-CH3 and -CH2CH3 surfaces decreased the rate of coagulation activation. The pattern of complement activation was entirely similar to that of surface-induced coagulation. All material coated surfaces resulted in clots with thicker fibrin in a denser network at the clot/material interface and a significantly slower initial fibrinolysis when compared to uncoated glass surfaces. The amounts of platelet-derived growth factor-AB (PDGF-AB) and transforming growth factor-ß (TGF-ß1) released from an intact clot were higher than a lysed clot. The release of PDGF-AB was found to be correlated with the fibrin density. This study demonstrated that surface chemistry can significantly influence the activation of blood coagulation and complement system, resultant clot structure, susceptibility to fibrinolysis as well as release of growth factors, which are important factors determining the bone healing process.

Combined VEGF and PDGF treatment reduces secondary degeneration after spinal cord injury.

Trauma to the spinal cord creates an initial physical injury damaging neurons, glia, and blood vessels, which then induces a prolonged inflammatory response, leading to secondary degeneration of spinal cord tissue, and further loss of neurons and glia surrounding the initial site of injury. Angiogenesis is a critical step in tissue repair, but in the injured spinal cord angiogenesis fails; blood vessels formed initially later regress. Stabilizing the angiogenic response is therefore a potential target to improve recovery after spinal cord injury (SCI). Vascular endothelial growth factor (VEGF) can initiate angiogenesis, but cannot sustain blood vessel maturation. Platelet-derived growth factor (PDGF) can promote blood vessel stability and maturation. We therefore investigated a combined application of VEGF and PDGF as treatment for traumatic spinal cord injury, with the aim to reduce secondary degeneration by promotion of angiogenesis. Immediately after hemisection of the spinal cord in the rat we delivered VEGF and PDGF and to the injury site. One and 3 months later the size of the lesion was significantly smaller in the treated group compared to controls, and there was significantly reduced gliosis surrounding the lesion. There was no significant effect of the treatment on blood vessel density, although there was a significant reduction in the numbers of macrophages/microglia surrounding the lesion, and a shift in the distribution of morphological and immunological phenotypes of these inflammatory cells. VEGF and PDGF delivered singly exacerbated secondary degeneration, increasing the size of the lesion cavity. These results demonstrate a novel therapeutic intervention for SCI, and reveal an unanticipated synergy for these growth factors whereby they modulated inflammatory processes and created a microenvironment conducive to axon preservation/sprouting.

Biomechanical analysis of a synthetic, biodegradable impaction graft substitute.

Impaction bone grafting in revision arthroplasty is a common and successful procedure to restore primary bone stock. Reducing the amount of bone needed to fill large grafts has been a driving force for the use of synthetic materials that can act as extenders or substitutes. To this end, we evaluated the mechanical properties of a new class of biodegradable polymer beads with and without donor bone to determine its suitability for use in impaction grafting. Biodegradable methacrylated anhydride beads were synthesized using thermal polymerization techniques. The mechanical properties of the beads were then tested in an impaction grafting test chamber and compared with morsellised porcine allograft. The beads, porcine allograft and a 50/50 combination all had similar mechanical properties, both in compression and relaxation. Pure polymer beads compacted significantly less than pure allograft and retained macroporosity after impaction. Our results suggest that the biodegradable beads have sufficient mechanical properties to be considered as an impaction grafting substitute or extender. Their ability to fill space whilst retaining macroporosity may be advantageous for tissue ingrowth and remodeling.

Transplanted abdominal granulation tissue induced bone formation--an in vivo study in sheep.

Many wounds to both soft and hard tissues heal via the formation of a granulation tissue bed. This bed is supportive of neoangiogenesis and releases proangiogenic, migratory, and proliferative growth factors and cytokines. In this study granulation tissue was grown on an intraperitoneal implant (4 mm diameter, 20 mm length) in a sheep. After 2 weeks, this implant was removed and transplanted into a femoral bone defect (4 mm diameter, 20 mm length). The sheep were sacrificed after 3 months, and the implant site examined using micro-CT and histology. A bone plaque formed adjacent to the implant, only in the presence of the peritoneal granulation tissue. This suggests that the formation of granulation tissue is a relatively conserved response at various locations in the body and its transplantation from one location to another can be used to induce tissue healing. This technique may prove useful as a method of improving physiological response to biomaterials.

Shear properties of bilaminar polymethylmethacrylate cement mantles in revision hip joint arthroplasty.

Although cement-within-cement revision arthroplasty minimizes the complications associated with removal of secure PMMA, failure at the interfacial region between new and old cement mantles remains a theoretical concern. This article assesses the variability in shear properties of bilaminar cement mantles related to duration of postcure and the use of antibiotic cements. Bilaminar cement mantles were 15% to 20% weaker than uniform mantles (P < .001) and demonstrated variability in shear strength related to duration of postcure of the freshly applied cement (P < .001). The use of Antibiotic Simplex did not significantly influence interfacial cement adhesion (P = .52). Interfacial adhesion by mechanisms other than mechanical interlock plays a significant role in the bond formed between new and old PMMA cements, with an important contribution by diffusion-based molecular interdigitation. In the presence of a secure cement-bone interface, we recommend cement-within-cement revision techniques in suitable patients.