Friction and wear behavior of poly(vinyl alcohol)/poly(vinyl pyrrolidone) hydrogels for articular cartilage replacement

A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications

Polyvinyl alcohol (PVA) is a synthetic polymer derived from polyvinyl acetate through partial or full hydroxylation. PVA is commonly used in medical devices due to its low protein adsorption characteristics, biocompatibility, high water solubility, and chemical resistance. Some of the most common medical uses of PVA are in soft contact lenses, eye drops, embolization particles, tissue adhesion barriers, and as artificial cartilage and meniscus. The purpose of this review is to evaluate the available published information on PVA with respect to its safety as a medical device implant material for cartilage replacement. The review includes historical clinical use of PVA in orthopedics, and in vitro and in vivo biocompatibility studies. Finally, the safety recommendation involving the further development of PVA cryogels for cartilage replacement is addressed.

Analysis of the in vitro swelling behavior of poly(vinyl alcohol) hydrogels in osmotic pressure solution for soft tissue replacement

An osmotic solution was used to evaluate poly(vinyl alcohol) (PVA) hydrogels as potential non-degradable soft tissue replacements in vitro. Osmotic solutions are necessary in order to mimic the swelling pressure observed in vivo for soft tissues present in load-bearing joints. In vitro studies indicated that PVA hydrogels experience minimal changes in swelling with a polymer concentration of 20 wt.% PVA in phosphate-buffered saline solution (0 atm) and between 30 and 35 wt.% PVA in osmotic solution with a pressure of 0.95 atm. Swelling in osmotic pressure solutions caused decreases in the equilibrium hydrogel hydration. An investigation of hydrogel compressive modulus indicated that PVA hydrogels are within the range of articular cartilage, meniscal tissue, and the temporomandibular joint disk. Furthermore, it is possible to tailor PVA hydrogels through careful modification of the polymer concentration and freeze-thaw cycles during hydrogel preparation to match both a desired swelling ratio and a desired compressive modulus or porosity. The microstructure of the PVA hydrogels was also evaluated as a function of freeze-thaw cycles and polymer concentration to give an insight into the processes occurring during synthesis and swelling in osmotic solutions.

Shape, loading, and motion in the bioengineering design, fabrication, and testing of personalized synovial joints

With continued development and improvement of tissue engineering therapies for small articular lesions, increased attention is being focused on the challenge of engineering partial or whole synovial joints. Joint-scale constructs could have applications in the treatment of large areas of articular damage or in biological arthroplasty of severely degenerate joints. This review considers the roles of shape, loading and motion in synovial joint mechanobiology and their incorporation into the design, fabrication, and testing of engineered partial or whole joints. Incidence of degeneration, degree of impairment, and efficacy of current treatments are critical factors in choosing a target for joint bioengineering. The form and function of native joints may guide the design of engineered joint-scale constructs with respect to size, shape, and maturity. Fabrication challenges for joint-scale engineering include controlling chemo-mechano-biological microenvironments to promote the development and growth of multiple tissues with integrated interfaces or lubricated surfaces into anatomical shapes, and developing joint-scale bioreactors which nurture and stimulate the tissue with loading and motion. Finally, evaluation of load-bearing and tribological properties can range from tissue to joint scale and can focus on biological structure at present or after adaptation.

Formulations of polyvinyl alcohol cryogel that mimic the biomechanical properties of soft tissues in the natural lumbar intervertebral disc

STUDY DESIGN

An original investigation that characterizes polyvinyl alcohol cryogel (PVA-C) in the context of the human lumbar intervertebral disc (IVD).

OBJECTIVES

To evaluate the mechanical properties of PVA-C under physiological conditions; to assess PVA-C's suitability as a key component in a tissue-mimicking artificial lumbar intervertebral disc; and to identify suitable formulations that mimic the nucleus pulposus and anulus fibrosus.

SUMMARY OF BACKGROUND DATA

Current lumbar intervertebral disc prostheses provide suboptimal symptom relief and do not restore natural load-cushioning. PVA-C is a promising material due to its high water content, excellent biocompatibility, and versatile mechanical properties.

METHODS

PVA-C samples were prepared with different PVA concentrations and number of freeze-thaw cycles (FTC). Unconfined compression was conducted to characterize various PVA-C formulations. Compressive stress relaxation and creep were performed to assess the stability of PVA-C under loading. The results were compared to the mechanical properties of human lumbar intervertebral discs obtained from literature.

RESULTS

PVA-C compressive elastic modulus increased with increasing PVA concentration and number of FTC's. The 3% 3FTC is the optimal formulation for mimicking the nucleus pulposus in compression. In general, compressive stress relaxation and creep decreased with increasing PVA concentration and number of FTC's. Compressive stress relaxation and creep were lower for PVA-C than human lumbar intervertebral discs, suggesting that PVA-C will likely exhibit stable and predictable mechanical response in vivo. All formulations provided good mimicry of the human IVD in stress relaxation and creep. PVA-C also provided good match to the anulus fibrosus matrix.

CONCLUSION

Good unconfined compression, stress relaxation and creep behavior, combined with excellent biocompatibility, makes PVA-C a suitable choice as a major component of a tissue-mimicking artificial IVD. A potential artificial IVD design combining two or more different PVA-C formulations could provide excellent overall mimicry of the human IVD. Results of this investigation provide a solid foundation for future work in this area.

Biotribology of articular cartilage--a review of the recent advances

A brief review of the advances in the biotribology of articular cartilage in the last decade or so are presented. The review is limited to experimental friction and wear studies involving articular cartilage. The importance of developing in vitro models as tools not only to understand the cartilage tribological characteristics, but to evaluate current and future cartilage substitution and treatment therapies is discussed.

Superporous hydrogels for cartilage repair: Evaluation of the morphological and mechanical properties

Despite extensive research in the design of biomaterials for articular cartilage repair, there remains a need for the development of materials with the mechanical compliance to function synergistically with healthy cartilage, but porous enough to allow for tissue integration. In this study, superporous hydrogels of poly(vinyl alcohol) and poly(vinyl pyrrolidone) were prepared using a novel technique consisting of a double emulsion process. The hydrogel emulsions were physically cross-linked by freeze-thaw cycling. The hydrogels had a high degree of porosity, determined using environmental scanning electron microscopy, a technique superior to any method that involves dehydrating the samples. Increasing the volume of organic solvent increased porosity, due to cross-linking of the hydrogel solution around the droplets in the emulsion, leaving pores where the organic solvent was present. Poly(lactic-co-glyclic acid) microparticles formed and were embedded in the matrix. The mechanical properties, measured in confined creep and in unconfined, uniaxial compression, were similar to native articular cartilage. The permeability of the samples was unaffected by changing solvent content, despite changes in porosity. These materials are good candidates for tissue engineering of cartilage because they can mimic mature cartilage mechanically while providing a porous matrix through which cells can migrate and proliferate.