Towards a synthetic articular cartilage

Special Issue: Nanocomposite Hydrogels for Biomedical Applications

A hydrogel consists of a three-dimensional network of polymer chains, with water as a solvent in the system [...]

High modulus hydrogels for ophthalmic and related biomedical applications

This paper presents three families of semi-interpenetrating polymer network (SIPN) hydrogels based on an ester-based polyurethane (EBPU) and hydrophilic monomers: N,N-dimethylacrylamide (NNDMA), N-vinyl pyrrolidone (NVP) and acryloylmorpholine (AMO) as potential materials for keratoprosthesis, orthokeratology and mini-scleral lens application. Hydrogels sheets were synthesized via free-radical polymerization with methods developed in-house. SIPN hydrogels were characterized for their equilibrium water content, mechanical and surface properties. Three families of optically clear SIPN-based hydrogels have been synthesized in the presence of water with >10% of composition attributable to EBPU. Water contents of SIPN materials ranged from 30% to 70%. SIPNs with ≤15% EBPU of total composition showed little influence to mechanical properties, whereas >15% EBPU contributed significantly to an increase in material stiffness. In the hydrated state, SIPNs with ≤15% EBPU of total composition show little difference in polar component (γ^p ) of surface free energy, whereas for >15% EBPU there is a decrease in γ^p . The EBPU SIPN hydrogels display complementary material properties for keratoprosthesis, orthokeratology, and mini-scleral applications. © 2018 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1645-1653, 2019.

Cross-Linking and Degradation Properties of Plasma Enhanced Chemical Vapor Deposited Poly(2-hydroxyethyl methacrylate)

Plasma Enhanced Chemical Vapor Deposition (PECVD) of poly-2-hydroxyethyl methacrylate (pHEMA) biocompatible, biodegradable polymer films were produced alone and cross-linked with ethylene glycol diacrylate (EGDA). Degree of cross-linking was controlled via manipulation of the EGDA flow rate, which influenced the amount of swelling and the extent of degradation of the films in an aqueous solution over time. Noncross-linked pHEMA films swelled 10% more than cross-linked films after 24 h of incubation in an aqueous environment. Increasing degree of film cross-linking decreased degradation over time. Thus, PECVD pHEMA films with variable cross-linking properties enable tuning of gel formation and degradation properties, making these films useful in a variety of biologically significant applications.

Markedly different effects of hyaluronic acid and chondroitin sulfate-A on the differentiation of human articular chondrocytes in micromass and 3-D honeycomb rotation cultures

A source of morphologically and functionally available human cartilagenous tissue for implantation is required in the field of tissue engineering. To achieve this goal, we evaluated the effects of hyaluronic acid (HA-810 and 1680 kDa), and chondroitin sulfate (CS-A 16 and C-34 kDa) on human articular chondrocytes (HC) in micromass and rotation culture conditions. Cell proliferation was increased by CS-A 16 kDa under micromass and rotation cultures, while cell differentiation was increased under rotation but not micromass conditions. Proliferation and differentiation due to CS-C 34 kDa were very similar to the control under both culture conditions. With HA, cell proliferation was increased depending on the molecular weight under micromass and rotation conditions. In contrast, chondrocyte differentiation was enhanced under rotation conditions, but decreased under micromass conditions depending on the molecular weight of HA. In both culture conditions, aggrecan gene was continuously expressed. However, the collagen type II gene was more weakly expressed in rotation than the micromass culture conditions. Thus, the chemical structures of polysaccharides, and the culture condition, rotation or micromass, caused differences in chondrogenesis.

In vitro release of transforming growth factor-beta 1 from gelatin microparticles encapsulated in biodegradable, injectable oligo(poly(ethylene glycol) fumarate) hydrogels

This research investigates the in vitro release of transforming growth factor-beta1 (TGF-beta1) from novel, injectable hydrogels based on the polymer oligo(poly(ethylene glycol) fumarate) (OPF). These hydrogels can be used to encapsulate TGF-beta1-loaded-gelatin microparticles and can be crosslinked at physiological conditions within a clinically relevant time period. Experiments revealed that OPF formulation and crosslinking time may be adjusted to influence the equilibrium swelling ratio, elastic modulus, strain at fracture, and mesh size of these hydrogels. Studies with OPF-gelatin microparticle composites revealed that OPF formulation and crosslinking time, as well as microparticle loading and crosslinking extent, influence composite swelling. In vitro TGF-beta1 release studies demonstrated that burst release from OPF hydrogels with a mesh size of 136 A was approximately 53%, while burst release from hydrogels with a mesh size of 93 A was only 34%. For hydrogels with a large mesh size (136 A), encapsulation of loaded gelatin microparticles allowed burst release to be reduced to 29-32%, depending on microparticle loading. Likewise, final cumulative release after 28 days was reduced from 71% to 48-66% by encapsulation of loaded microparticles. However, inclusion of gelatin microparticles within OPF hydrogels of smaller mesh size (93 A) was seen to increase TGF-beta1 release rates. The equilibrium swelling ratio of the microparticle component of these composites was shown to be greater than the equilibrium swelling ratio of the OPF component. Therefore, increased release rates are the result of disruption of the polymer network during swelling. These combined results indicate that the kinetics of TGF-beta1 release can be controlled by adjusting OPF formulation and microparticle loading, factors affecting the swelling behavior these composites. By systematically altering these parameters, in vitro release rates from hydrogels and composites loaded with TGF-beta1 at concentrations of 200 ng/ml can be varied from 13 to 170 pg TGF-beta1/day for days 1-3 and from 7 to 47 pg TGF-beta1/day for days 6-21. Therefore, these studies demonstrate the potential of these novel hydrogels and composites in the sustained delivery of low dosages of TGF-beta1 to articular cartilage defects.

Selection of elastomeric materials for compliant-layered total hip arthroplasty

A selection procedure has been developed to identify suitable commercial materials for use in compliant-layer artificial hip joints. Mechanical requirements, notably hardness and strength, as well as biocompatibility, constituted the specification for the compliant layer. Applying these constraints, candidate materials were identified in a broad range of polymeric material classes. Detailed sourcing and literature searching helped to identify materials appropriate to the application, with suitable mechanical and physical properties, as well as a history of successful clinical use. Some likely materials were identified but were prohibited from further consideration by limited commercial availability. Physical and mechanical characterization together with literature data were used to determine the relative ranking of the candidate materials and through a weighted materials property selection procedure the materials of choice were identified. The linear segmented aromatic polyurethanes, Tecothane 1085 and Estane 5714F1, emerged as the preferred materials.

Mechanical properties of a novel PVA hydrogel in shear and unconfined compression

Poly(vinyl alcohol) (PVA) hydrogels have been proposed as promising biomaterials to replace diseased or damaged articular cartilage. A critical barrier to their use as load-bearing tissue replacements is a lack of sufficient mechanical properties. The purpose of this study was to characterize the functional compressive and shear mechanical properties of a novel PVA hydrogel. Two formulations of the biomaterial were tested, one with a lower water content (75% water), and the other with higher water content (80% water). The compressive tangent modulus varied with biomaterial formulation and was found to be statistically strain magnitude and rate dependent. Over a strain range of 10-60%, the compressive modulus increased from approximately 1-18 MPa, which is within the range of the modulus of articular cartilage. The shear tangent modulus (0.1-0.4 MPa) was also found to be strain magnitude dependent and within the range of normal human articular cartilage, but it was not statistically dependent on strain rate, This behavior was attributed to the dominance of fluid flow and related frictional drag on the viscoelastic behavior. Compressive failure of the hydrogels was found to occur between 45 and 60% strain, depending on water content.

Rationalizing the design of polymeric biomaterials

Polymers are a promising class of biomaterials that can be engineered to meet specific end-use requirements. They can be selected according to key 'device' characteristics such as mechanical resistance, degradability, permeability, solubility and transparency, but the currently available polymers need to be improved by altering their surface and bulk properties. The design of macromolecules must therefore be carefully tailored in order to provide the combination of chemical, interfacial, mechanical and biological functions necessary for the manufacture of new and improved biomaterials.