Reinforcing mechanically weak hydrogels with fibers is a promising route to obtain strong and tough materials for biomedical applications while retaining a favorable cell environment. The resulting hierarchical structure recreates structural elements of natural tissues such as articular cartilage, with fiber diameters ranging from the nano- to microscale. Through control of properties such as the fiber diameter, orientation, and porosity, it is possible to design materials which display the nonlinear, synergistic mechanical behavior observed in natural tissues. In order to fully exploit these advantages, it is necessary to understand the structure–property relationships in fiber-reinforced hydrogels. However, there are currently limited models which capture their complex mechanical properties. The majority of reported fiber-reinforced hydrogels contain fibers obtained by electrospinning, which allows for limited spatial control over the fiber scaffold and limits the scope for systematic mechanical testing studies. Nevertheless, new manufacturing techniques such as melt electrowriting and bioprinting have emerged, which allow for increased control over fiber deposition and the potential for future investigations on the effect of specific structural features on mechanical properties. In this review, we therefore explore the mechanics of fiber-reinforced hydrogels, and the evolution of their design and manufacture from replicating specific features of biological tissues to more complex structures, by taking advantage of design principles from both tough hydrogels and fiber-reinforced composites. By highlighting the overlap between these fields, it is possible to identify the remaining challenges and opportunities for the development of effective biomedical devices.
This report introduces a unique method of significantly improving toughness in highly swollen block copolymer-based thermoplastic elastomer (TPE) hydrogels by converting an intrinsically large population of dangling chain ends into a mechanically active second network. In one form, the TPE hydrogels developed by our group are based on swelling of a vitrified melt-blend of two amphiphilic block copolymer species, sphere-forming polystyrene-poly(ethylene oxide) (SO-H) diblock and triblock (SOS) copolymers. Here, the PEO midblock in the SOS triblock copolymer serves to tether adjacent PS spherical aggregates, producing hydrogel networks that are incredibly elastic and mechanically robust, preserving their shape even at the very high intrinsic swelling ratios produced at low SOS concentrations (e.g, 37 g of H2O/(g of polymer) at 3.3 mol % SOS). In this report, we advance the utility of this framework by exploiting the hundreds of dangling PEO chain ends per spherical aggregate to form a second, mechanically active network. The approach is based on a stepwise installation of two tethering SOS triblock copolymer populations. The first is present directly during melt-state self-assembly of the original diblock/triblock copolymer blend and inherently determines the equilibrium swelling ratio of resulting hydrogel. The second population is then introduced postswelling, by simply coupling the dangling SO diblock copolymer chain ends under conditions largely free of the mechanical stress osmotically imposed on the primary network. Notably, this action simply shifts the ratio of diblock and triblock copolymer without compromising the thermoplasticity of the network. Here, we use the facile water-based coupling of PEO-terminal azide and alkyne groups to demonstrate the scale of toughness enhancements possible through conversion of dangling ends into a second network. The dangling-end double networks produced exhibit remarkable improvements in tensile properties (tensile modulus, toughness, strain at break, and stress at break), including a 58-fold increase in mean toughness (to 361 kJ/m3) and a 19-fold increase in mean stress to break (to 169 kPa) in highly swollen samples containing up to 95% (g/g) water. Importantly, these improvements could be realized without altering water content, shape, small-strain dynamic shear, and unconfined compressive properties of the original TPE hydrogels.
Understanding how human meniscal tissue responds to loading regimes mimetic of daily life as well as how it compares to larger animal models is critical in the development of a functionally accurate synthetic surrogate. Seven human and 8 ovine cadaveric meniscal specimens were regionally sectioned into cylinders 5mm in diameter and 3 mm thick along with 10 polystyrene-b-polyethylene oxide block copolymer-based thermoplastic elastomer (TPE) hydrogels. Samples were compressed to 12% strain at 1 Hz for 5000 cycles, unloaded for 24 hours, and then retested. No differences were found within each group between test one and test two. Human and ovine tissue exhibited no regional dependency (p<0.05). Human samples relaxed quicker than ovine tissue or the TPE hydrogel with modulus values at cycle 50 not significantly different from cycle 5000. Ovine menisci were found to be similar to human menisci in relaxation profile but had significantly higher modulus values (3.44MPa instantaneous and 0.61MPa after 5000 cycles compared to 1.97MPa and 0.11MPa found for human tissue) and significantly different power law fit coefficients. The TPE hydrogel had an initial modulus of 0.58MPa and experienced less than a 20% total relaxation over the 5000. Significant differences in the magnitude of compressive modulus between human and ovine menisci were observed, however the relaxation profiles were similar. Although statistically different than the native tissues, modulus values of the TPE hydrogel material were similar to those of the human and ovine menisci, making it a material worth further investigation for use as a synthetic replacement.
Hydrogels are a class of synthetic biomaterials composed of a polymer network that swells with water and as such they have both an elastic and viscous component making them ideal for soft tissue applications. This study characterizes the compressive, tensile, and shear properties of a thermoplastic elastomer (TPE) hydrogel and compares the results to published literature values for soft tissues such as articular cartilage, the knee meniscus, and intervertebral disc components. The results show the TPE hydrogel material is viscoelastic, strain rate dependent, has similar surface and bulk properties, displays minimal damping under dynamic load, and has tension-compression asymmetry. When compared to other soft tissues it has a comparable equilibrium compressive modulus of approximately 0.5MPa and shear modulus of 0.2MPa. With a tensile modulus of only 0.2MPa though, the TPE hydrogel is inferior in tension to most collagen based soft tissues. Additional steps may be necessary to reinforce the hydrogel system and increase tensile modulus depending on the desired soft tissue application. It can be concluded that this material could be a viable option for soft tissue replacements.
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