Thermoplastic Elastomer Hydrogels
Applications from Clean Energy to Biomedical Devices
At A Glance
Researchers at Colorado State University have developed patented hydrogel polymer compositions that exploit reversible energy absorption to achieve exceptional fatigue resistance, long-term mechanical stability, and elastic recovery ideal for mechanically-demanding applications, particularly those involving repetitive cyclic loading. Sacrificial bond breaking and slow recovery dynamics of most advanced hydrogel systems are avoided. The modular network assembly, including in-situ toughening capabilities, allow the direct tuning of the mechanical properties including elasticity, network mesh density, the uptake of liquid media, mass transfer characteristics, fatigue resistance, strength, and mechanical hysteresis. The assembly of the network uses melt-state fabrication compatible with thermoplastic processing technologies and avoids solution-based cross-linking reactions that limit manufacturing integration.
Mechanical performance in hydrogels ranges from very soft and brittle gels to extremely tough and stiff gels, all which may be widely applied. Conventional hydrogels, such as highly crosslinked polyvinyl alcohols, have been particularly successful in low – load bearing biomedical applications including drug delivery, wound dressings, and injectable fillers, due to their high-water content and biocompatibility. The low – load bearing limits of conventional hydrogels are a product of the intrinsic heterogeneity and brittle primary network. During synthesis, the initial differences in reactivity between monomers and crosslinkers create densely crosslinked microgels, which later assemble into macrogels. This inhomogeneous gel formation causes regions of high strain on the polymer system exacerbated during swelling. Thus, the system has few mechanisms available to distribute load effectively, leading to network failure.
Great interest exists for expanding the scope of hydrogel applications in more mechanically demanding environments by bridging the gap between these conventional, weaker hydrogel networks and tough polymeric elastomers. Double network hydrogels (DN) can absorb large amounts of energy and attain a high modulus but with significant disadvantages. These DN hydrogels compensate for the brittleness and low modulus of conventional single network hydrogels by reinforcing them with a second or third interpenetrating network, introduced at reduced osmotic stress relative to the primary network produces an enhanced modulus while the secondary and tertiary networks provide enhanced structure and some additional absorption of energy even after the high modulus primary network fails. This method of toughening through additional networks has even been incorporated into non-hydrated elastomers with good success, but it is not compatible with fatigue resistance as the primary network is no longer intact for subsequent loading cycles. Like single network hydrogels with low fatigue resistance, the failure mode of the primary network is also correlated to its heterogeneity, leading fragmentation into microgels. These microgels then act as sliding crosslinkers for the secondary network, giving additional energy absorption capabilities, but no longer providing access to the high strain and stress capabilities exhibited under initial loading.
Many advanced applications of hydrogels need to undergo very high levels of cyclic loading. The permanent mechanical fatigue present in many DN and TN gels is incompatible with such applications. Healable primary networks may remedy this issue if incorporated into a DN hydrogel; however, rates of recovery are usually very slow, making them impractical for applications requiring high frequency cycle loading. There is a long-felt need for hydrogel systems that possesses the significant modulus and fatigue resistance (toughness) of such DN and TN systems, while also being able to undergo high levels of cyclic loading without experiencing permanent mechanical fatigue.
- Long-term mechanical stability
- Fatigue resistant
- Elastic recovery / Highly elastic
- Allows for direct tuning of mechanical properties
- Withstands cyclic loading (both compression and tension)
- Industrially viable CO2/N2 separations
- Clean Energy (e.g. Industrial scale CO2/N2 separations)
- Soft tissue mimetics (e.g. meniscus)
Wijayasekara, Dilanji B., et al. “Elastic Free-Standing RTIL Composite Membranes for CO2/N2 Separation Based on Sphere-Forming Triblock/Diblock Copolymer Blends.” Journal of Membrane Science, Elsevier, 29 Mar. 2016, www.sciencedirect.com/science/article/pii/S0376738816301831.
Wijayasekara, Dilanji B. “Elastic Free-Standing RTIL Composite Membranes for CO2/N2 Separation Based on Sphere-Forming Triblock/Diblock Copolymer Blends” (Doctoral dissertation). Mountain Scholar Home, Colorado State University. Libraries, 2016, mountainscholar.org/handle/10217/178868.
Last updated: January 2020
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