Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99986

99986 - Biomimetic, Hierarchical Hydrogel Fibers With Independently Tunable Nano- and Micro Scale Ordering and Actuation Response 

While hierarchically-ordered materials mimicking natural tissues are highly sought after for applications in tissue engineering and biomedical research, researchers have had limited success in producing and controlling structures on multiple length scales. Here, we propose a non-equilibrium method of producing soft, biomimetic hydrogels from amphiphilic triblock copolymers in a facile, two-step process.

Though the self-assembly of dilute and semi-dilute solutions of linear amphiphilic triblock copolymers in water has been extensively studied in the literature, the primary method of producing gels from these copolymers typically involves a simplistic “equilibrium” approach, where the dry polymer is hydrated at near-steady state. Conversely, in this research, the triblock copolymer is dissolved in a water-miscible organic solvent above the entanglement concentration, then injected into a water bath in a rapid, “non-equilibrium” process. As a representative system we have chosen glassy polystyrene to be the hydrophobic end-block, and semicrystalline poly(ethylene oxide) to be the hydrophilic mid-block. On contact with water, the hydrophobic polystyrene end-blocks self-assemble at the nanoscale to minimize their interaction with water, resulting in a network of polystyrene micelle cores bridged by poly(ethylene oxide) chains. Simultaneously, the rapid diffusion of the water-miscible organic solvent produces pores in the gel with diameters up to tens of microns.

The porosity of the gels significantly increases the water content of the hydrogels, allowing them to extend to over ten times their original length. Furthermore, the gels exhibit a strain-hardening response at higher strains similar to that of natural tissue. The pores allow initial, reversible deformation at the microscale, while the micelle network remains largely intact. At high strain, stress is transferred to the poly(ethylene oxide) chains bridging the micelles. The dimensions of the pores and the resulting mechanical properties are strongly influenced by the solvent used during injection, as well as the initial concentration of polymer in the solvent. In contrast, the nanoscale micelle network is dictated by the composition of the block copolymer itself, which allows the nano- and micro- levels of ordering to be tuned independently.

Interestingly, if the gels are dried while under strain, the resulting aligned crystallization of the hydrophilic poly(ethylene oxide) mid-blocks produces a strong and repeatable actuation response triggered by melting the crystals, either by hydration or heating. The energy density of the actuators is greater than mammalian skeletal tissue, and the materials are fully recyclable due to the absence of chemical cross-links in the material. This research presents an exciting class of new materials with potential for diverse applications in both tissue engineering and soft robotics and provides an opportunity to further explore the complex physics of block copolymer self-assembly.

Funds: NSF DMREF CMMI 2119717

Presenting Author: Elisabeth Lloyd Pennsylvania State University

Presenting Author Biography: Elisabeth Lloyd received her B.S. in Materials Science from North Carolina State University in 2019, where she conducted undergraduate research with Dr. Richard Spontak and Dr. Jan Genzer. She is now conducting her doctoral thesis research in Materials Science at Pennsylvania State University with Dr. Robert Hickey. Her thesis research concerns the structure-processing-property relationships of biomimetic, hierarchically structured, porous hydrogels with high water content, high extensibility, and a strain-hardening response similar to natural tissue.

Authors:

Elisabeth Lloyd Pennsylvania State University
Robert Hickey Pennsylvania State University
Chao Lang Pennsylvania State University