Thrust 3: Autoregenerative Materials
Self-healing and shape-shifting polymeric materials transform their geometries in response to stimuli. Common stimuli include heat and light, which usually effect modest changes in the material. The most interesting of these materials undergo shape transformations akin to polymer origami. We envision that the incorporation of living systems into these types of materials will be game-changing in that it will allow for more complexity in geometric changes (e.g., two types of engineered cells respond to two independent stimuli with differing responses) and increase the diversity of feed-stocks used for self-healing and folding through new biosynthetic pathways (e.g., new monomers based on isoprenes, acrylates, and polyols).
Thrust 3 will integrate cells engineered to produce new polymer feedstocks into films, membranes, and gels, which will be polymerized to heal material defects/damage (auto-regeneration) or patterned to create asymmetric forces to facilitate genetically encoded shape-shifting materials. In the area of auto-regenerative materials, polymer substrates will be polymerized by RAFT or ATRP so that the scaffolding material contains active chain ends. Films will be composed of acrylate polymers like poly(methyl acrylate) (PMA) and gels will be made from poly((oligoethylene glycol) methyl ether acrylate). Engineered cells will be integrated through dip-coating or spin-coating onto mechanically robust materials, like PMA, or incorporated during the gel-forming process for hydrated substrates. To enable healing of defects or damage, we will employ several techniques to make materials truly auto-regenerative. S. Golden will upregulate the naturally produced cyanobacterial alkaloid, phenazine, which can serve as a metal-free photopolymerization catalyst for ATRP and RAFT. This catalyst will be secreted to the surrounding media in response to optogenetic regulation. The material will also incorporate newly engineered biosynthetic routes to acrylates and isoprene in single-celled algae (Burkart). We envision creation of a bio-synthetic mechanism where PET-RAFT catalysts (phenazine) and monomers are produced genetically, and the light-induced gene-synthesis also polymerizes the material. Oxidase-type enzymes may be needed to preclude oxygen termination, although PET-RAFT has been known to be oxygen tolerant. In the ultimate regenerative step, S. Golden will incorporate her pioneering work in phototaxis of cyanobacteria, where light directs migration of cyanobacteria on polymeric surfaces; this concentrates cells at the site of defect and enhances deposition of regenerative polymer. The complexity of these materials will require Dutton to develop a positive synergy among microbial populations, as described previously.
An alternative approach to biosynthetic polymer synthesis was pioneered by Burkart and Mayfield to generate a wide array of polyols from algae in order to make sustainable polyurethane foams (a newsworthy application of this was renewable surfboards). The tolerance of algae to desiccation is especially advantageous because the microbes proposed can be cycled between dry and hydrated. This allows us to biosynthesize polyols in a hydrated living material and then dry the material for polyurethane synthesis, where water would typically terminate polymerization. By applying the above techniques to plant cell callus cultures, we anticipate that we can grow synthetically enhanced plant communities (Steinmetz). These techniques will provide biohybrid materials that combine the beneficial properties of synthetic and biopolymers, with potential applications in advanced ‘cyborg’ building materials.
Based on the capabilities developed in auto-regeneration research, Bae will lead the team to design shape-shifting materials. Bae will model and fabricate materials that will integrate genetically encoded algae (Burkart/Mayfield) capable of producing unique feedstocks. Once polymerized, the polymers will vary in modulus, but if the stiffness of the biosynthetic material is higher than the base matrix or if asymmetric swelling can be achieved, a force will be generated on the material. These engineered microbes will be patterned, either through additive manufacturing or soft-lithography, such that when they produce new polymers, they will generate asymmetric forces to enable complex material folding.