Highly Cross-Linked, Physiologically Responsive, Mechanically Adaptive Polymer Networks Made by Photopolymerization

Photolithographic Fabrication of Mechanically Adaptive Devices

Water-responsive polymers, which enable the design of objects whose mechanical properties or shape can be altered upon moderate swelling, are useful for a broad range of applications. However, the limited processing options of materials that exhibit useful switchable mechanical properties generally restricted their application to objects having a simple geometry. Here we show that this problem can be overcome by using a negative photoresist approach in which a linear hydrophilic polymer is converted into a highly transparent cross-linked polymer network. The photolithographic process allows the facile production of objects of complex shape and permits programming of the cross-link density, the extent of aqueous swelling, and thereby the stiffness and refractive index under physiological conditions over a wide range and with high spatial resolution. Our findings validate a straightforward route to fabricate mechanically adaptive devices for a variety of (biomedical) uses, notably optogenetic implants whose overall shape, mechanical contrast, and optical channels can all be defined by photolithography.

Mechanically adaptive implants fabricated with poly(2-hydroxyethyl methacrylate)-based negative photoresists

Neural implants that are based on mechanically adaptive polymers (MAPs) and soften upon insertion into the body have previously been demonstrated to elicit a reduced chronic tissue response than more rigid devices fabricated from silicon or metals, but their processability has been limited. Here we report a negative photoresist approach towards physiologically responsive MAPs. We exploited this framework to create cross-linked terpolymers of 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate and 2-ethylhexyl methacrylate by photolithographic processes. Our systematic investigation of this platform afforded an optimized composition that exhibits a storage modulus E' of 1.8 GPa in the dry state. Upon exposure to simulated physiological conditions the material swells slightly (21% w/w) leading to a reduction of E' to 2 MPa. The large modulus change is mainly caused by plasticization, which shifts the glass transition from above to below 37 °C. Single shank probes fabricated by photolithography could readily be implanted into a brain-mimicking gel without buckling and viability studies with microglial cells show that the materials display excellent biocompatibility.

Asymmetric water transport in dense leaf cuticles and cuticle-inspired compositionally graded membranes

Most of the aerial organs of vascular plants are covered by a protective layer known as the cuticle, the main purpose of which is to limit transpirational water loss. Cuticles consist of an amphiphilic polyester matrix, polar polysaccharides that extend from the underlying epidermal cell wall and become less prominent towards the exterior, and hydrophobic waxes that dominate the surface. Here we report that the polarity gradient caused by this architecture renders the transport of water through astomatous olive and ivy leaf cuticles directional and that the permeation is regulated by the hydration level of the cutin-rich outer cuticular layer. We further report artificial nanocomposite membranes that are inspired by the cuticles' compositionally graded architecture and consist of hydrophilic cellulose nanocrystals and a hydrophobic polymer. The structure and composition of these cuticle-inspired membranes can easily be varied and this enables a systematic investigation of the water transport mechanism.