(615b) Engineering Thermoresponsive Biointerfaces Using Graft-to Strategies: Connecting Conjugation Strategy to Surface Wettability

Thermoresponsive surfaces exhibiting temperature-dependent hydrophobicity allow for control over bioadhesive processes, offering a powerful strategy for dynamically modulating interactions at the biointerface. At physiological temperature, biological entities such as proteins, cells, and tissues adhere, but detach as the temperature is lowered and the surface becomes hydrophilic. Tuning the degree of surface wettability and the sharpness of this hydrophobic transition is critical for optimizing the performance of these platforms: surface properties impact interactions with biomolecules, and thus, biological outcomes such as protein adsorption and cell behavior. In the development of temperature-responsive surfaces, thermoresponsive polymer chemistry plays an important role in dictating these surface properties. Previous studies have demonstrated that variables such as polymer composition, molecular weight, and grafting density impact cell adhesion and detachment, underscoring the importance of developing platforms that can be tailored to specific biological systems. Most of these variables are modulated in graft-from approaches where polymers are grown from the surface, complicating molecular characterization. To better connect molecular properties to surface wettability, we explore the impact of varying the type, density, and distribution of surface anchoring units in a graft-to approach.

We employed reversible addition-fragmentation chain transfer (RAFT) polymerization to synthesize a library of copolymers comprised of thermoresponsive di(ethylene glycol) methyl ether methacrylate (DEGMA) units and surface anchoring units. Initial surface anchoring comonomers included methacrylic acid (MAA) and N-hydroxysuccinimide ester methacrylate (NHSMA) designed to react with aminosilane-functionalized glass surfaces. We observed the aminosilane glass coatings to undergo hydrolysis, as indicated by a fluorescamine assay: we observed a loss of fluorescent signal following incubation in phosphate buffered saline, corresponding to a decrease in free surface amines. The instability of these amine-functionalized substrates is suspected to hinder the effective attachment of copolymers, thus prompting us to explore alternate conjugation strategies. To stabilize the aminosilane coatings, we immediately capped the amines with excess difunctional aldehyde, leaving residual aldehyde groups capable of reacting with amine-functionalized polymers. Towards this goal, DEGMA and aminoethyl methacrylate (AEMA) were copolymerized via RAFT in acidic aqueous conditions, with AEMA incorporated along the polymer backbone in random or block architectures. Additionally, holding molecular weight constant, we vary the density of surface anchoring units. For random architecture polymers, we expect this to modulate the grafted loop size, and for block architectures, the brush length. The resulting differences in chain mobility between architectures are expected to impact the extent of hydration, influencing temperature-dependent surface wettability. We use X-ray photoelectron spectroscopy (XPS) and contact angle measurements to evaluate polymer attachment to aldehyde-functionalized surfaces, with contact angle also used to characterize the thermoresponsive behavior of each substrate. By advancing understanding of how polymer grafting architecture and conjugation strategy impacts the fabrication and surface properties of temperature-responsive substrates, this work expands the tunability of platforms that can be adapted to unique biological environments.