Functionalized hydrogel systems for molecular sensing and transport

Hydrogels are a unique class of materials. The combination of many outstanding properties, including responsiveness, elasticity, transparency, porosity, high water content, biocompatibility, and the ability to immobilize functional species makes hydrogels attractive multifunctional materials for a broad range of applications. This dissertation focuses on modulating hydrogel properties for applications in molecular sensing and transport. There are two focus areas under the hydrogel theme within this dissertation. The first area is focused on the design, synthesis, and characterization of hydrogel glucose sensor materials with a liner fast response, small hysteresis and good stability under simulated physiological conditions. The first step was to study the interactions of glucose with phenylboronic acid (PBA) which is the glucose sensing functionality inside hydrogels. We find that (i) linear and slow responses are observed on hydrogels containing PBAs of low reactivity to glucose, (ii) linear and slow responses are coupled, (iii) the intramolecular ring formed in such PBAs is responsible for the low reactivity, (iv) PBAs of low pKa generally lead to low sensitivity, (v) hydrogels containing PBAs without intramolecular ring show fast and nonlinear response, (vi) nonlinear and fast responses are coupled, (vii) 1:1 and 2:1 PBA-glucose complexes exist universally in PBA-modified hydrogels and their coexistence leads to the nonlinear response. Based on these observations, a linear and fast sensor design is developed by tailoring the binding of nonlinear PBAs with glucose. A volume resetting agent is incorporated into the PBA-modified hydrogels to crosslink the PBA moieties and shrink the hydrogel volume in the absence of glucose. When glucose is present, the crosslinks are displaced into 1:1 PBA-glucose complexes, leading to a linear volume expansion. The coexistence of 1:1 and 2:1 PBA-glucose complexes during glucose sensing is largely eliminated. The sensor performance in terms of sensitivity, dynamic range, kinetics, hysteresis and stability is evaluated under simulated physiological conditions. The structure-property correlation of the sensor material is studied to allow customizing the sensor performance for different applications. The second primary area of this dissertation focuses on developing autonomic hydrogel materials that can manipulate molecular transport without external input. To anisotropically move molecules, enthalpy gradients are incorporated into the hydrogel. The molecules of interest undergo a downhill movement to lower enthalpy locations in the hydrogel, enabling the spatial iii control of molecular transport, even when such directed transport leads to the concentration of molecules. The enthalpy of molecules are essentially from interactions with the chemical gradients built inside the hydrogel medium. The chemical gradients are fabricated by localized chemical modification of hydrogels. The diffusion of reactive species inside hydrogel enables a gradual variation in the hydrogel chemical composition. With varied fabrication conditions, chemical gradients of different functionalities, strengths and geometries can be generated. The gradient functionality can be tuned to interact with molecules of interest, generating a chemically specific enthalpy gradient. Enthalpy gradients of electrostatic and supramolecular interactions are selected to study the gradient-driven molecular transport. Directional gradients can anisotropically transport molecules several millimeters over a few hours. The directed concentration of molecules is demonstrated by radially symmetric gradients. By varying the structural parameters of radially symmetric gradients, such as gradient width and binding constant with molecules, the directed concentration performance is modulated. Using enthalpy gradients based on different types of chemical interactions, molecular separation is achieved on a binary mixture. In general, the molecular transport performance depends on the hydrogel structure, enabling the flexibility in the design of autonomic materials that can manipulate a broader range of molecules. This dissertation explores the structure-property correlation of hydrogel materials. The interactions of functionalized hydrogels with molecules of interest can be quantitatively controlled to achieve specific materials performance. Two important aspects of hydrogel materials, stimuli-responsiveness and diffusion medium, are both studied in this dissertation. By a rational design of the hydrogel chemical structure, molecular sensing and transport can be precisely achieved