Chemomechanical properties and the applications of semi-confined hydrogel microstructures

Zhenwen Ding, Purdue University

Abstract

Environmentally sensitive hydrogels are three-dimensional crosslinked polymer networks capable of undergoing reversible volume change in response to different stimuli such as temperature, pH, ionic strength, electric field, etc. The integration of these soft polymeric materials with conventional micromachined hard materials (such as silicon and glass) offers unique opportunities to enhance their applications and functionalities. Many such applications require the hydrogel to be trapped in a semi-confined structure. This calls for a careful study of hydrogel thermodynamics, kinetics, and chemomechanical properties associated with such a confinement. This physiochemical understanding is essential in order to design hydrogel-based smart sensing and actuating microdevices. This Ph.D. thesis consists of two parts. The first part will be focused on the physical concepts and measurements of the physical parameters of hydrogels. We will discuss the hydrogel network structure, kinetics of hydrogel volume phase transition (VPT), biomolecule diffusion in hydrogel network, and characterizations of hydrogel properties. In the second part, we will discuss the applications of semi-confined hydrogel structures in microsystems. A high-resolution technique for fabricating hydrogel microstructures will be presented. In this method, squeeze-film is used to generate hydrogel thin film on a smooth substrate. Parylene passivation and dry etching are utilized for micropatterning. This method allows for the integration of hydrogel with MEMS and NEMS microstructures in order to fabricate miniaturized devices for sensing and actuating. Subsequently, two applications will be discussed using the patterning technique: a hydrogel diffraction grating and a hydrogel-based integrated-antenna pH sensor. Microscale patterning of biomolecules (DNA, antibody, enzyme, etc.) on solid surfaces is necessary for the successful development of many biotechnological microdevices. We developed a hydrogel stamper with built-in reservoirs for soft-printing of biomolecules on specific spots. Due to the hydrogel inherent softness, the biomolecule printing with adjustable feature size can be achieved by using a single micromachined hydrogel stamper without the need to design stampers with different feature size. Utilizing this method, we successfully stamped bovine serum albumin conjugated with fluorescein isothiocyanate (BSA-FITC) proteins on hydrophilic silicon substrate with a feature dimension ratio of 20:1 using a single hydrogel stamper. As another example of semi-confined hydrogel microstructure, we also demonstrate a two-step-casting process to fabricate a bifunctional hydrogel-based microlens array that responds to both temperature (turns opaque above a certain temperature) and pH (changes its focal length at different pH levels) and can be operated in air for extended periods of time. Each lens in the array is 1 mm in diameter and its focal length changes from 4.5 to 55 mm when the environmental pH is varied between 2.0 and 5.0. The light-switching capability is measured to be ∼ 92% when temperature increases from 25 to 35°C.

Degree

Ph.D.

Advisors

Giordano, Purdue University.

Subject Area

Biomedical engineering|Electrical engineering|Solid State Physics

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