Yuanye Chen

Polymer gels with engineered environmentally responsive surface patterns

The polymer gels called hydrogels may be induced to swell or shrink (taking up or expelling water between the crosslinked polymer chains) in response to a variety of environmental stimuli, such as changes in pH or temperature, or the presence of a specific chemical substrate. These gels are being explored for several technological applications, particularly as biomedical materials. When hydrogels swell or shrink, complex patterns may be generated on their surfaces. Here we report the synthesis and controlled modulation of engineered surface patterns on environmentally responsive hydrogels. We modify the character of a gel surface by selectively depositing another material using a mask. For example, we use sputter deposition to imprint the surface of an N-isopropylacrylamide (NIPA) gel with a square array of gold thin films. The periodicity of the array can be continuously varied as a function of temperature or electric field (which alter the gels volume), and so such an array might serve as an optical grating for sensor applications. We also deposit small areas of an NIPA gel onthe surface of an acrylamide gel; the patterned area can be rendered invisible reversibly by switching the temperature above or below the lower critical solution temperature of the NIPA gel. We anticipate that these surface patterning techniques may find applications in display and sensor technology.

Shape memory gels made by the modulated gel technology

Shape memory gels based on interpenetrating only part of one gel network with another gel network have been synthesized. These gels consist of two parts: a control element, which is responsive to a designated environmental stimulus, and a nonresponsive substrate element. By designing the pattern in the gelation process, a variety of shapes are obtained including “spiral,” “square,” “fish,” “numbers,” “alphabets,” and “tube.” The change between two different shapes can be controlled by external stimuli such as temperature and is reversible.

Turbidity investigation of the sol–gel transition in carrageenan gels under physiologic conditions

A spectrophotometer was used to measure the turbidity of a carrageenan gel as a function of temperature. The optical transmission of the gels was found to decrease as the gels undergo the sol–gel phase transition. The differential of transmission (I) with respect to temperature (T), dI/dT, exhibits peaks for both the cooling and the heating runs with the peak positions corresponding to temperatures of gelation and melting, respectively. The full-width at half-height of the dI/dT peak obtained from the heating curve is about 2.5 times broader than that from the cooling curve. This indicates that the melting of gels may involve multiple relaxation mechanisms. The area of the hysteresis loop covered by the cooling and the heating curves increases with a decrease in the scanning rate. The thermal cycling has little impact on the sol–gel transition in the gels. The experiments show that turbidity is a powerful tool for studying the sol–gel transition in carrageenan gels.

Kinetics of ion‐induced gelation in carrageenan gels under physiologic conditions

The kinetics of gelation of an ungelled carrageenan solution exposed to a gel-inducing ionic solution was studied using a turbidity technique. Alkali metal ions were allowed to diffuse through a dialysis membrane into the solution of biopolymer. Optical transmission was measured as a function of distance away from the membrane. In the early stage of 12 h, the transmission increases with distance. After 96 h, the transmission is independent of distance. This indicates that gelation is completed everywhere inside the gel and the gels structure is homogeneous, in agreement with previous results. Time-dependent transmission indicates the presence of two relaxation processes occurring during ion-induced gelation: a primary relaxation process related to the gelling zone movement, and a secondary relaxation process related to local diffusion of polymer, bound ions, and water molecules.

Bending and shape memory effects of modulated gels

The smart materials based on spatial modulation of the chemical nature of gels have been synthesized. The modulation is achieved by interpenetrating only part of one gel network with another gel network. Therefore, these gels have an internally heterogeneous, or modulated structure. The simplest modulated structure is a bigel. The bending of the bigel has been studied as a function of temperature. A theoretical model has been used to analyze the bending stress in the bigels. A variety of shapes, including sinusoidal and spiral ones, of the gels at various temperatures can be obtained by designing the modulation pattern of the system. The novel gel functions obtained from the modulation method are based on the fact that the volumes of different gels are sensitive to different environmental aspects.