Yuandong Gu

A hydrogel-actuated environmentally sensitive microvalve for active flow control

This paper reports on the fabrication and test of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. The microvalve consists of a thin hydrogel, sandwiched between a stiff porous membrane and a flexible silicone rubber diaphragm. Swelling and deswelling of the hydrogel, which results from the diffusion of chemical species through the porous membrane is accompanied by the deflection of the diaphragm and hence closure and opening of the valve intake orifice. A phenylboronic-acid-based hydrogel was used to construct a smart microvalve that responds to the changes in the glucose and pH concentrations. The fastest response time (for a pH concentration cycle) achieved was 7 min using a 30-/spl mu/m-thick hydrogel and a 60-/spl mu/m-thick porous membrane with 0.1 /spl mu/m pore size and 40% porosity.

Hard and soft micro- and nanofabrication: An integrated approach to hydrogel-based biosensing and drug delivery

We review efforts to produce microfabricated glucose sensors and closed-loop insulin delivery systems. These devices function due to the swelling and shrinking of glucose-sensitive microgels that are incorporated into silicon-based microdevices. The glucose response of the hydrogel is due to incorporated phenylboronic acid (PBA) side chains. It is shown that in the presence of glucose, these polymers alter their swelling properties, either by ionization or by formation of glucose-mediated reversible crosslinks. Swelling pressures impinge on microdevice structures, leading either to a change in resonant frequency of a microcircuit, or valving action. Potential areas for future development and improvement are described. Finally, an asymmetric nano-microporous membrane, which may be integrated with the glucose-sensitive devices, is described. This membrane, formed using photolithography and block polymer assembly techniques, can be functionalized to enhance its biocompatibility and solute size selectivity. The work described here features the interplay of design considerations at the supramolecular, nano, and micro scales.

A hydrogel-based wireless chemical sensor

In this paper, we report on the fabrication and characterization of a new hydrogel-based wireless chemical sensor. The basic device structure is a passive LC resonator coupled to a stimuli-sensitive hydrogel which is confined between a stiff porous membrane and a thin glass diaphragm. As small molecules pass through the porous membrane, hydrogel swells and deflects the flexible glass diaphragm which is the movable plate of the variable capacitor in the totally integrated passive LC resonator. The corresponding change in resonant frequency can be remotely detected. pH-sensitive hydrogels were loaded and tested with the measured sensitivity of 394 kHz/pH at the pH of 7.4. The swelling pressure of the confined hydrogel was also determined using the calibration curve with applied air pressure.

Modulation of drug delivery rate by hydrogel-incorporating MEMS devices

We describe two approaches for modulating fluid flow and drug delivery rate in response to external stimuli using hydrogels incorporated in MEMS devices. The first design, a hydrogel-gated flow controller (HFC), consists of two components, a 3-dimensional crosscut structure and a loaded hydrogel. For a temperature-sensitive HFC, temperature cycling between 25 and 40/spl deg/C results in a flow rate change between 0 and 12 ml/minute, with a 30 second response time. In the second design, a hydrogel-actuated microvalve (HAM) was constructed. In such a device, a hydrogel disc is sandwiched between a porous plate and a flexible silicone rubber membrane. Swelling of the hydrogel produced by diffusion of chemical species through the porous plate, resulting in the deflection of the membrane and closure of the valve intake orifice. A HAM loaded with phenylboronic acid (PBA)-based glucose-sensitive hydrogel was tested. This glucose-sensitive HAM opens and closes in response to changes in glucose concentration and pH. The fastest response time achieved was 16 minutes using a 70 /spl mu/m thick hydrogel and a 60 /spl mu/m porous back plate.

A hydrogel-actuated smart microvalve with a porous diffusion barrier back-plate for active flow control

This paper reports on the fabrication and testing of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. It consists of a hydrogel disc sandwiched between a porous plate and a flexible silicone rubber membrane. Swelling of the hydrogel produced by diffusion of chemical species through the porous plate results in the deflection of the membrane and closure of the valve intake orifice. A phenylboronic acid based hydrogel was used to construct a smart microvalve that opens and closes in response to the changes in the glucose concentration and pH. The fastest response time achieved was 16 minutes using a 70 /spl mu/m thick hydrogel and a 60 /spl mu/m porous back plate.

Soft mold-dry etch: a novel hydrogel patterning technique for biomedical applications

This work introduces a patterning technique for environmentally sensitive hydrogels using a combination of soft mold and dry etch. This method alleviates the need for photoinitiators used in conventional approaches and is applicable to a broad range of hydrogels. This technique is also compatible with the traditional microfabrication methods thus allowing the integration of hydrogels with microelectronics and MEMS microstructures. Hydrogels of different shapes and sizes were patterned in a batch scale (wafer-level) with resolutions of up to 2.5 /spl mu/m. The patterned pH-sensitive hydrogels with micron-sized dimensions were able to respond within seconds. Deposited aluminum thin films on top of hydrogel microstructures was used to fabricate environmentally sensitive free standing micromirrors with a vertical displacement sensitivity of 7 /spl mu/m/pH at pH 5.0. These structures open the possibility of fabricating on-chip photonic-based hydrogel microsensors.

An environmentally responsive microflow controller with double side tethered structure for the entrapment of hydrogel

In this paper, we report on the fabrication and characterization of a new hydrogel-based microflow controller (HFC). The basic HFC structure is a silicon membrane having an array of orifices with an internal structure designed to confine the hydrogel while allowing it to control the flow across the membrane. Each orifice (140 /spl mu/m diameter) in the membrane has a central post held by two sets of tethers on each side. The hydrogel polymerized inside such orifices contracts around the post in the shrunken state, allowing the pass of fluid through the perimeter. In the swollen state, the hydrogel occupies the whole volume of the orifice completely blocking the flow. Fabrication of the structure involve a combination of deep trench etch and KOH etch. Two different hydrogels, temperature-sensitive and glucose-sensitive, have been tested in the HFC. The measured response times were 10 seconds and 10 minutes respectively.

Swelling, Mechanics, and Thermal/Chemical Stability of Hydrogels Containing Phenylboronic Acid Side Chains

We report here studies of swelling, mechanics, and thermal stability of hydrogels consisting of 20 mol % methacrylamidophenylboronic acid (MPBA) and 80 mol % acrylamide (AAm), lightly crosslinked with methylenebisacrylamide (Bis). Swelling was measured in solutions of fixed ionic strength, but with varying pH values and fructose concentrations. Mechanics was studied by compression and hold. In the absence of sugar or in the presence of fructose, the modulus was mostly maintained during the hold period, while a significant stress relaxation was seen in the presence of glucose, consistent with reversible, dynamic crosslinks provided by glucose, but not fructose. Thermal stability was determined by incubating hydrogels at pH 7.4 at room temperature, and 37, 50, and 65 °C, and monitoring swelling. In PBS (phosphate buffered saline) solutions containing 9 mM fructose, swelling remained essentially complete for 50 days at room temperature, but decreased substantially with time at the higher temperatures, with accelerated reduction of swelling with increasing temperature. Controls indicated that over long time periods, both the MPBA and AAm units were experiencing conversion to different species.