Electroactive Polymers

Figure 1. Schematics of operation of the Polypyrrole (PPy)/Gold actuator

Figure 2. (a) SEM images of opened valve and aperture (top) and schematic model of the drug delivery device (bottom) and (b) Illustration of the cross-section of a single reservoir and bilayer valve in the drug delivery device

Electroactive Polymers and Polypyrrole (PPy)/Gold Microactuator

Electroactive polymers are polymers that exhibit shape or size changes in respond to electrical stimulation. The mechanism for this phenomenon, in the case of redox polymers, is based on the migration of counter ions in and out of the polymer matrix during redox cycling (resulting in swelling and shrinking of the polymer matrix). When the polymer is adhered to a thin metal layer, redox cycling of the polymer can induce bending of the polymer/metal bilayer (see schematic drawings and video below).

In our research, polypyrrole/gold bilayer actuators are utilized in numerous biomedical applications such as responsive drug delivery systems, extended life biosensor platforms, micro-mixers designed for Lab on a Chip (LOC) devices and microreactors.

Polypyrrole is an attractive material choice for its biocompatibility, low actuation voltage (1V or less), and potential to operate in liquid electrolytes such as biological fluids. In addition, polypyrrole can undergo 1-3% strain and thereby generate a high stress (100-1000 times greater than a skeletal muscle!).

Applications of Polypyrrole Microactuators

1. Responsive Drug Delivery System

Response(s) to a drug or combination of drugs differ from person to person. Individualized drug therapy puts the central focus on the treatment on the patient, taking into considerations unique differences between individuals. Though it has been a long pursued goal, individualized therapy has eluded scientists and researchers. An effective approach to individualized therapy involves active monitoring of disease cues such as a marker molecule indicative of a disease state and regulating the progression of the disease with an appropriate drug. Currently our lab is developing a responsive drug delivery system that integrates biosensors with drug delivery components. We are fabricating arrays of drug reservoirs covered with small (several hundred micron wide) polypyrrole/gold valves (see Figure 2).

 

2. Extended-Life Biosensor Platform

The successful and convenient self-monitoring of glucose levels is a key component for outpatient diabetes therapy. For those diabetic patients admitted to a hospital, controlling glucose levels within normal range has been associated with better clinical outcomes and shorter hospital stay. Technical limitations prohibit currently available devices from monitoring glucose in real-time in vivo for more than seven days and accurately measuring glucose levels in the hypoglycemic range. To that end, our team is developing a miniaturized microfabricated biosensing platform that is based on microfabricated arrays of sensor-containing microreservoirs. When the sensing capability of the sensor begins to deteriorate, activation of a new sensor, which has been protected in a closed microreservoir, will ensure the uninterrupted and reliable long-term monitoring of glucose. As an example, an array of 30 sensors can provide up to two months of continuous sensing if each sensor operates and is replaced by a new sensor every several days.

Optimization of the device with respect to response times, lifetimes, reproducibility, robustness, and dynamic range of the sensor, as well as with regard to biocompatibility and biofouling of the device is underway.

Figure 3. Schematic drawing of device showing PPy/Au valve and location of sensor

Figure 4. Fabricated device arrays (left) and device after polypyrrole deposition (right) with view of aperture (top right)

3. Micromixers

Micromixing (mixing in microchannels and microreservoirs) is often a diffusion-limited process due to low Reynold’s number associated with the microdomain. To create chaotic advection and promote mixing, polypyrrole actuators are utilized as micromixers. These devices, which can be fabricated in microscopic size, are flexible structures which combined in different spatial and actuation configurations. The structures promote flows of sufficient complexity to improve micromixing in isolated volumes of fluid (not necessarily along a microchannel), and enhance the rate of surface bounded reactions in microspots (for example, for DNA hybridization enhancement).

 

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