Conducting polymers undergo volumetric expansion through redox-mediated ion exchange with its electrolytic environment. The ion transport processes resulting from an applied electrical field controls the conformational relaxation in conducting polymer and regulates the generated stress and strain. In the last two decades, significant contributions from various groups have resulted in methods to fabricate, model and characterize the mechanical response of conducting polymer actuators in bending mode. An alternating electrical field applied to the polymer electrolyte interface produces the mechanical response in the polymer. The electrical energy applied to the polymer is used by the electrochemical reaction in the polymer backbone, for ion transport at the electrolyte-polymer interface and for conformational changes to the polymer. Due to the advances in polymer synthesis, there are multitudes of polymer-dopant combinations used to design an actuator. Over the last decade, polypyrrole (PPy) has evolved to be the most common conducting polymer actuator. Thin sheets of polymer are electrodeposited on to a substrate, doped with dodecylbezenesulfonate (DBS-) and microfabricated into a hermetic, air operated cantilever actuator. The electrical energy applied across the thickness of the polymer is expended by the electrochemical interactions at the polymer-electrolyte interface, ion transport and electrostatic interactions of the backbone. The widely adopted model for designing actuators is the electrochemically stimulated conformational relaxation (ESCR) model. Despite these advances, there have been very few investigations into the development of a constitutive model for conducting polymers that represent the input-output relation for chemoelectromechanical energy conversion. On one hand, dynamic models of conducting polymers use multiphysics-based non-linear models that are computationally intensive and not scalable for complicated geometries. On the other, empirical models that represent the chemomechanical coupling in conducting polymers present an over-simplified approach and lack the scientific rigor in predicting the mechanical response. In order to address these limitations and to develop a constitutive model for conducting polymers, its coupled chemomechanical response and material degradation with time, we have developed a constitutive model for polypyrrole-based conducting polymer actuator. The constitutive model is applied to a micron-scale conducting polymer actuator and coupling coefficients are expressed using a mechanistic representation of coupling in polypyrrole (dodecylbenzenesulfonate) [PPy(DBS)].

Conducting polymer actuators and sensors utilize electrochemical reactions and associated ion transport at the polymer-electrolyte interface for their engineering function. Similarly, a bioderived active material utilizes ion transport through a protein and across a bilayer lipid membrane for sensing and actuation functions. Inspired by the similarity in ion transport process in a bilayer lipid membrane (BLM) and conducting polymers, we propose to build an integrated ionic device in which the ion transport through the protein in the bilayer lipid membrane regulates the electrolytic and mechanical properties of the conducting polymer. This article demonstrates the fabrication and characterization of a DPhPC planar BLM reconstituted with alamethicin and supported on a polypyrrole bridge measuring 100 μm × 500 μm and formed across micro-fabricated gold pads. The assembly is supported on silicon dioxide coated wafers and packaged into an electronic-ionic package for electrochemical characterization. The various ionic components in the integrated ionic device are characterized using electrical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA) measurements. The results from our experimental studies demonstrate the procedure to fabricate a rugged electro active polymer supported BLM that will serve as a platform for chemical, bioelectrical sensing and VOC detection.

“Catheterization” is becoming one of the important medical operations since it engages minimally invasive medical procedures. It is a common practice that a catheter is inserted into body-cavities to administrate diversified medical functions such as diagnosis and surgery. Current method to deal with catheters is based upon the manual mechanisms that are time consuming and, sometimes, involving high risk to the patient. In this work, we explore the use of a promising electroactive polymer (EAP), called ionic polymer-metal composite (IPMC) as a material for use in active catheter-platforms. The configuration of our interest is a rod-shaped IPMC with 2-DOF electromechanical actuation capability. The desired functionality was achieved by fabricating inter-digitated electrodes. Firstly, the 3-D Finite Element (FE) model was introduced as a design tool to optimize the important parameters including electrode configurations. The FE model is based upon the physical transport processes — field induced migration and diffusion of ions. Secondly, based upon the FE modeling we fabricated an initial prototype exhibiting desired electromechanical output. The prototype of rod-shaped IPMC has a diameter of 1 mm and a 20 mm length. We have successfully demonstrated that the 2-DOF bending of the fabricated IPMC is feasible.

Our membrane transport protein laboratory has worked with material scientists, computational chemists and electrical and mechanical engineers to design bioactuators and sensing devices. The group has demonstrated that it is possible to produce materials composed native and engineered biological transport proteins in a variety of synthetic porous and solid materials. Biological transport proteins found in nature include pumps, which use energy to produce gradients of solutes, ion channels, which dissipate ion gradients, and a variety of carriers which can either transport substances down gradients or couple the uphill movement of substances to the dissipation of gradients. More than one type of protein can be reconstituted into the membranes to allow coupling of processes such as forming concentration gradients with ion pumps and dissipating them with an ion channel. Similarly, ion pumps can provide ion gradients to allow the co-transport of another substance. These systems are relevant to bioactuation. An example of a bioactuator that has recently been developed in the laboratory was based on a sucrose-proton exchanger coupled to a proton pump driven by ATP. When coupled together, the net reaction across the synthetic membrane was ATP driven sucrose transport across a flexible membrane across a closed space. As sucrose was transported, net flow of water occurred, causing pressure and deformation of the membrane. Transporters are regulated in nature. These proteins are sensitive to voltage, pH, sensitivity to a large variety of ligands and they can be modified to gain or lose these responses. Examples of sensors include ligand gated ion channels reconstituted on solid and permeable supports. Such sensors have value as high throughput screening devices for drug screening. Other sensors that have been developed in the laboratory include sensors for membrane active bacterial products such as the anthrax pore protein. These materials can be self assembled or manufactured by simple techniques, allowing the components to be stored in a stable form for years before (self) assembly on demand. The components can be modified at the atomic level, and are composed of nanostructures. Ranges of sizes of structures using these components range from the microscopic to macroscopic scale. The transport proteins can be obtained from natural sources or can be produced by recombinant methods from the genomes of all kingdoms including archea, bacteria and eukaryotes. For example, the laboratory is currently studying an ion channel from a thermophile from deep sea vents which has a growth optimum of 90 degrees centigrade, and has membrane transport proteins with very high temperature stability. The transport proteins can also be genetically modified to produce new properties such as activation by different ligands or transport of new substances such as therapeutic agents. The structures of many of these proteins are known, allowing computational chemists to help understand and predict the transport processes and to guide the engineering of new properties for the transport proteins and the composite membranes. Supported by DARPA and USARMY MURI Award and AFOSR.