Electromechanical transduction with charged polyelectrolyte membranes

Electroactive polymers for sensing

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a unique class of materials, with low-moduli high-strain capabilities and the ability to conform to surfaces of different shapes. These features make them attractive for applications such as wearable sensors and interfacing with soft tissues. Here, we review the major types of EAPs and their sensing mechanisms. These are divided into two classes depending on the main type of charge carrier: ionic EAPs (such as conducting polymers and ionic polymer–metal composites) and electronic EAPs (such as dielectric elastomers, liquid-crystal polymers and piezoelectric polymers). This review is intended to serve as an introduction to the mechanisms of these materials and as a first step in material selection for both researchers and designers of flexible/bendable devices, biocompatible sensors or even robotic tactile sensing units.

Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer

The interstitium describes the fluid, proteins, solutes, and the extracellular matrix (ECM) that comprise the cellular microenvironment in tissues. Its alterations are fundamental to changes in cell function in inflammation, pathogenesis, and cancer. Interstitial fluid (IF) is created by transcapillary filtration and cleared by lymphatic vessels. Herein we discuss the biophysical, biomechanical, and functional implications of IF in normal and pathological tissue states from both fluid balance and cell function perspectives. We also discuss analysis methods to access IF, which enables quantification of the cellular microenvironment; such methods have demonstrated, for example, that there can be dramatic gradients from tissue to plasma during inflammation and that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma. Accumulated recent data show that IF and its convection through the interstitium and delivery to the lymph nodes have many and diverse biological effects, including in ECM reorganization, cell migration, and capillary morphogenesis as well as in immunity and peripheral tolerance. This review integrates the biophysical, biomechanical, and biological aspects of interstitial and lymph fluid and its transport in tissue physiology, pathophysiology, and immune regulation.

Analysis of pH and electrically controlled swelling of hydrogel-based micro-sensors/actuators

In this paper, we carry out the theoretical electro-chemo-mechanical investigation into water swollen ionic polymer gels under the simultaneous influence of electrical and chemical stimuli. In addition to these hydrogels being deployed as active sensing/actuating elements in MEMS/BioMEMS devices, this work can also serve as the basis of illustrating synthetic analogs of physiological muscles with possible applications in orthotics, prosthetics, and as artificial muscles. An electro-chemo-mechanical model or Multi-Effect-Coupling of pH Stimulus (MECpH) model, which was developed earlier by the present authors, is significantly extended to handle nonlinear deformation and implemented numerically to simulate the deformation characteristics of the pH-stimulus responsive hydrogel under the application of an externally applied voltage in different buffered pH solutions. The nonlinear deformation theory provides more accurate results especially when the deformations are large. The hydrogel is observed to experience swelling and bending when pH and external electric field stimuli coexist. The mode and degree of deformation are found to be highly dependent on changes of environmental pH, external electrical potential and bathing ionic strength. As an anionic hydrogel is considered in the simulation, it shows larger changes in deformation characteristic in basic than in acidic solutions. More importantly, the average curvatures of the swollen hydrogel are found to be a linear function with the applied electric potential, making the hydrogel an ideal actuator. However, we also note a significant decrease in the swelling equilibrium degree as the ionic strength becomes concentrated.

Tensorial electrokinetics in articular cartilage

Electrokinetic phenomena contribute to biomechanical functions of articular cartilage and underlie promising methods for early detection of osteoarthritic lesions. Although some transport properties, such as hydraulic permeability, are known to become anisotropic with compression, the direction-dependence of cartilage electrokinetic properties remains unknown. Electroosmosis experiments were therefore performed on adult bovine articular cartilage samples, whereby fluid flows were driven by electric currents in directions parallel and perpendicular to the articular surface of statically compressed explants. Magnitudes of electrokinetic coefficients decreased slightly with compression (from approximately -7.5 microL/As in the range of 0-20% compression to -6.0 microL/As in the 35-50% range) consistent with predictions of microstructure-based models of cartilage material properties. However, no significant dependence on direction of the electrokinetic coupling coefficient was detected, even for conditions where the hydraulic permeability tensor is known to be anisotropic. This contrast may also be interpreted using microstructure-based models, and provides insights into structure-function relationships in cartilage extracellular matrix and physical mediators of cell responses to tissue compression. Findings support the use of relatively simple isotropic modeling approaches for electrokinetic phenomena in cartilage and related materials, and indicate that measurement of electrokinetic properties may provide particularly robust means for clinical evaluation of cartilage matrix integrity.

Streaming potential measurements at low ionic concentrations reflect bone microstructure

Streaming potentials (SPs) have been proposed as one transduction pathway for mechanically driven bone remodeling. The fluid spaces in which SPs are generated will determine, in part, the structural information that they can provide to bone cells. Streaming potential measurements across cortical bone strips soaked in a range of saline concentrations were used to estimate the mean radii of fluid spaces that contribute to generation of electrokinetic fields. Using a cylindrical pore model, a pore radius of less than 200 A fit SP magnitude as a function of concentration. This pore size was shown to be consistent with estimates obtained from data reported earlier for SP as a function of concentration using a non-specific model, but was smaller than previously reported estimates for pore radius. A pore size in this range indicates that flow either in bone microporosity, or canaliculi that are substantially occluded by cellular material, must generate streaming potentials. Further, the fact that such small pores generate SPs in bone indicates that SPs could provide information regarding local matrix structure to bone cells.