The soft and compliant nature of ionic polymer-metal composite (IPMC) sensors has recently been investigated for various applications in soft robotic and mechatronic devices. Recent results of physics-based chemoelectromechanical modeling suggest that IPMC asymmetric surface roughening may enhance the sensitivity under compression. This paper presents initial experimental results on IPMC compression sensors fabricated with varying degrees of asymmetric surface roughness. The roughness is created through a simple mechanical sanding process on the base polymer material, referred to as "polymer abrading technique'", followed by traditional electroless plating to create electrodes. Sample sensors are characterized by measuring the voltage response under different compressive loads. The results show consistently increased sensor sensitivity of the asymmetrically roughened IPMCs versus a control sample. Sensitivity increases non-monotonically with rougher electrode surfaces, where maximum sensitivity of about 0.0433 mV/kPa is achieved with sensor electrodes with 53-74~micrometer abrasions. More variability is also observed through augmented electrode roughness, suggesting greater flexibility for IPMC sensor design. These results align with predictions from the existing physics-based chemoelectromechanical model.
In this paper, we analyze the effect of electrode surface roughness on the capacitance of Ionic Polymer Metal
Composites (IPMCs). We use the linearized Poisson-Nernst-Planck (PNP) model to describe the steady-state
spatial distribution of the electric potential and counterion concentration in the polymer region. We account
for the electrode surface roughness by solving the PNP model in a three-dimensional region, whose planar
dimensions are infinite and whose transverse dimension is varying in the neighborhood of a nominal constant
thickness. In this framework, the electrode roughness is described by a zero-mean function whose key-features,
such as spatial correlation and peak-to-peak variation, can be potentially inferred by IPMC microscopy. We use
the method of asymptotic expansions to determine a second-order accurate solution of the PNP model in terms
of the statistical properties of the electrode surface. Further, we establish a handleable closed-form expression
for the IPMC capacitance that elucidates the interplay among the IPMC nominal dimensions, the statistical
properties of the electrode surface, and the Debye screening length. We specialize our findings to isotropic
surface roughness models, including random and fractal roughness. We validate our theoretical findings through
extensive experimental work on Nafion-based IPMCs.
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