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1. Ionic polymer metal composite compression sensors with 3D-structured interfaces

   Rebecca Histed, Justin Ngo (November 2021, Smart materials and structures)

   In this paper, we report the development of tailored 3D-structured (engineered) polymer-metal interfaces to create enhanced 'engineered ionic polymer metal composite' (eIPMC) sensors towards soft, self-powered, high sensitivity strain sensor applications. We introduce a novel advanced additive manufacturing approach to tailor the morphology of the polymer-electrode interfaces via inkjet-printed polymer microscale features. We hypothesize that these features can promote inhomogeneous strain within the material upon the application of external pressure, responsible for improved compression sensing performance. We formalize a minimal physics-based chemoelectromechanical model to predict the linear sensor behavior of eIPMCs in both open-circuit and short-circuit sensing conditions. The model accounts for polymer-electrode interfacial topography to define the inhomogeneous mechanical response driving electrochemical transport in the eIPMC. Electrochemical experiments demonstrate improved electrochemical properties of the inkjet-printed eIPMCs as compared to the standard IPMC sensors fabricated from Nafion polymer sheets. Similarly, compression sensing results show a significant increase in sensing performance of inkjet-printed eIPMC. We also introduce two alternative methods of eIPMC fabrication for sub-millimeter features, namely filament-based fused-deposition manufacturing and stencil printing, and experimentally demonstrate their improved sensing performance. Our results demonstrate increasing voltage output associated to increasing applied mechanical pressure and enhanced performance of the proposed eIPMC sensors against traditional IPMC based compression sensors. 

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2. Engineered ionic polymer metal composites (eIPMCs) under dynamic compression loading conditions: theory and experiments

   https://doi.org/10.1088/1361-665X/adaab6  

   Fakharian, Omid; Nagel, William S; Leang, Kam K; Aureli, Matteo (January 2025, Smart Materials and Structures)

   Engineered Ionic Polymer Metal Composites (eIPMCs) represent the next generation of IPMCs, soft electro-chemo-mechanically coupled smart materials used as actuators and sensors. Recent studies indicate that eIPMC sensors, featuring unique microstructures at the interface between the ionic polymer membrane and the electrode, exhibit enhanced electrochemical behavior and sensitivity under compression, as compared to traditional IPMCs. However, a complete and experimentally-validated model of how eIPMCs behave under dynamic compression loads is currently missing. In this paper, we develop an analytical model for eIPMC sensors, elucidating the role of the engineered interface, modeled as a separate material layer with unique mechanical and electrochemical properties. Theoretical predictions focus on the mechanical-to-electrochemical transduction response under dynamic compressive loads. Experimental verification is conducted on conventional IPMC and novel eIPMC samples fabricated using the polymer abrading technique. Electrochemical impedance spectroscopy is performed to study the effect of the engineered interface on the electrochemical properties. Open-circuit (OC) voltage and short-circuit (SC) current are measured under external compressive loads in different loading scenarios to demonstrate sensing performance. Results show good qualitative agreement between experimental trends and model predictions. Experiments over the frequency range 1-18 Hz demonstrate an increase of 220%-290% in open-circuit voltage and 17%-166% in SC current sensitivity for eIPMCs over IPMCs. 

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3. Ionic Polymer Metal Composite Sensors With Engineered Interfaces (eIPMCs): Compression Sensing Modeling and Experiments

   https://doi.org/10.1115/DSCC2020-3289  

   Histed, Rebecca; Ngo, Justin; Hussain, Omar A.; Lapins, Chantel; Leang, Kam K.; Liao, Yiliang; Aureli, Matteo (October 2020, ASME 2020 Dynamic Systems and Control Conference)

   null (Ed.)

   Abstract In this paper, we examine the development of tailored 3D-structured (engineered) polymer-metal interfaces to create enhanced ionic polymer-metal composite (eIPMC) sensors towards soft, self-powered, high sensitivity strain sensor applications. First, a physics-based chemoelectromechanical model is developed to predict the sensor behavior of eIPMCs by incorporating structure microfeature effects in the mechanical response of the material. The model incorporates electrode surface properties, such as microscale feature thickness, size and spacing, to help define the mechanical response and transport characteristics of the polymer-electrode interface. Second, two novel approaches are described to create functional samples of eIPMC sensors using fused deposition manufacturing and inkjet printing technologies. Sample eIPMC sensors are fabricated for experimental characterization. Finally, experimental results are provided to show superior performance in the sensing capabilities compared to traditional sensors fabricated from sheet-form material. The results also validate important predictive aspects of the proposed minimal model. 

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4. Engineered IPMC sensors: modeling, characterization, and application towards wearable postural-tactile measurement

   https://doi.org/10.1088/1361-665X/ad142b  

   Nagel, William S; Fakharian, Omid; Aureli, Matteo; Leang, Kam K (December 2023, Smart Materials and Structures)

   Abstract This paper focuses on the modeling and development of engineered ionic polymer-metal composite (eIPMC) sensors for applications such as postural and tactile measurement in mechatronics/robotics-assisted finger rehabilitation therapy. Specifically, to tailor the sensitivity of the device, eIPMCs, fabricated using a polymer-surface abrading technique, are utilized as the sensing element. An enhanced chemoelectromechanical model is developed that captures the effect of the abrading process on the multiphysics sensing behavior under different loading conditions. The fabricated sensors are characterized using scanning electron microscopy imaging and cyclic voltammetry and chronoamperometry. Results show significant improvement in the electrochemical properties, including charge storage, double layer capacitance, and surface conductance, compared to the control samples. Finally, prototype postural-tactile finger sensors composed of different eIPMC variants are created and their performance validated under postural and tactile experiments. The tailored eIPMC sensors show increased open-circuit voltage response compared to control IPMCs, with 7.7- and 4.7-times larger peak-to-peak bending response under postural changes, as well as a 3.2-times more sensitive response under compression during tactile loading, demonstrating the feasibility of eIPMC sensors. 

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5. Surface roughness effects on ionic polymer-metal composite (IPMC) sensitivity for compression loads

   https://doi.org/10.1117/12.2613127  

   Nagel, William S.; Hussain, Omar A.; Fakharian, Omid; Aureli, Matteo; Leang, Kam K. (January 2022, Electroactive Polymer Actuators and Devices (EAPAD) XXIV)

   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. 

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