Additive manufacturing techniques are used to create three-dimensional structures with complex shapes and features from polymer and/or metal materials. For example, fused filament three-dimensional (3D) printing utilizes non-electroactive polymers, such as acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), to build structures and components in a layer-by-layer fashion for a wide variety of applications. Presented here is a summary of recent work on a fused filament 3D-printing technique to create 3D ionic polymer-metal composite (IPMC) structures for applications in soft robotics. The 3D printing technique overcomes some of the limitations of existing manufacturing processes for creating IPMCs, such as limited shapes and sizes and time-consuming manufacturing steps. In the process described, first a precursor material (non-acid Nafion precursor resin) is extruded into a thermoplastic filament for 3D printing. Then, a custom-designed 3D printer is described that utilizes the precursor filament to manufacture custom-shaped structures. Finally, the 3D-printed samples are functionalized by hydrolyzing them in an aqueous solution of potassium hydroxide and dimethyl sulfoxide, followed by application of platinum electrodes. Presented are example 3D-printed single and multi-degree-of-freedom IPMC actuators and characterization results, as well as example soft-robotic devices to demonstrate the potential of this process.
A three-dimensional (3D) fused filament additive manufacturing (AM) technique (3D printing) is described for creating ionic polymer-metal composites (IPMC) actuators. The 3D printing technique addresses some of the limitations of existing manufacturing processes for creating IPMCs, which includes limited shapes and sizes and time-consuming steps. In this paper, the 3D printing process is described in detail, where first a precursor material (non-acid Nafion precursor resin) is extruded into a thermoplastic filament for 3D printing. A custom designed 3D printer is described which utilizes the filament to manufacture custom-shaped IPMC actuators. The 3D printed samples are hydrolyzed in an aqueous solution of potassium hydroxide and dimethyl sulfoxide, followed by application of platinum electrodes. The performance of 3D-printed IPMC actuators with different infill patterns are characterized. Specifically, experimental results are presented for electrode resistance, actuation performance, and overall effective actuator stiffness for samples with longitudinal (0 degrees) and transverse (90 degrees) infill pattern.
It is generally understood that increasing the specific surface area of the electrodes of IPMC leads to improved electromechanical performance of the material. Most physics based models compensate the effect of high surface area of the electrodes by increasing both diffusion constant and dielectric permittivity values, while using flat electrode approximation in calculations. Herein, a model was developed to take into account the shape and area of the electrodes. High surface area of the electrodes in the model was achieved by designing 2D polymer-electrode interface as a Koch fractal structure – different generation depths and both unidirectional and random directional generations were studied. The calculations indicate that increasing the generation depth of fractals, thus surface area of the electrodes results in more overall transported charge during the actuation process. Based on the model, the effect of the specific surface area of the electrodes on the electromechanical performance was experimentally investigated. IPMCs with different Pd-Pt electrode structures were prepared and their electromechanical and electrochemical properties were examined and discussed. The methods to manipulate the surface structure of Pd-Pt electrodes were proposed.
Experiments indicate that the electrodes affect the charge dynamics, and therefore actuation of ionic polymermetal
composite (IPMC) via three different types of currents - electric potential induced ionic current, leakage
current, and electrochemical current if approximately higher than 2 V voltage is applied to a typical 200 μm
thick IPMC. The ionic current via charge accumulation near the electrodes is the direct cause of the osmotic
and electrostatic stresses in the polymer and therefore carries the major role in the actuation of IPMC. However,
the leakage and the electrochemical - electrolysis in case of water based IPMCs - currents do not affect the
actuation dynamics as directly but cause potential gradients on the electrodes. These in turn affect the ionic
current. A physics based finite element (FE) model was developed to incorporate the effect of the electrodes and
three different types of currents in the actuation calculations. The Poisson-Nernst-Planck system of equations is
used in the model to describe the ionic current and the Butler-Volmer relation is used to describe the electrolysis
current for different applied voltages and IPMC thicknesses. To validate the model, calculated tip deflection,
applied net current, and potential drop in case of various IPMC thicknesses and applied voltages are compared
to experimental data.
This paper presents a study of IPMCs for twisting motion. To accomplish the twisting electromechanical transduction
of IPMC, patterned electrodes were used. Here we present a three dimensional (3D) finite element (FE)
model based on the fundamental physical principles. Due to very high aspect ratio of the dimensions of IPMC
materials, constructing a full scale 3D model that includes charge transport, continuum mechanics, and electrostatics
equations for the electrodes is challenging. Therefore, a process where some of the data is calculated in
a scaled 2D domain and is later used to calculate the mechanoelectrical transduction in a full scale 3D domain
is presented. The modeling results are compared to experimentally measured data. In the second part of the
paper, the twisting mechanoelectrical transduction study of the IPMCs is introduced. A 3D FE model, again
based on the fundamental physical principles, was developed to estimate the generated charge. In case of the
mechanoelectrical transduction simulations, the full model was calculated in a 3D domain.
The current paper presents the latest advancements in manufacturing, modeling and applications of ionic
polymer-metal composite (IPMC) materials at University of Nevada, Reno. The paper highlights the newest
techniques used in making the novel IPMCs. This includes the dimension control and patterning the electrodes
so that the multiaxial bending of the material can be achieved. The novel concept of strain sensing and also more
energy efficient actuation is discussed. Moreover, we have been working on improving the modeling of IPMC.
The focus has been creating a physical model for design purposes. This has lead to developing a first full scale
3-dimensional model of IPMC material. Additionally, electrode effect has been presented and new techniques
have been explored to take the Finite Element (FE) modeling of IPMC to the next level.
The development of ionic polymer-metal composite (IPMC) actuators with sectored electrodes for bending and
twisting motion is discussed. The application of such IPMCs include highly-dexterous and efficient biorobotic
systems and biomedical devices (e.g. , active catheters). The sectored-electrode pattern on the IPMC actuator
is created using two techniques: masking and surface machining. The bending and twisting performance
of a prototype actuator is evaluated and compared to a finite-element model. Experimental results demonstrate
twisting angle exceeding 7 degrees for an IPMC actuator with four pairs of electrodes having dimensions
50 mm×25 mm×0.23 mm. Technical design challenges and performance limitations are also discussed.
Ionic-polymer metal composites are innovative materials that offer combined sensing and actuating ability in
lightweight and flexible package. As such, they have been exploited in robotics and a wide variety of biomedical
devices, for example, as fins for propelling aquatic robots and as an injector for drug delivery. One of the
main challenges of IPMC-based actuators is precision control of their movements, especially at high operating
speed (frequency) because of dynamic effects. As the frequency increases, the dynamics cause vibration which
leads to significant tracking error. A model-based feedforward controller is applied to control the position of a
custom-made Nafion-based IPMC actuator. The feedforward controller was designed to account for the linear
dynamics, and the feedforward input was computed by considering the magnitude of the input signal and the
tracking precision. To account for unmodeled effects not captured by the linear model, a feedback controller
was integrated with the feedforward controller. The feedback controller provides robustness. Experimental
results show a significant improvement in the tracking performance using feedforward control. In particular, the
feedforward controller resulted in over 75% improvement in the tracking error compared to the case without
dynamic compensation. Then by adding a proportional-integral feedback controller, the tracking error was less
than 10% at 18 Hz scan frequency.
In this paper, we study the no-load behavior of a lightweight piezo-composite curved actuator (LIPCA) subjected
to voltage and charge control. First, we examine the effect of hysteresis and creep when the actuator is voltage
controlled at a slow scan speed. The experimental results show that creep increases the displacement hysteresis by
over 25% when scanning at 1/60 Hz. Afterwards, we discuss the design and implementation of a charge-feedback
circuit to control the displacement of the actuator. The hysteresis curves between voltage- and charge-control
modes are compared for the scan frequencies of 1 and 5 Hz. The results show that charge control (compared to
voltage control) of a LIPCA device exhibits significantly less hysteresis, over 80% less.