Conducting Polymer Actuators Research Articles

Tear Fracture Analysis of Fiber-Reinforced Conducting Polymer-based Soft Actuator

Conducting polymer (CP) is an electroactive polymer that displays specific electronic properties, including conductivity. The utility of CP-based soft actuators in various biomedical applications has recently been motivated due to their low voltage-driven specialty compared to the widely used high voltage-driven dielectric elastomers. In some biomedical applications, highly delicate CP actuators may be torn or damaged for unknown reasons. In this regard, this study develops a tear fracture model for fiber-reinforced CP actuators to investigate a specific fracture test of mode III, namely, the trousers test, which involves pulling two legs of a cut specimen horizontally apart. The development of the tear fracture model adopts a well-known Griffith criterion along with the thermodynamically consistent continuum mechanics approach. Additionally, prominent strain energy capturing elastomer strain-stiffening at a moderate strain range is used in conjunction with an empirically established correlation to couple the two internal phenomena, ion diffusion and mechanical deformation of the CP actuators. Later, the effects of various electrical and geometrical parameters on the tearing of the actuator are also addressed.

Subsurface Profiling of Ion Migration and Swelling in Conducting Polymer Actuators with Modulated Electrochemical Atomic Force Microscopy.

Understanding the dynamics of ion migration and volume change is crucial to studying the functionality and long-term stability of soft polymeric materials operating at liquid interfaces, but the subsurface characterization of swelling processes in these systems remains elusive. In this work, we address the issue using modulated electrochemical atomic force microscopy as a depth-sensitive technique to study electroswelling effects in the high-performance actuator material polypyrrole doped with dodecylbenzenesulfonate (Ppy:DBS). We perform multidimensional measurements combining local electroswelling and electrochemical impedance spectroscopies on microstructured Ppy:DBS actuators. We interpret charge accumulation in the polymeric matrix with a quantitative model, giving access to both the spatiotemporal dynamics of ion migration and the distribution of electroswelling in the electroactive polymer layer. The findings demonstrate a nonuniform distribution of the effective ionic volume in the Ppy:DBS layer depending on the film morphology and redox state. Our findings indicate that the highly efficient actuation performance of Ppy:DBS is caused by rearrangements of the polymer microstructure induced by charge accumulation in the soft polymeric matrix, increasing the effective ionic volume in the bulk of the electroactive film for up to two times the value measured in free water.

Analyzing the structural behavior of conducting polymer actuators and its interdependence with the electrochemical phenomenon

This work reports the deformation behavior of a conducting polymer, poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)/bacterial cellulose (BC) bi-layered cantilever type actuator. Herein, it was found that the type (i.e. bending and torsion) of deformation of (PEDOT:PSS)/BC actuator was non-trivially dependent on its dimensions (width and length). Increasing the actuator’s width resulted in larger torsional deformation along the longitudinal axis against the increased area moment of inertia. The actuator with a width of 7.75 mm rotates ∼90° (i.e. the bottom cross-section) with respect to its top end. It was noticed that torsional motion dominated the deformation when the bending in the lateral direction was restricted. Further, the maximum tip displacement trivially increased with the length from 5.40 mm for an actuator of length 10 mm–12.40 mm for a length of 59.00 mm. However, the curvature of bending, which was proportional to the induced strain, was higher for smaller lengths. The change in the dimension of the actuator involves change in the stress field distribution (i.e. induced through electrochemical process) and simultaneously the resistance to deformation, resulting in a non-trivial relationship between the deformation and the dimensions. This can be advantageous from the design perspective in realizing different types of motions without incorporating additional materials. Structural theory and electrochemical impedance Spectroscopy were used to understand the mechanism of deformation dependence on the dimensions. The electrochemical impedance spectroscopy results indicated that electrolytic ions penetrate deeper into the PEDOT:PSS layer for actuators of smaller lengths. The increase in the curvature of the actuator could be explained based on the constancy of the strain produced due to the volume change per ion. The torsional motion increased because the stresses were being induced further away from the center in wider actuators. These observations and analyses reveal the interdependence of the structural behavior (i.e. dimensions) and the electrochemical phenomenon (i.e. deformation) in a conducting polymer actuator.

Modeling of a fiber-reinforced tubular conducting polymer actuator

This study presents the analytical modeling of a fiber-reinforced tubular conducting polymer (FTCP) actuator. The FTCP actuator is a low voltage-driven electroactive polymer arranged in an electrochemical cell. The electrochemical model is developed following an electrical circuit analogy that predicts the charge diffused inside the actuator for an applied voltage. An empirical relation is applied to couple the two internal phenomena, viz., diffusion of the ions and mechanical deformation. Further, the finite deformation theory is applied to predict the blocked force and free strain of the FTCP actuator. The developed model is consistent with existing experimental results for an applied voltage. In addition, the effect of various electrical and geometrical parameters on the performance of the actuator is addressed. The model predicts an axial elongation or contraction for different fiber properties at same input voltage.

(Invited) Breath and Skin Gas Sensing for Rapid POC Medical Diagnostics

Antiquity’s first known physician, Hippocrates of Cos (460–370 BC), smelled his patients’ breath to diagnose disease and recommend the appropriate remedy. Some medical terms coined then survive today, such as “fetor hepaticus,” which describes the sweet, ketone- and ammonia- rich scent that indicates the late stages of liver failure. Yet, neither breath nor skin analysis, as a noninvasive means of disease detection, are common practice among physicians to-date nor are they available to the general population. Ceramic sensor nanotechnology and nanomedicine are, however, capable of making breath-based diagnostics. VOCs in breath and skin are products of core metabolic processes, while inorganic molecules are related to other health conditions and can be indicators of a potential disease, recent exposure to a drug or an environmental pollutant. Therefore, an abnormally high or low measured concentration of certain breath or skin gases, so-called biomarkers, could potentially provide clues for diagnosing corresponding diseases. This work focuses on electronic olfaction systems which we have developed and used in breath-based testing of Volatile Organic Compound (VOC) biomarkers with great success. It also addresses the concept of skin-gas “smelling” of disease. These gas sensing nanotechnologies allow for the rapid and reliable output signals which indicate the host’s response to infection or metabolic changes. Using the library of specific prints/unique patterns for each of the targeted diseases, the following types of diseases could be potentially diagnosed: Metabolic diseases: diabetes, cholesterol-induced heart disease; Neurological diseases: Alzheimers, Parkinson’s; Chronic diseases: obesity, sleep apnea; Pulmonary diseases: CF, asthma, COPD; GI tract diseases: IBS, colitis; Cancers: breast, lung, pancreatic, colon, and Infectious diseases: flu, COVID-19; as well as diseases commonly found in ICU patients: Urinary tract infections, pneumonia, infections of the blood stream. Focusing on the most common gaseous biomarkers in breath and skin, that is Nitric Oxide and Carbon Monoxide and VOCs (acetone, isoprene, ammonia, alcohols, sulfides) effective discrimination between the diseases mentioned above is possible, by capturing the relative sensor output signals from the detection of each of these biomarkers and analyzing them to reveal the distinct breath print for each disease. We focus here on an electronic olfaction device based on resistive gas sensing utilizing a small number, just four at a time, of metal-oxide sensors that are specifically sensitive to the biomarkers of interest.A single-scan skin testing device comprises an array of gas sensors, each of which satisfies the following stringent conditions: high sensitivity to the target gas, high selectivity, stable response over extended periods of time and rapid response. The concentration of a target gas over a selective sensor is quantified by measurement of the sensor resistance. We also present a novel wearable device interfacing with a smart phone for disease detection and monitoring. This device utilizes conducting polymer actuators, since conductive polymers exhibit a low operating potential (typically <2V), and a high mechanical strength. Quantitative medical diagnostics require simultaneous monitoring of multiple gases because markers are affected differently in different diseases and this device allows for continuous measurement of acetone for diabetes monitoring.

Modeling and Control of Conducting Polymer Actuator

Conducting polymer (CP) actuator has nonlinear dynamic characteristics during its charge process. In this study, we proposed an electromechanic model and an optimal controller for a type of ionic electroactive polymer (IEPA) actuator with submillimeter scale, which can produce large deformation under low actuation voltage. The electronic model is to describe the evolution of charge state in time domain. The mechanic model is to calculate the deformation of CP actuator under the actuation force and external force. Based on the electromechanic coupling model, a parameter identification method is proposed to estimate the nonlinear parameter of CP actuator. The experiments show that our electromechanic model successfully predicts the deformation of actuator under different input voltages with the identified parameters. In the last step, an optimal controller is designed to control the orientation of IEAP actuator, which achieves at a high control performance in our experiments. The success of the modeling and control lays the foundation work for the subsequent biomedical applications.

Wireless electromechanical enantio-responsive valves.

Microfluidic valves based on chemically responsive materials have gained considerable attention in recent years. Herein, a wireless enantio-responsive valve triggered by bipolar electrochemistry combined with chiral recognition is reported. A conducting polymer actuator functionalized with the enantiomers of an inherently chiral oligomer was used as bipolar valve to cover a tube loaded with a dye and immersed in a solution containing chiral analytes. When an electric field is applied, the designed actuator shows a reversible cantilever-type deflection, allowing the release of the dye from the reservoir. The tube can be opened and closed by simply switching the polarity of the system. Qualitative results show the successful release of the colorant, driven by chirality and redox reactions occurring at the bipolar valve. The device works well even in the presence of chemically different chiral analytes in the same solution. These systems open up new possibilities in the field of microfluidics, including also controlled drug delivery applications.

Micropipette‐Based Fabrication of Free‐Standing, Conducting Polymer Bilayer Actuators

AbstractElectrically driven actuators have found widespread applications in biomimetics and soft robotics. Among different actuation materials, conducting polymers (CPs) have stood out due to their unique doping‐based actuation mechanism. Fabricating actuators at the microscale is particularly important, not only in the manufacturing of delicate biomimetic/robotic devices but also in advanced microphysiological studies. However, the choice for microfabrication techniques is limited, with the reported CP microactuators being mainly planar. To overcome this issue, a new micropipette‐based method is developed for the fabrication of free‐standing 3D CP actuators. The two‐layer actuator consists of a layer of PPy:CF3SO3, fabricated by localized electropolymerization, and a layer of PEDOT:PSS, fabricated by a “direct writing” technique. Due to the opposite contraction and expansion behavior of these two CPs, determined by the size of dopants, the electrically driven bending actuators have been demonstrated. This fabrication approach provides unprecedented capability for fabricating high aspect ratio microactuators: the 360° bending orientation of the actuators can be controlled by the relative position of the two layers. As a proof‐of‐principle, we demonstrate CP “micro‐tweezers” and the manipulation of a PDMS microsphere. The technique developed in this work opens exciting opportunities to manufacture advanced artificial muscles and sophisticated soft microrobotics.

A fast-response electroactive actuator based on TEMPO-oxidized cellulose nanofibers

Cellulose-based electroactive actuators are promising candidates for biomimetic robots and biomedical applications due to their lightweight, high mechanical strength, and natural abundance. However, cellulose-based electroactive actuators exhibit lower actuation performance than traditional conductive polymer actuators. This work reports a fast-response cellulose-based electroactive actuator based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized nanocellulose (TOCNF) film with layered structure fabricated by evaporation, and gold electrodes prepared by ion sputtering. The residual ions during the TEMPO oxidation process and the layered structure due to self-assembly accelerate the ion migration efficiency in actuators. The proposed actuator can reach a tip displacement of 32.1 mm at a voltage of 10 V and deflect 60° in 5 s. After applying a reverse 10 V voltage, the actuator can also be quickly deflected (42.5 mm). In addition, the actuator also shows high electrical actuation performance at low voltage (5 V). The excellent electroactive performance of as-prepared TOCNF/Au enables the feasibility to be applied to actuators.

Ion Mobility in Thick and Thin Poly-3,4 Ethylenedioxythiophene Films-From EQCM to Actuation.

Conductive polymer actuators and sensors rely on controlled ion transport coupled to a potential/charge change. In order to understand and control such devices, it is of paramount importance to understand the factors that determine ion flux at various conditions, including the synthesis potential. In this work, the ion transport in thinner poly-3,4-ethylenedioxythiophene (PEDOT) films during charge/discharge driven by cyclic voltammetry is studied by consideration of the electrochemical quartz crystal microbalance (EQCM) and the results are compared to the actuation responses of thicker films that have been synthesized with the same conditions in the bending and linear expansion modes. The effects of polymerization potentials of 1.0 V, 1.2 V, and 1.5 V are studied to elucidate how polymerization potential contributes to actuation, as well the involvement of the EQCM. In this work, it is revealed that there is a shift from anion-dominated to mixed to cation-dominated activity with increased synthesis potential. Scanning electron microscopy shows a decrease in porosity for the PEDOT structure with increasing synthesis potential. EQCM analysis of processes taking place at various potentials allows the determination of appropriate potential windows for increased control over devices.

Electro-interpenetration as tool for high strain trilayer conducting polymer actuator

Electrochemical actuators based on electronic conducting polymers can convert electrical energy into mechanical work. The amplitude of the electromechanical response is related on volumetric charge density of conducting polymer electrodes and strain-to-charge ratio, an empiric electromechanical coupling parameter. In this work we describe the elaboration of high performance trilayer bending actuators by the electro-interpenetration of a conducting polymer, i.e. poly(3,4-ethylenedioxythiophene) (PEDOT), within a host membrane. The host membrane is first elaborated as an interpenetrating polymer networks combining nitrile butadiene rubber network and poly(ethylene oxide) network. An oxidative chemical prepolymerization of EDOT allows turning the host membrane into conductive substrate suitable for subsequent electropolymerization. The electropolymerization of EDOT from the chemically polymerized layer allows controlled interpenetration of PEDOT underneath the surface. A significant increase of local concentration of PEDOT in the electrodes, compared to those obtained from oxidative polymerization lead to a significant increase of the charge density. The polymerization conditions do not have significant impact on sign and absolute value of strain-to-charge ratio, but it can be improved by the nature of the electrolyte. The combination of these two effects leads to an actuator with up to 15% of strain difference.

Shape‐Controlled Flexible Microelectronics Facilitated by Integrated Sensors and Conductive Polymer Actuators

The next generation of biomedical tools requires reshapeable electronics to closely interface with biological tissues. This will offer unique mechanical properties and the ability to conform to irregular geometries while being robust and lightweight. Such devices can be achieved with soft materials and thin‐film structures that are able to reshape on demand. However, reshaping at the submillimeter scale remains a challenging task. Herein, shape‐controlled microscale devices are demonstrated that integrate electronic sensors and electroactive polymer actuators. The fast and biocompatible actuators are capable of actively reshaping the device into flat or curved geometries. The curvature and position of the devices are monitored with strain or magnetic sensors. The sensor signals are used in a closed feedback loop to control the actuators. The devices are wafer‐scale microfabricated resulting in multiple functional units capable of grasping, holding, and releasing biological tissues, as demonstrated with a neuronal bundle.

Designing tubular conducting polymer actuators for wireless electropumping.

Rational design and shaping of soft smart materials offer potential applications that cannot be addressed with rigid systems. In particular, electroresponsive elastic materials are well-suited for developing original active devices, such as pumps and actuators. However, applying the electric stimulus requires usually a physical connection between the active part and a power supply. Here we report about the design of an electromechanical system based on conducting polymers, enabling the actuation of a wireless microfluidic pump. Using the electric field-induced asymmetric polarization of miniaturized polypyrrole tubes, it is possible to trigger simultaneously site-specific chemical reactions, leading to shrinking and swelling in aqueous solution without any physical connection to a power source. The complementary electrochemical reactions occurring at the opposite extremities of the tube result in a differential change of its diameter. In turn, this electromechanical deformation allows inducing highly controlled fluid dynamics. The performance of such a remotely triggered electrochemically active soft pump can be fine-tuned by optimizing the wall thickness, length and inner diameter of the material. The efficient and fast actuation of the polymer pump opens up new opportunities for actuators in the field of fluidic or microfluidic devices, such as controlled drug release, artificial organs and bioinspired actuators.

Consistent response from conducting polymer actuators: Potential window and embedded charges to avoid mixed ion transport

A soft ionic electroactive polymer actuator does not differ from other types of actuators in the requirement of reliable, controllable and consistent performance. In ionic systems, like those of conducting polymers, the direction of actuation (as well as - naturally - the amplitude) depends on the dominance of one type of ions (cations over anions or vice versa) as the mobile species, maintaining charge neutrality. Here we demonstrate that the same polypyrrole (PPy) films can have rather different response in terms of linear actuation amplitude as well as direction under different voltage ranges. A system designed to mitigate such uncertainties – PPy blended with polymerized ionic liquid – is shown as a comparison. Three reversible and charge-balanced potential ranges in cyclic voltammetry and square wave voltage steps experiments were chosen to demonstrate the importance of selecting the right voltage range, especially if no embedded charges acting as “filters” are being used.

Soft parallel manipulator fabricated by additive manufacturing

Conducting polymer (CP) based soft actuators are good candidates for miniaturised manipulation systems and soft robotic applications. We present the fabrication, characterization, and modelling, of a novel ionically driven soft, flat, parallel manipulator with minimal footprint actuated by CP actuators. This three degrees of freedom (3Dof) manipulator consists of four trilayer actuators with poly (3, 4- ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) electrodes on both sides of PVDF separator membrane with 1-Ethyl-3-methylimidazoliummethyl imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid used as the electrolyte. The complete manipulator is fabricated as a monolithic structure using commercially available off the shelf materials by additive manufacturing technique including a syringe type printer. Its workspace and dynamics are characterised and the results are compared with a multiphysics model based on the finite element method. The model uses two types of charge storage mechanism namely electrical double layer and redox reactions to describe the electrode kinetics. Through simulation the charge contributed by each of the processes is separated and presented providing new insights in the underlying kinetics in this type of actuators. It is found the double layer charge is the dominant phenomenon driving these actuators compared to the redox process. Finally, to demonstrate the versatile applications, the manipulator is explored for a four-way laser steering application. This work has demonstrated high levels of manipulability along three degrees of freedom from the printed CP actuators that are outstanding within the class of soft ionic actuators while using off the shelf commercially available materials keeping the fabrication method simple, scalable and cost-effective along with the electro-chemo-mechanical model providing an insightful view of the charge storage mechanism.

A square tubular conducting polymer actuator for smart catheter application

Flexible micro-catheter for minimally invasive medical diagnosis and therapy is highly desirable, but still a challenge. Here, an active interventional micro-catheter based on square tubular conducting polymer actuator is developed. This actuator is composed of two conducting polymer composite electrodes and a square tubular gel polymer electrolyte layer between the electrodes layer. To fabricate the square tubular gel polymer electrolyte layer, a simple, solution-based, gradual phase inversion technique was used. A high ionic conductivity and low tensile modulus Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) square tube that could act as the actuator body and electrolyte layer to allow the actuator operate without the need of external ions was fabricated. Also, since the electrodes are supposed to be largely deformed under low voltage, which has great significance for the safe application of the catheter for the human body, conducting polymers with good electrical and mechanical properties are great choice for the catheter. Therefore, we developed a PEDOT:PSS/carboxylic SWCNT (SWCNT-COOH)/ionic liquid (IL) composite electrode film. With the addition of SWCNT-COOH and IL, the conductivity reached more than ten times higher than that of pristine PEDOT:PSS and the specific capacitance was three times higher than that of PEDOT:PSS film. Additionally, the stretchability and flexibility of the electrode film were highly enhanced because of the doping of IL. Due to the high electrical conductivity of composite electrode and low tensile modulus of actuator body, the obtained square tubular actuator can bend in two dimensions under a low voltage (∼1 V) in open air. A simulated vessel model was constructed and the square tubular actuator succeeded in real-time active bending and guiding, which will have broad application prospects in the interventional medicine field.

Conducting Polymer Microtubes for Bioactuators.

Conducting polymer (CP) actuators are promising devices for biomedical applications such as artificial muscles and drug delivery systems. Here, we report a tri-layer actuator based on poly(pyrrole) (PPy) microtubes (PPy MTs) doped with poly(sodium-p-styrenesulfonate) (PSS) and constructed on a passive layer of gold-coated poly-propylene (PP) film. The PPy MTs were fabricated using electrochemical deposition of PPy around poly(lactic-co-glycolic acid) (PLGA) fiber templates, followed by template removal. The PPy MTs were subjected to a redox process using cyclic voltammetry in 0.1 M NaPSS electrolyte solution as the potential was swept between -0.8 V and +0.4 V for 5 cycles at the scan rates of 10, 50, 100, and 200 mV/s. The bending behavior of the PPy MTs actuator was investigated by measuring the deflection of actuator tip resulting from the expansion/contraction strain of PPy MTs. The PPy MTs actuator showed a reversible bending movement during each potential cycle. The maximum deflection of actuator decreased by increasing the scan rate that was confirmed by calculating the actuation strain generated during each cycle at various scan rates.

Intrinsically passivated polypyrrole/polyol-borate soft electroactuators

Electroactive conducting polymer films have attracted widespread interests as artificial muscles in soft robots and biomedical devices. The predominant bi- and multi-layered conducting polymer actuators often include an inert base to prevent strain offset between two opposite sides of an electroactive film, but the rigidity of the base adds an extra load to the actuator thus compromising its performance. This study introduces an intrinsically single-side-passivated polypyrrole/polyol-borate composite film capable of generating large and endurable deformations in saline. We utilize a mechanism of insufficient polymerization to achieve a monolithic structure comprising an electroactive side and an intrinsically passivated opposite side. With minimum strain offset, the resulting actuator demonstrates larger bending strain and higher electrochemical stability than unpassivated samples and conventional polypyrrole/dodecylbenzenesulfonate bilayers. This type of biocompatible and mechanically robust electroactuator may serve as a promising functional material for biomedical devices targeting for long-term in-vivo operation.

Experimental Studies and Numerical Simulation of Polypyrrole Trilayer Actuators.

Conducting polymer actuators have shown wide application prospects in the field of biomedical sensors and micro-/nanorobotics. In order to explore more applications in biomedical sensing and robotics, it is essential to understand the actuator static behavior from an engineering perspective, before incorporating them into a design. In this article, we have established the mathematical model of a trilayer polypyrrole (PPy) cantilever actuator and validated it experimentally. The model helps in enhancing the efficiency and in improving the performance, predictability, and control of the actuator. The thermal expansion analogy, which is similar to volume change of the multilayer PPy actuator due to ion migration, has been considered to develop a mathematical model in COMSOL Multiphysics. To further validate the actuator deformation predicted by the mathematical modeling, a multilayer PPy actuator was fabricated by electrochemical synthesis and the experimentally determined deflection of the actuator was compared to simulation data. Both the theoretical and experimental results depict that the model is effective for predicting the bending behavior of multilayer PPy actuators at different input voltages.

Cover Feature: Wireless Coupling of Conducting Polymer Actuators with Light Emission (ChemPhysChem 7/2019)

Cover Feature: Wireless Coupling of Conducting Polymer Actuators with Light Emission (ChemPhysChem 7/2019)