We propose a 3D-printable soft, stretchable, and transparent hydrogel-elastomer device that is able to detect simulated ‘nerve’ signals. The signal is passed to a conductive hydrogel electrode through a non-contact method of capacitive coupling through polydimethylsiloxane (PDMS). We demonstrate that the device is able to detect sinusoidal waveforms passed through a simulated ‘nerve’ made from conductive hydrogel over a range of frequencies (1 kHz – 1 MHz). Analysis of signal detection showed a correlation to the electrode contact area and a Vin/Vout of larger than 10%. This provides the framework for the future development of a soft, 3D-printable, capacitive coupling device that can be used as a cuff electrode for detecting peripheral nerve signals.

Soft robots are being developed to mimic the movement of biological organisms and as wearable garments to assist human movement in rehabilitation, training, and tasks encountered in functional daily living. Stretchable artificial muscles are well suited as the active mechanical element in soft wearable robotics, and here the performance of highly stretchable and compliant polymer coil muscles are described and analyzed. The force and displacements generated by a given stimulus are shown to be determined by the external loading conditions and the main material properties of free stroke and stiffness. Spring mechanics and a model based on a single helix are used to evaluate both the coil stiffness and the mechanism of coil actuation. The latter is directly coupled to a torsional actuation in the twisted fiber that forms the coil. The single helix model illustrates how fiber volume changes generate a partial fiber untwist, and spring mechanics shows how this fiber untwist generates large tensile strokes and high gravimetric work outputs in the polymer coil muscles. These analyses highlight possible as yet unexplored means for further enhancing the performance of these systems.

The past few years have seen the introduction of a number of 3D and 4D printingtechniques used to process tough hydrogel materials. The use of‘color’ 3D printing technology where multiple inks areused in the one print allows for the production of composite materials andstructures that can further enhance the mechanical performance of the printedhydrogel. This article reviews a number of 3D and 4D printing techniques forfabricating functional hydrogel based devices.

The mechanical characteristics of ionic-covalent entanglement hydrogels consisting of combinations of the biopolymers gellan gum and kappa-carrageenan, and the synthetic polymers polyacrylamide and an epoxy amine were investigated. Compression testing showed that these gels exhibited “double network” behavior, i.e. strong tough gels.

Carbon nanotubes (CNTs) with macroscopically ordered structures (e.g., aligned or patterned mats, fibers, and sheets) and associated large surface areas have proven promising as new CNT electroactive polymer materials (CNT-EAPs) for the development of advanced chemical and biological sensors. The functionalization of CNTs with many biological species to gain specific surface characteristics and to facilitate electron transfer to and from them for chemical- and bio-sensing applications is an area of intense research activity.

Mechanical actuation generated by CNT-EAPs is another exciting electroactive function provided by these versatile materials. Controlled mechanical deformation for actuation has been demonstrated in CNT mats, fibers, sheets, and individual nanotubes. This article summarizes the current status and technological challenges for the development of electrochemical sensors and electromechanical actuators based on carbon nanotube electroactive materials.

Conducting polymers (CP) are a promising area in the field of micro actuators, and have potential applications in micro robotics. Their properties are modelled as having an electro-chemical active component and a passive viscoelastic component. Methods exist to model the passive component as a configuration of springs and dashpots and the electro-active effects as a strain generator. Typically, the strain is assumed to be proportional to the charge transferred, and the two components are assumed to be independent. We show that there is a significant interaction between the two components for polypyrrole actuators, by observing the dynamic elastic modulus whilst varying the electric potential. The elastic modulus was measured in-situ by applying a high frequency rectangular isotonic stress input and recording the corresponding strain output. Two separate potential control inputs, vs. a reference electrode, were used. In the first experiment, a triangular voltage signal with variable frequency was applied to the PPy helix tube actuator and in the second experiment; a step voltage signal was applied to the actuator. The value of total real modulus was calculated during both experiments to evaluate the effect of actuation on the mechanical properties of PPy actuator. The performance of the mentioned method was confirmed by comparing its results to that of a sinusoidal stress input during a temperature ramp through the glass transition of polyethylene terephthalate (PET). We show that polypyrrole actuators show a complex change in stiffness with contractile state, which mimic skeletal muscle [1].

Reversible actuation strains in excess of 2% in the sheet direction and over 300% in the thickness direction have been produced by single wall carbon nanotube mats when electrochemically charged to +1.5V (vs. SCE) in aqueous sodium chloride solution. The observed strains represent a ten-fold increase over that previously reported for carbon nanotube actuators, and is considerably larger than that achievable with polymer ferroelectric actuators. The enhanced actuator strains result from a new mechanism of electrochemically induced “pnuematic actuation” where high pressure gas forms within the porous structure of the nanotube mat causing partial delamination and swelling. An erasable “memory” effect was also observed for pneumatic actuation driven by hydrogen gas evolution/storage in the nanotube electrodes.