A simple model system towards an impedance-probing strain sensor based on conducting tough hydrogels is demonstrated. A poly(acrylamide) hydrogel, cross-linked with N,N’-methylene-bis(acrylamide) was contacted with carbon fibres for electrical impedance analysis. The conductivity of the salt-containing hydrogel was determined to be 114 ± 10 mS/cm. Upon stretching the hydrogel samples to their fourfold initial length, the impedance response increased according to a power law. This was used to establish a sensing equation for the relation between the resistive component of the impedance signal and the applied mechanical strain under tension. This work contributes to the development of highly stretchable and soft strain sensors for applications in soft robotics.

Plants produce energy in a sustainable way, they are very effective in converting light energy into a useable form. Utilising certain parts of plants in technology could become an efficient way to enhance energy production and improve sustainability. Integrating plants with technology would offer a ‘green’ way of producing elements for electronic circuits and reduce heavy metal waste. In this paper, we demonstrate that conducting polymers can be incorporated into living system such as celery. Electrical impedance analysis was used to establish the conductivity of celery with a conducting polymer (PEDOT:PSS) into its vascular system. It was demonstrated that electronic celery exhibited conductivity values of up to 0.55 ± 0.03 S/cm. This conductivity value was sufficient to demonstrate the potential of celery electronics where celery stalks are used as electrodes in simple circuits.

Edible devices are an emergent technology and in this paper the simplicity and efficacy that poly(acrylic acid)/calcium hydroxide possess in creating a pH sensitive ingestible actuator which responds to acidic environments such as gastric fluid is demonstrated. It was found that poly(acrylic acid)/calcium hydroxide hydrogels exhibit reversible actuation upon submerging in 0.1 M sodium citrate for 2 hours. Our results show that these hydrogels can restore their compressive stress to 0.19 ± 0.06 MPa, swelling ratio to 26 ± 2 and volume to 56% ± 3% of its original volume. This work offers new possibilities for developments in a variety of fields such as drug delivery, 4D printed materials, soft robotics and edible devices.

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.

We demonstrate that Additive Manufacturing (3D printing) is a viable approach to rapidly prototype personalised fins for surfboards. Surfing is an iconic sport that is extremely popular in coastal regions around the world. We use computer aided design and 3D printing of a wide range of composite materials to print fins for surfboards, e.g. ABS, carbon fibre, fibre glass and amorphous thermoplastic poly(etherimide) resins. The mechanical characteristics of our 3D printed fins were found to be comparable to commercial fins. Computational fluid dynamics was employed to calculate longitudinal (drag) and tangential (turning) forces, which are important for surfboard maneuverability, stability and speed. A commercial tracking system was used to evaluate the performance of 3D printed fins under real-world conditions (i.e. surfing waves). These data showed that the surfing performance of surfboards with 3D printed fins is similar to that of surfboards with commercial fins.

We report the use of a novel extrusion tip that allows for the omnidirectional printing of eutectic gallium-indium (eGaIn) alloy onto the surface of hydrogel materials into complex 2-dimensional patterns. The use of these printed soft “wires” as an electrothermal heating element for soft robotics purposes was explored. Heating of the eGaIn structures encapsulated in an alginate/acrylamide ionic-covalent entanglement hydrogel was measured by a thermal imaging camera. It was determined that eGaIn is a suitable material for use in future soft robotics applications as an electrothermal heating element to actuate thermally responsive N-isoproylacrylamide hydrogels.

We report on a hand-held reactive printing device used to pattern highlyconductive, edible hydrogel wires formed from gellan gum, gelatin, cross-linkersand a common salt (NaCl). The conductivity of the gels when printed (190± 20 mS/cm) closely matched the conductivity recorded for cast systems(200 ± 19 mS/cm). Printing was observed to reduce the elastic modulusand failure strains of hydrogels under compression, but printed gels retainedsufficient integrity for application as flexible conductive lines. Wedemonstrate that hand-held printing can utilize to pattern soft conductorelements within a simple electronic circuit.

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 development of highly conductive, robust edible hydrogels was investigated using a combination of the biopolymers gellan gum and gelatin, a common salt (NaCl) and plant-derived cross-linker (genipin). Robust strain gauge/pressure sensors were developed using edible materials to demonstrate the potential of these hydrogels. The hydrogels exhibited gauge factor and pressure sensitivity values of 7.6 ± 0.1 and 400 ± 7 μΩ/Pa for loads up to 3 kPa, respectively. Furthermore, these devices were able to operate under larger loads with gauge factor and pressure sensitivity values of 0.308 ± 0.002 and 7.17 ± 0.05 μΩ/Pa, respectively, for loads between 9 kPa and 280 kPa.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is an organic conducting polymer that has been the focus of significant research over the last decade, in both energy and biological applications. Most commonly, PEDOT is doped by the artificial polymer polystyrene sulfonate due to the excellent electrical characteristics yielded by this pairing. The biopolymer dextran sulphate (DS) has been recently reported as a promising alternative to PEDOT:PSS for biological application, having electrical properties rivaling PEDOT:PSS, complimented by the potential bioactivity of the polysaccharide. In this work we compared chemical and electrochemical polymerisations of PEDOT:DS in terms of their impact on the electrical, morphological and biological properties of the resultant PEDOT:DS films. Post-growth cyclic voltammograms and UV-Vis analyses revealed comparable redox behaviour and absorbance profiles for the two synthesis approaches. Despite good intrinsic conductivity of particles, the addition of chemically produced PEDOT:DS did not markedly enhance the bulk conductivity of aqueous solutions due to the lack of interconnectivity between adjacent PEDOT:DS particles at achievable concentrations. Scanning electron microscopy revealed significantly greater roughness in films cast from chemically produced PEDOT:DS compared to electropolymerised samples, attributable to the formation of solution phase nanoparticles prior to casting. In cell studies with the L929 cell line, electrochemical polymerisation of PEDOT:DS afforded better integrity of resultant films for surface seeding, whilst chemically polymerised PEDOT:DS appeared to localised at the proliferating cells, suggesting possible applications in drug delivery.

The electrical and mechanical characteristics of ionic-covalent entanglement hydrogels consisting of combinations of the edible biopolymers gellan gum and gelatin were investigated. Impedance analysis and compression testing showed that these hydrogels (with water content = 97%) exhibited conductivity values of up to 13 mS/cm and compressive stress at failure values of up to 1.0 MPa. These are suitable characteristics for printed and mechanically robust wet device components.

Emerging applications for hydrogels such as soft robotics and tissue engineering require hydrogels with enhanced mechanical performance. We report the mechanical characteristics of two types of hydrogels: i) ionic-covalent entanglement (ICE) network hydrogels based on calcium cross-linked gellan gum and genipin cross-linked gelatin and ii) ICE microsphere reinforced gelatin hydrogels. This investigation showed that ICE gels can recover up 80% of their mechanical behavior during 5 repeated compressions. In addition, the optimum mechanical performance of gelatin reinforced gels was achieved with inclusion of 40% of ICE microspheres.

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.

Hydrogels consisting of the conducting polymer PEDOT:PSS and the biopolymer gellan gum (GG) were characterized using electrical, mechanical and rheological methods. Compression testing and rheological analysis showed that the gels weakened with increasing PEDOT:PSS content. In contrast, the increasing PEDOT:PSS content resulted in an increasing electrical conductivity.

Inkjet printing of alternate layers of anionic and cationic polyelectrolytes allows organized gels to form with structures similar to those made by layer--by-layer dipping methods but very much faster. Structures of gels formed using slow and fast inkjet printing systems are compared using elemental analysis, swelling and diffusion kinetics as characterization methods. After printing and washing, most sodium or chloride counter-ions are last from the gel, leave only the polymer complex. The swelling properties of the printed and washed gel depend on the deposition rate and on the ratio of the two polymers as originally printed. The synthetic polyelectrolytes reported here can be compared with biological polyelectrolytes reported earlier by us.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

A new design for a field effect transistor able to push back the physical limits of Moore's Law is described. An ab initio computational approach is presented that can be further developed to characterize the ON/OFF states of such a device. Distributed response analysis (M. in het Panhuis, P.L.A. Popelier, R.W. Munn, J.G. Ágyán (2001), J. Chem. Phys. 114, 7951-7961) is employed to investigate conductive behavior. The method demonstrates that in analogy to conduction an electron can move across a possible conjugated molecular switching element (para-nitroaniline) in an electric field.

We have described a novel experimental technique to separate nanotubes from other unwanted carbon species in arc generated carbon soot. A conjugated polymer was used to bind to nanotubes in solution. The resultant hybrid was soluble while extraneous carbon material formed a sediment at the bottom of the sample bottle. This process was monitored using electron paramagnetic resonance (EPR) which showed that 63% of nanotubes were kept in solution while 98.1% of impurities were rejected. Optimal polymer characteristics for nanotube solubility were identified using geometry optimisation and experimental evidence. It was calculated that a successful polymer has a flat shaped helical backbone with solvent solubilising groups projected outwards. This is achieved with the following polymer characteristics, two solvent solubilising groups on a twist allowing π-conjugated backbone.