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Sensor-laden wearable systems that are capable of providing continuous measurement of key physiological parameters coupled with data storage, drug delivery and feedback therapy have attracted huge interest. Here we report a stretchable wireless system for sweat pH monitoring, which is able to withstand up to 53% uniaxial strain and more than 500 cycles to 30% strain. The stretchability of the pH sensor patch is provided by a pair of serpentine-shaped stretchable interconnects. The pH sensing electrode is made of graphite-polyurethane composite, which is suitable for biosensor application. The sensing patch validated through in-depth electrochemical studies, exhibits a pH sensitivity of 11.13 ± 5.8 mV/pH with a maximum response time of 8 s. Interference study of ions and analyte (Na + , K + and glucose) in test solutions shows negligible influence on the pH sensor performance. The pH data can be wirelessly and continuously transmitted to smartphone through a stretchable radio-frequency-identification antenna, of which the radiating performance is stable under 20% strain, as proved by vector network analyzer measurement. To evaluate the full system, the pH value of a human sweat equivalent solution has been measured and wirelessly transmitted to a custom-developed smart phone App. [ABSTRACT FROM AUTHOR]

A simple soluble salt-driven matrix swelling method was developed for fast fabrication of a silver nanoparticle-based conductive elastomer. Taking advantage of the synergistic effects of silver trifluoroacetate and methanol, the swelling capability of an elastomeric block copolymer polystyrene- block -polyisoprene- block -polystyrene (SIS) was greatly enhanced. The interaction of silver ions and double bonds of the copolymer enables fast infiltration of soluble salt into the polymer network, resulting in a high-content loading of silver ions, which is a prerequisite for fabrication of a highly conductive polymer. In-situ reduction was performed under a swollen state to allow the reducer to effectively penetrate into the polymer matrix, thoroughly reducing the silver salt into silver nanoparticles. A three-layered conductive elastomer was obtained, with a high concentration of silver nanoparticles formed on both surfaces of SIS copolymer. Typically, the conductivity of the reduced SIS elastomer was measured to be > 2 × 10 5 S/m when ε = 0, and can maintain to be over one seventh of its initial value even at a 200% elongation. The conductive elastomer can withstand over 1000 stretch-release cycles under a strain of 100%. A radio-frequency antenna was prepared with the conductive elastomer, which gives the ability of dimension tuning, to realize high-quality frequency-selective radiation at a wide bandwidth of larger than 2.6 GHz. [ABSTRACT FROM AUTHOR]

• 3-D LM composite with high electrical conductivity of 105–106 S/m is fabricated. • Soft antennas with ultimate strain of 300% are achieved based on 3-D LM composite. • The antenna presents a strain-independent reflection coefficient of around −30 dB. • The antenna exhibits stretch-tuned resonant frequency over wide frequencies. • Both softness of LM and protection from stiff layer contribute to the performance. We develop a class of stretchable dipole antennas based on embedding three-dimensional liquid metal network into an elastically soft elastomer as conductive branches, which can be highly stretched up to a strain of 300% while presenting high-quality reflection coefficient around −30 dB and a wide range of tunable resonant frequency from 1.55 to 0.45 GHz simultaneously. Neither mechanical damage nor performance degradation was observed during 100 stretch-release cycles under 100% strain for the antennas. The high electrical conductivity and deformation reversibility of the liquid metal, super stretchability of the soft elastomer and protection from the rigid layer are responsible for such a unique performance, enabling potential application for wireless strain sensors. [ABSTRACT FROM AUTHOR]