Wearable sensor enables continuous and ubiquitous real-time monitoring of the gait pattern of a patient and it eliminates the need of frequent visit to the clinics and the use of costly medical facilities. Attaching gait sensors on body, however, may not be easy as most of the reported gait sensors are made of non-stretchable materials and they are non-conformal to human skin. In this paper, we have presented an in-shoe pressure measurement system that is developed using multiple stretchable microfluidic sensors for monitoring gait parameters ergonomically. The sensor here has employed Eutectic Gallium-Indium (EGaIn) as the conductive liquid to fill up a spiral-patterned microchannel which is encapsulated inside an Ecoflex 00–50 elastomer. Fundamental studies have been performed to investigate the sensor’s behaviours when it is exposed to normal and axial applied forces. As proof-of-concept demonstration of the applicability of the proposed sensor for gait monitoring application, we have measured the localised foot pressure and dynamically by attaching two sensors on a shoe insole for detecting pressure changes around the heel and metatarsal regions. The experiment was then extended to evaluate the strain sensing by measuring the ankle angle. It has been shown that data can be captured wirelessly and displayed on a smartphone using the Bluetooth communication protocol. Owing to its unique and stretchable structure, the sensor is able to distinguish walking and running motions by analysing their stance and swing phases. This work has explored the possibility of applying a microfluidic-based structure for designing a wearable gait sensor that has good flexibility.

For the first time, liquid metal is incorporated with an elastomer platform for designing an EBG-backed wearable antenna that can be placed on human body. Here, the liquid metal (EGaIn) is injected into a soft and stretchable Ecoflex 0030 substrate for making a fluidic slot antenna and a liquid EBG (LEBG) structure. The LEBG is then placed at the back of the slot antenna for mitigating the effects of human body and improving the antenna performance. It has an antenna gain of 5.80 dBi in the boresight and a low specific absorption rate (SAR for 1g US standard) of 1.25 W/kg. Since liquid metal is highly dense, the ground plane of the LEBG is separated into 6 segments, which are connected through narrow bridges. Due to the use of such design, the wobbly effect of the dense liquid metal can be reduced significantly. The measured impedance bandwidth (3.23-3.59 GHz) of the proposed antenna is very stable and not affected much by its own structural deformation on human body. Since the elastomer platform is waterproof, it makes the antenna characteristics very stable in humid environment, which is a very desirable feature for wearable antennas.

For the first time, a deformable liquid metal patch, which is loaded with an air cavity underneath, is proposed for designing an air pressure-detecting antenna. Both the liquid metal (EGaIn) and air cavity of the antenna structure are constructed inside a soft and deformable polydimethylsiloxane (PDMS) substrate. Since liquid metal is highly dense, a PDMS pillar is tactfully embedded into the air cavity region for supporting the liquid patch. Due to the use of such structural support, the liquid metal patch can now be made into one whole piece and it does not cause the antenna structure to collapse. The proposed antenna structure does not require the use of microfluidic channels, which usually compromise the radiation performances, for designing the liquid metal patch. As a result, it is able to attain a high antenna gain of 8.02 dBi. The existence of the air cavity has been proven to be crucial for making the antenna structure deformable so that it is useable for detecting ambient air pressure.

The zeroth-order resonance of a compact patch structure is explored for designing a metal-mountable tag antenna in the US UHF RFID passband. This zeroth-order resonator (ZOR) structure comprises a cap-shaped patch loaded with two L-shaped slots. Also, the patch is coupled to the ground through two inductive stubs which provide enough parasitic reactance to support the zeroth-order resonance. The proposed single-element ZOR can generate sufficient antenna impedance for achieving conjugate match with the RFID chip impedance. Good omnidirectional radiation characteristic with vertically polarized fields can be attained in the azimuth plane when the tag antenna is placed on a metallic surface. The proposed tag antenna’s realized gain at 915 MHz is –1.382 dB (EIRP 4 W). A stable read distance of ~ 15 m is obtainable with even spatial coverage in the entire azimuth plane. The resonance frequency of the proposed tag does not vary much with the object being tagged.

Rapid advances in sensing technologies have brought about the fast development of wearable electronics for biomedical applications. Since its conception, over the years, the ionic polymer metal composite (IPMC) is a new man-made material that has demonstrated its great potential for wearable devices due to self-powered sensing capabilities. Here, for the first time, a novel Kirigami technique with unique cut patterns has been employed for designing a stretchable IPMC sensor with enhanced performance. As Nafion itself exhibits the characteristic of shape memory polymer, the Kirigami structure that is built using the IPMC can be buckled up by loading and heating the IPMC above the deformation temperature, T _def. To further enhance the memory effect, the Kirigami structure has further been locked by immersing it in potassium hydroxide for the formation of deprotonated Nafion. The voltage output of the proposed IPMC with Kirigami shows a superior performance with 3 times improvement over the conventionally planar electrodes. Dynamic tests with a range of displacements have been performed to validate the sensor design and the robustness of the Kirigami structure. This novel Kirigami-based IPMC sensor has been successfully demonstrated for braille sensing by designing 7 independent electrodes.

A frequency-switchable semi-active tag antenna that can perform well on both the on- and in-metal platforms is proposed. A double H-slot antenna has been employed for designing the tag, and it can be easily made using a copper-clad polyimide film. The proposed antenna structure has a backing metallic cavity, which has made the read performance very robust even being used for the in-metal platform. To achieve frequency reconfiguration, the ionic polymer-metal composite (IPMC) actuators are coated with metal, and they are tactfully integrated with a double H-slot antenna for changing its shape. The IPMC flaps can be actuated with a low operating voltage for switching the tag antenna across the US and EU RFID passbands. A water-embedded substrate that consists of a polydimethylsiloxane (PDMS) container filled up with water has also been employed for miniaturizing the antenna. Good maximum read ranges (> 10 m) have been achieved for all cases.