The necessity to continuously and seamlessly monitor human health is calling for compliant, comfortable, and safe wearables. The employment of piezoelectric biopolymers in form of thin film perfectly matches with these needs due to their inherent flexibility, sensitivity, and biocompatibility. Among them, chitosan is a low cost, highly sustainable, and biocompatible material with a great potential for applications in compliant wearables. However, chitosan and biopolymers in general show relatively low piezoelectric coefficients and processing difficulties. Here, it is shown a facile approach to increase more than twice the piezoelectric coefficient of thin chitosan film and to process this promising biomaterial for the fabrication of the first set of thin chitosan film‐based ultrasound transducers. This work leverages the exploitation of environmental‐friendly biopolymers in the development of compliant wearable transducers and thus represents a step forward in the development of completely biodegradable, transparent, thin, and flexible piezoelectric macro‐ and microtransducers.

The majority of wearable and flexible 5G and 6G devices are based on plastic substrates, that are harmful to the environment. Therefore, the development of sustainable and plastic-free radio frequency (RF) devices becomes a crucial issue. In this regard, we present a fully biocompatible Planar Inverted-F Antenna (PIFA), fabricated on a 55 μm-thick chitosan substrate. Chitosan has a relative dielectric constant of 5. This antenna is working at 4.5 GHz in the sub-6 GHz band of the 5G spectrum. It has a very compact footprint of 14 x 23 mm ² and a Specific Absorption Rate (SAR) of 0.41 W/kg. The prototype has been fabricated using an innovative fabrication protocol. A very good agreement between numerical and experimental results has been obtained. The measured realized gain is equal to 1 dBi at the resonant frequency. Our results demonstrate the suitability of chitosan as a dielectric substrate for the fabrication of plastic-free antennas and paves the way for the development of sustainable wearable devices for the Internet of Healthcare Things (IoHT) applications.

Piezoelectric microelectromechanical systems (MEMS) meet the growing demand for sensors with small sizes and low power consumption. For tactile applications and pressure measurements, they are applied in various fields from robotics to healthcare. In this context, flexible devices are very important for their high responsivity and ability to conform to the analyzed surface. This work reports on miniaturized flexible and compliant piezoelectric devices to increase the number of integrable sensors for detecting and discriminating localized pressures and contacts per unit area with minimal crosstalk. For this purpose, a series of Aluminium Nitride-based (AlN) piezoelectric sensors with different diameters (from 5 μm to 500 μm) was realized and the generated electrical signal by sensor deformation was amplified by a differential voltage amplifier circuit. By the analysis of the shape of the piezoelectric signal as a function of the applied pressure, oscillations due to piezoelectric deformations, superimposed on the initial peak signal corresponding to the touch event, have been observed. The integral of the electrical signal was calculated for the most accurate representation to describe the sensor’s response. The best responsivity was obtained in samples with diameters of 200 μm and 500 μm. Furthermore, it was also found that the minimum distance between the edges of closed sensors, observing crosstalk below -20 dB, was around 500 μm.

Arterial pulse waves contain a wealth of parameters indicative of cardiovascular disease. As such, monitoring them continuously and unobtrusively can provide health professionals with a steady stream of cardiovascular health indices, allowing for the development of efficient, individualized treatments and early cardiovascular disease diagnosis solutions. Blood pulsations in superficial arteries cause skin surface deformations, typically undetectable to the human eye; therefore, Microelectromechanical systems (MEMS) can be used to measure these deformations and thus create unobtrusive pulse wave monitoring devices. Miniaturized ultrathin and flexible Aluminium Nitride (AlN) piezoelectric MEMS are highly sensitive to minute mechanical deformations, making them suitable for detecting the skin deformations caused by cardiac events and consequently providing multiple biomarkers useful for monitoring cardiovascular health and assessing cardiovascular disease risk. Conventional wearable continuous pulse wave monitoring solutions are typically large and based on technologies limiting their versatility. Therefore, we propose the adoption of 29.5 μm-thick biocompatible, skin-conforming devices on piezoelectric AlN to create versatile, multipurpose arterial pulse wave monitoring devices. In our initial trials, the devices are placed over arteries along the wrist (radial artery), neck (carotid artery), and suprasternal notch (on the chest wall and close to the ascending aorta). We also leverage the mechano-acoustic properties of the device to detect heart muscle vibrations corresponding to heart sounds S1 and S2 from the suprasternal notch measurement site. Finally, we characterize the piezoelectric device outputs observed with the cardiac cycle events using synchronized electrocardiogram (ECG) reference signals and provide information on heart rate, breathing rate, and heart sounds. The extracted parameters strongly agree with reference values as illustrated by minimum Pearson correlation coefficients (r) of 0.81 for pulse rate and 0.95 for breathing rate.

Fabricating plasmonic nanostructures with good optical performances often requires lengthy and challenging patterning processes that can hardly be transferred to unconventional substrates, such as optical fiber tips or curved surfaces. Here we investigate the use of a single Ga focused ion beam process to fabricate 2D arrays of gold nanoplatelets for nanophotonic applications. While observing that focused ion beam milling of crossing tapered grooves inherently produces gaps below 20 nm, we provide experimental and theoretical evidence for the spectral features of grooves terminating with a sharp air gap. We show that transmission near 10% can be obtained via two-dimensional nano-focusing in a finite subset of 2D arrays of gold nanoplatelets. This enables the application of our nanostructure to detect variations in the refractive index of thin films using either reflected or transmitted light when a small number of elements are engaged.

Innovative biocompatible organic materials with piezoelectric properties have great potential for the development of wearable sensors for monitoring physiological parameters. Among them, Chitosan (CS) is a natural, biodegradable, antibacterial and low cost biopolymer that shows interesting piezoelectric behaviour. In this context, this work reports on a protocol where plain chitosan films (CS-F) are exploited to easily create a flexible, wearable piezoelectric patch. By adapting a previously reported simple drop casting method, we here demonstrate that a 70 μm thick CS-F can exhibit good piezoelectric properties. The structure of CS-F was analysed via the XRD technique: the spectrum reveals peaks of partially crystalline chitosan film, indicating the presence of organized polymeric chains (Suppl. Ppt. Slide 8). Piezoresponse Force Microscopy scans confirmed the presence of domains with opposite polarization directions with an extrapolated value of the piezoelectric coefficient d₃₃ of 2.54 pC/N. A microfabrication process for patch realization has been set up. The top electrode was created by the simple thermal evaporation of gold directly onto the free-standing CS-F (Suppl. Ppt. Slide 10). This bilayer was then precisely cut using a cutting plotter and assembled on the copper bottom electrode (Suppl. Ppt. Slide 11). The complete patch can be conformally applied on the skin. The ability of the device to sense physiological movements was validated by an ad hoc measurement set up generating strain pulses; open circuit voltage peaks up to 20 mV were detected (Suppl. Ppt. Slide 13). This sensor represents an important step towards totally biocompatible and biodegradable wearable devices.