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This paper presents a Chitosan-Graphene Oxide (CS-GO)-based array of ultra-thin biosensors with gold (Au)-based microgap ( $60~\mu \text{m}$ ) electrode. The cross-linked GO is shown to improve the stability of chitosan substrate in aqueous medium and compatibility with microfabrication steps. The sensor patch has been evaluated for label-free monitoring by immobilizing the CS-GO surface with human dermal fibroblast (HDF) cells. The cyclic voltammetry (CV) of HDF cell immobilized CS-GO surface show quasi-reversible nature with a characteristic cathodic peak at +300 mV and anodic peak at −300 mV. Both peaks are stable and repeatable up to 50-scan cycle without any potential shift. The device shows a steady-state peak enhancement (1.923-11.195nA) during the DHF cell growth period (0-96h). The redox peak enhancement correlates with the cell proliferation rates over time, indicating that it could be employed for investigation of the cyto-physiological state against any endo and exogenous stimulation. In addition, the developed sensor-patch was used to detect a wide range of glucose from $1~\mu \text{M}$ to 20mM in vitro with a sensitivity of $0.17~\mu \text{A}$ /mM. Considering these, the presented sensor-patch has a great potential for the detection of glucose level, cell-health proliferation rate at the wound site, and diabetic wound monitoring applications.

Triboelectric nanogenerators (TENGs) are flexible, efficient, and cost-effective energy harvesters. Here, we develop high-performance ferroelectric-assisted TENGs using electrospun fibrous surfaces based on P(VDF-TrFE) with dispersed BaTiO3 (BTO) nanofillers in either cubic (CBTO) or tetragonal (TBTO) form. TENGs with three types of tribo-negative surface (pristine P(VDF-TrFE), P(VDF-TrFE)/CBTO and P(VDF-TrFE)/TBTO) in contact with PET were investigated and output increased progressively from pristine (0.75 W/m2) to CBTO (2 W/m2) and to TBTO (2.75 W/m2). Accounting for contact pressure, the max output (Voc = 315 V & Jsc = 6.7 µA/cm2) is significantly higher than for TENGs having spin-coated P(VDF-TrFE)/BTO. It is hypothesized that electrospinning increases dipole alignment due to high applied voltages, but also aids the formation of a highly oriented crystalline β-phase via uniaxial stretching. Essentially, tribo-charge transfer is boosted due to increased surface potential owing to enhanced ferroelectric polarization. P(VDF-TrFE)/TBTO produced higher output than P(VDF-TrFE)/CBTO even though permittivity is nearly identical. Thus, it is shown that BTO fillers boost output, not just by increasing permittivity, but also by enhancing the crystallinity and amount of the β-phase (as TBTO produced a more crystalline β-phase present in greater amounts). Finally, the ferroelectric-assisted TENG was integrated with a flexible graphene electrode-based supercapacitor to produce a self-charging system capable of charging to 1.25 V in just 5 min. These results demonstrate that this technology can be valuable in wearable applications where higher power output, more efficient charging and flexibility are paramount.

This work presents a screen-printed LC resonant tank-based strain sensor. The resonant tank consists of a planar inductor and an interdigitated capacitor connected in parallel. The inductor was screen printed on flexible polyamide substrate and the capacitor is printed on the piezoelectric Poly-L-lactide (PLLA) nanofibers obtained by electrospinning. The resonant frequency of the tank is tuned for ~13.56 MHz. The inductors and capacitors were characterized using an impedance analyzer to evaluate the frequency characteristic of both. As the interdigitated capacitor is realized on a piezoelectric substrate the permittivity and hence the capacitance varies with the application of dynamic pressure. For the dynamic pressure, less than 2 kPa the sensitivity S 1 is 8.55 kPa-1. The sensitivities in the range of 2-4 kPa-1 and 4-7 kPa-1 are calculated as S 2 = 31.27 kPa-1 and S 3 = 8.61 kPa-1 respectively. The fabricated flexible sensor shows the potential for wearable applications such as sub-bandage pressure monitoring while also exploiting the piezoelectric properties of PLLA to accelerate the wound healing through electrical stimulation.

This work presents a soft capacitive pressure sensor fabricated by sandwiching a porous elastomeric dielectric layer between two electrodes on flexible polyimide (Kapton) substrate. The fabricated sensor was tested over a wide pressure sensing range (0-300 kPa), similar to pressures experienced up to approx. 30m below the sea level. In this range, the sensor presented a near-linear response (94.5%) and a sensitivity of 0.0007 kPa−1. The sensor also featured fast response and recovery times (0.8 s and 1.2 s, respectively), as well as good repeatability and stability - thus proving its applicability for a wide range of pressure sensing applications, particularly underwater robotics for real-time monitoring and surveillance operations.

This work presents a flexible potentiometric pH sensor consisting of a graphite-CuO sensitive electrode and an Ag/AgCl reference electrode. The pH sensor was fabricated by using a simple stencil hand printing technique at room temperature. The results demonstrate that the flexible printed graphite-CuO based pH sensor exhibits a linear response in a wide range of pH 4 to 10.5 with the sensitivity of 22.60 ± 1.3 mV/pH and a response time of 5 s. The developed flexible and miniaturized pH sensor is stable in the water for a long time (more than 1 month) and can be used for online water quality monitoring.

The recent advent of biodegradable materials has offered huge opportunity to transform healthcare technologies by enabling sensors that degrade naturally after use. The implantable electronic systems made from such materials eliminate the need for extraction or reoperation, minimize chronic inflammatory responses, and hence offer attractive propositions for future biomedical technology. The eco-friendly sensor systems developed from degradable materials could also help mitigate some of the major environmental issues by reducing the volume of electronic or medical waste produced and, in turn, the carbon footprint. With this background, herein we present a comprehensive overview of the structural and functional biodegradable materials that have been used for various biodegradable or bioresorbable electronic devices. The discussion focuses on the dissolution rates and degradation mechanisms of materials such as natural and synthetic polymers, organic or inorganic semiconductors, and hydrolyzable metals. The recent trend and examples of biodegradable or bioresorbable materials-based sensors for body monitoring, diagnostic, and medical therapeutic applications are also presented. Lastly, key technological challenges are discussed for clinical application of biodegradable sensors, particularly for implantable devices with wireless data and power transfer. Promising perspectives for the advancement of future generation of biodegradable sensor systems are also presented.

This work presents a biodegradable flexible pressure sensor using a piezoelectric layer sandwiched between Mg electrodes. The active layer of the sensor is a piezoelectric composite of β-glycine amino acid and natural chitosan polymer, fabricated by self-assembly of glycine molecules inside polymer. As a result, a one-step solution casting method have been used to fabricate the biodegradable piezoelectric composite. The x-ray diffraction confirms the structure and crystalline nature of β-glycine crystals inside the chitosan matrix and hence its piezoelectric nature. The sensitivity of the fabricated pressure was about ~ 1.42 mV in the range of 5-25 kPa. The in-vitro degradability of the device has been confirmed in cell culture medium. The fabricated biodegradable sensor offers potential for point-of-care and implantable healthcare applications.

Textile based wearable, biocompatible and low-cost energy storage devices are highly in demand to overcome the issue of powering wearable sensors and electronic devices. In this work, we designed an environmentally friendly textile supercapacitor (SC) which operates with sweat equivalent electrolyte. For investigating the influence of conductivity of the electrodes we present two types of SCs which are based on electrodes of: (1) pure PEDOT: PSS and (2) PEDOT: PSS with DMSO. The increasing conductivity of PEDOT: PSS with DMSO shows significant influence performance enhancement of the SC. The SC exhibited a capacitance of 10 mF.cm−2 for PEDOT: PSS with DMSO and 3.8 mF.cm−2 for PEDOT: PSS in sweat equivalent solution. For a real human sweat the SC exhibited a capacitance of 9 mF.cm-2, thus showing the capability for powering the low-power wearable sensors.

This work presents biocompatible flexible piezoelectric composite fabricated by self-assembly of amino acid glycine molecules inside natural chitosan polymer. Piezoelectric composite film consists of glycine spherulite structure embedded in chitosan matrix. The x-ray diffraction confirms the crystallization of stable $\beta$-glycine inside the composite and hence its piezoelectric nature. As a result, a simple solvent-casting technique have been used to fabricate the biodegradable composite of $\beta$-glycine/chitosan with significant piezoelectric response. We show that the piezoelectric sensor can precisely measure pressure in the range of 0-40 kPa with sensitivity of $\sim 4.7$ mV kPa$^{-1}$. The devices based on bio-based functional material such as glycine offer huge potential for disposable wearable health applications such as monitoring the pressure in a compression bandage.

A sweat-based flexible supercapacitor (SC) for self-powered smart textiles and wearable systems is presented. The developed SC uses sweat as the electrolyte and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the active electrode. With PEDOT:PSS coated onto cellulose/polyester cloth, the SC shows specific capacitance of 8.94 F g-1 (10 mF cm-2 ) at 1 mV s-1 . With artificial sweat, the energy and power densities of the SC are 1.36 Wh kg-1 and 329.70 W kg-1 , respectively for 1.31 V and its specific capacitance is 5.65 F g-1 . With real human sweat the observed energy and power densities are 0.25 Wh kg-1 , and 30.62 W kg-1 , respectively. The SC performance is evaluated with different volumes of sweat (20, 50, and 100 µL), bending radii (10, 15, 20 mm), charging/discharging stability (4000 cycles), and washability. With successful on-body testing, the first demonstration of the suitability of a sweat-based SC for self-powered cloth-based sensors to monitor sweat salinity is presented. With attractive performance and the use of body fluids, the presented approach is a safe and sustainable route to meet the power requirements in wearable systems.

This paper presents flexible pressure sensors based on free-standing and biodegradable glycine-chitosan piezoelectric films. Fabricated by the self-assembly of biological molecules of glycine within a water-based chitosan solution, the piezoelectric films consist of a stable spherulite structure of β-glycine (size varying from a few millimeters to 1 cm) embedded in an amorphous chitosan polymer. The polymorphic phase of glycine crystals in chitosan, evaluated by X-ray diffraction, confirms formation of a pure ferroelectric phase of glycine (β-phase). Our results show that a simple solvent-casting method can be used to prepare a biodegradable β-glycine/chitosan-based piezoelectric film with sensitivity (∼2.82 ± 0.2 mV kPa-1) comparable to those of nondegradable commercial piezoelectric materials. The measured capacitance of the β-glycine/chitosan film is in the range from 0.26 to 0.12 nF at a frequency range from 100 Hz to 1 MHz, and its dielectric constant and loss factor are 7.7 and 0.18, respectively, in the high impedance range under ambient conditions. The results suggest that the glycine-chitosan composite is a promising new biobased piezoelectric material for biodegradable sensors for applications in wearable biomedical diagnostics.

Our skin, and many other tissue types, display electrical and electromechanical properties, which play an important role in tissue regeneration and healing. This work explored a new route for artificial skin with similar properties and presents an opportunity for further enhancing tissue repair. To this end, the electrical properties and piezoelectricity of a collagen-mesh scaffold are investigated. This method has demonstrable success in the rapid fabrication of 3D tissue scaffold equivalents for a skin substitute. As hydration is known to reduce the piezoelectric response of biological materials, as well as alter the conductance and charge storage capabilities, efforts were made to characterize the scaffold in both the dehydrated and hydrated states. The measured capacitance, at 100Hz frequency, was 37nF and 0.45nF for the hydrated and dehydrated scaffold, respectively. The piezoelectric sensitivity of the dehydrated scaffold was ~17 mV N -1 which is large enough to be detected by cells. The results of this study could open a pathway for advancing human-like sense of touch in eSkin with piezoelectric collagen fibrils.

Electromechanical coupling, including piezoelectricity, ferroelectricity, and flexoelectricity, is present in a wide range of organic materials. Such phenomena have been postulated to have a functional role in biological systems, where e.g. conformational changes in proteins can be electrically activated. Investigating the electromechanical properties of biological materials at the micro to nanoscale is therefore crucial for understanding the possible biofunctionality of piezoelectricity and for exploiting such properties in, e.g. sensing, actuating or energy harvesting applications. This chapter provides an overview of the use of piezoresponse force microscopy in the investigation of biomaterials to further our understanding of bioelectromechanics.

Main results of the modeling and computational studies of two the most interesting non-centro-symmetric polymorphic glycine structures: β-glycine (space group P21) and γ-glycine (space group P32), are presented in this work. These structures reveal piezoelectric and polar properties. But the value of polarization is not clear as well it's switching behaviour. In this work computational modelling of both glycine polymorphic crystal structures were performed using combined method with Local Density Approximation (LDA) first principle calculations of atomic optimized crystal structures on Linux cluster and with molecular semi-empirical PM3 calculations by HyperChem 8.0 This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM / PFM).

The results are based on the first principal modeling and calculations for HAP nanostructures as native as well surface modified, charged and having various defects (H and OH vacancies, H inter-nodes). HAP structures were studied using Local Density Approximation (LDA) method with calculations of Density of States (DOS) allow us analyzes the experimental work function data. Molecular modeling by HyperChem is confirmed by photo-electron monochromatic measurements up 6.5 eV and photo-luminescence data from synchrotron DESY experimental data up 30 eV values. Brief analysis of the influence of heating, microwave radiation, hydrogenation, x-rays and synchrotron radiation on Hydroxyapatite (HAP) surface is presented in this work. New data of the structure of hydroxyapatite are obtained.

This work presents an advanced soft robotic finger with integrated strain sensor based on carbon nanotubes (CNTs). The interdigitated strain sensor is obtained by dielectrophoretic assembled CNT network. The sensor is connected to stretchable interconnects to ensure robust electrical connection during movements of the soft finger. The sensor is highly sensitive with up to 1300% change in the resistance for 11% strain. Finally, the CNT strain sensor is integrated with a soft robotic finger to monitor the bending for real time kinesthetic tactile feedback.

The application of controlled mechanical stress on a wound site can accelerate the wound healing by improving the cellular proliferation during tissue regeneration. To facilitate this there is need to monitor the applied stress around the wound site and hence there is need to develop a biocompatible stress sensor. Here we present the glycine-based biocompatible and flexible piezoelectric stress sensor. The sensor is based on novel piezoelectric composite comprising of piezoelectric y-glycine micro-crystals embedded in PDMS. On application of external force the sensor generates an average output voltage of 250 mV, with current density of 0.010 µA/cm2 and a power density of 2.5 nW/cm2. While the produced piezoelectric voltage helps accelerate the wound healing, the energy generation by the sensor could also find potential applications in self-powered touch sensors. The successful demonstration of presented stress sensor, we will pave the way for their integration on compression bandages and dressings used for chronic wound management.