The flexible pressure sensor is one of the essential components of the wearable device, which is a critical solution to the applications of artificial intelligence and human−computer interactions in the future. Due to its simple manufacturing process and measurement methods, research related to piezoresistive mechanical sensors is booming, and those sensors are already widely used in industry. However, existing pressure sensors are almost all based on negative resistance variations, making it difficult to reach a balance between the sensitivity and the detection range. Here, we demonstrated a low-cost flexible pressure sensor with a positive resistance−pressure response based on laser scribing graphene. The sensor can be customized and modulated to achieve both an ultrahigh sensitivity and a broad detection range. Furthermore, the device possesses the signal amplification property like a mechanical triode under the external pressure bias. Based on its amplification ability, varieties of physiological signals and human movements have been detected using our devices; then, an integrated gait monitoring system has been realized. The reported positive graphene pressure sensor has outstanding capability, showing a wide application range such as intelligent perception, an interactive device, and real-time health/motion monitoring.

Strain sensors are devices used in applications such as electronic skin, prosthetic limbs, and e-textile applications, etc., for the purpose of measuring the physical elongation of a desired structure under a given or applied force. An artificial throat, using a strain sensor, was recently developed as an aid for speech impaired individuals. Strain sensors have been developed using graphene and polydimethylsiloxane (PDMS), with a reported gauge factor ranging from (5~120). We have developed a strain sensor through laser scribing. Using laser scribing is a recent and facile technology, used for printed electronics. Complex geometries and patterns can be drawn very easily using this method. The laser scribing method relies on the property of certain materials to form a graphene-like conductive material upon irradiation by lasers. Polyimide and graphene oxide (GO) are two such materials.In these experiments, 2×2 cm sheet of polyimide were taken and printed 1×1 cm box on the sheet using a laser patterning setup of 450 nm wavelength. Graphene oxide solution was drop-casted on the reduced polyimide sheet of 1×1cm, to increase its sensitivity, and then the drop-casted graphene oxide was reduced using the same laser. The strain sensor was characterized by a micro-strain testing machine. The normalized resistance was plotted against strain and the gauge factor was calculated. The effect of the laser intensity was investigated and different gauge factors were calculated by varying the intensity of the laser. The gauge factors were found to be in the range of 49-54 and was compared with the polyimide reduced strain sensor (without drop-casting the GO).

Recently, supercapacitors have attracted a tremendous amount of attention as energy-storage devices due to their high-power density, fast charge–discharge ability, excellent reversibility, and long cycling life. In this research work, we demonstrate a laser scribed super capacitor based on polyimide (PI) substrate for the storage of electrical energy. PI substrate of thickness 200μm and area 1cm × 1cm was reduced by a laser engraver with a 450 nm wavelength in the form of stackable supercapacitor electrodes. Although, PI itself exhibits non-conductive behavior; however, by laser irradiation we change the surface properties of PI and reduce its resistance. The chemical property of irradiated PI was characterized with XRD where the carbon peak was observed at 2*theta = 25.44, which confirms the reduction of PI material in to a graphene-like substance. The electrical conductivity was analyzed with a probe station and observed to be 1.6mS. Two conductive regions were assembled into a capacitor device by sandwiching a PVA/H3PO4 electrolyte in between. During the charging and discharging characterization of the capacitor device, current density was observed to be 1.5mA/cm2. Capacitance versus voltage analysis was carried out and the device showed 75mF/cm2 against a voltage sweep of ±2V. The galvanostatic charging and discharging curve shows a symmetric behavior with respect to time exhibiting the stability and durability of the device.

Skin is one of the most complex structures in the body, with many physiological functions. Skin acts as the barrier or an interface between the external environment and internal organs. Hydration within the skin is varied, known as the skin's water-loading. Perspiration occurs when watery fluid is secreted through the eccrine and apocrine glands. Flexible epidermal sensors are fabricated, which can be used to measure skin hydration and perspiration (sweat) as these sensors need to be skin-conformable. Polyimide (PI) and Polydimethylsiloxane (PDMS) are used as they are flexible and skin compliant, and the sensing layer is formed on them. The sensitivity of hydration sensors was in the range of 0.002-0.0046/%, while for sweat sensors, it was in the range of 0.092μl-1 0.116 μl-1. Stability tests indicated that external factors such as environment or physical deformation and skin curvature do not affect the performance of the as-prepared sensors. The sensitivity and stability results of the planar capacitor are highly suitable for flexible hydration and sweat-sensing applications. The proposed sensors offer an outstandingly good option for incorporation into wearable systems for physical personal health monitoring. In the future, we plan to integrate these sensors on a single substrate to create a multimodal device.