Bingwei Lu

Breathable and Stretchable Temperature Sensors Inspired by Skin

Flexible electronics attached to skin for healthcare, such as epidermal electronics, has to struggle with biocompatibility and adapt to specified environment of skin with respect to breath and perspiration. Here, we report a strategy for biocompatible flexible temperature sensors, inspired by skin, possessing the excellent permeability of air and high quality of water-proof by using semipermeable film with porous structures as substrate. We attach such temperature sensors to underarm and forearm to measure the axillary temperature and body surface temperature respectively. The volunteer wears such sensors for 24 hours with two times of shower and the in vitro test shows no sign of maceration or stimulation to the skin. Especially, precise temperature changes on skin surface caused by flowing air and water dropping are also measured to validate the accuracy and dynamical response. The results show that the biocompatible temperature sensor is soft and breathable on the human skin and has the excellent accuracy compared to mercury thermometer. This demonstrates the possibility and feasibility of fully using the sensors in long term body temperature sensing for medical use as well as sensing function of artificial skin for robots or prosthesis.

Ultra-flexible Piezoelectric Devices Integrated with Heart to Harvest the Biomechanical Energy.

Power supply for medical implantable devices (i.e. pacemaker) always challenges not only the surgery but also the battery technology. Here, we report a strategy for energy harvesting from the heart motion by using ultra-flexible piezoelectric device based on lead zirconate titanate (PZT) ceramics that has most excellent piezoelectricity in commercial materials, without any burden or damage to hearts. Experimental swine are selected for in vivo test with different settings, i.e. opened chest, close chest and awake from anesthesia, to simulate the scenario of application in body due to their hearts similar to human. The results show the peak-to-peak voltage can reach as high as 3 V when the ultra-flexible piezoelectric device is fixed from left ventricular apex to right ventricle. This demonstrates the possibility and feasibility of fully using the biomechanical energy from heart motion in human body for sustainably driving implantable devices.

Epidermal Inorganic Optoelectronics for Blood Oxygen Measurement

Flexible and stretchable optoelectronics, built-in inorganic semiconductor materials, offer a wide range of unprecedented opportunities and will redefine the conventional rigid optoelectronics in biological application and medical measurement. However, a significant bottleneck lies in the brittleness nature of rigid semiconductor materials and the performances extreme sensitivity to the light intensity variation due to human skin deformation while measuring physical parameters. In this study, the authors demonstrate a systematic strategy to design an epidermal inorganic optoelectronic device by using specific strain-isolation design, nanodiamond thinning, and hybrid transfer printing. The authors propose all-in-one suspension structure to achieve the stretchability and conformability for surrounding environment, and they propose a two-step transfer printing method for hybrid integrating III-V group emitting elements, Si-based photodetector, and interconnects. Owing to the excellent flexibility and stretchability, such device is totally conformal to skin and keeps the constant light transmission between emitting element and photodetector as well as the signal stability due to skin deformation. This method opens a route for traditional inorganic optoelectronics to achieve flexibility and stretchability and improve the performance of optoelectronics for biomedical application.

Biocompatible and Ultra-Flexible Inorganic Strain Sensors Attached to Skin for Long-Term Vital Signs Monitoring

Wearable electronics have attracted much attention and are experiencing rapid growth in recent years. Such devices are expected to stay closer to the human body (i.e., attached to the skin) for better performance. Therefore, ultra-flexibility of such devices is necessary in order to make the sensor conform to the human body when the devices are used for healthcare monitoring. Here, we present a biocompatible and ultra-flexible strain sensor for pulse and body motion real-time and long-term measurement. The sensor, fabricated and integrated on a semi-permeable substrate with good biocompatibility and waterproofness, is mechanically invisible for the human. It owns good linearity (r2 = 0.997), good repeatability, low resistance (350 Ω), and short response time (less than 100 ms). The sensor is designed with the shear lag theory, obtaining greater measuring range but still with good linearity. The liquid transfer printing method is used for thin-film sensing part and soft substrate integration in order to avoid damage. The sensor shows better performance and higher precision in motion and pulse monitoring than other similar sensors. The in vitro experiments demonstrate that the sensor is more suitable for long-term health monitoring at medical grade.

Skin-like biosensor system via electrochemical channels for noninvasive blood glucose monitoring

Electrochemical twin channels make glucose in vessels measurable by noninvasive ultrathin skin-like highly sensitive biosensors. Currently, noninvasive glucose monitoring is not widely appreciated because of its uncertain measurement accuracy, weak blood glucose correlation, and inability to detect hyperglycemia/hypoglycemia during sleep. We present a strategy to design and fabricate a skin-like biosensor system for noninvasive, in situ, and highly accurate intravascular blood glucose monitoring. The system integrates an ultrathin skin-like biosensor with paper battery–powered electrochemical twin channels (ETCs). The designed subcutaneous ETCs drive intravascular blood glucose out of the vessel and transport it to the skin surface. The ultrathin (~3 μm) nanostructured biosensor, with high sensitivity (130.4 μA/mM), fully absorbs and measures the glucose, owing to its extreme conformability. We conducted in vivo human clinical trials. The noninvasive measurement results for intravascular blood glucose showed a high correlation (>0.9) with clinically measured blood glucose levels. The system opens up new prospects for clinical-grade noninvasive continuous glucose monitoring.

Interfacial Failure in Flexible Electronic Devices

Flexible inorganic electronics with the delicate integration of stiff electronic films on soft substrate have attracted much attention recently. For the large mismatch of mechanical properties between the soft substrate and stiff electronic films in such devices, interfacial failures are always extremely dangerous. In this letter, we present theoretical analysis for the interfacial failures based on fracture mechanics and establish the criteria for the failure modes. It is found that the thin films always slip first and then transit to delamination from the substrate as the increasing of applied loading. The theoretical prediction is consistent with the experiment, which can guide the design and evaluate the reliability of flexible electronics.

Direct Laser Writing-Based Programmable Transfer Printing via Bioinspired Shape Memory Reversible Adhesive

Flexible and stretchable electronics offer a wide range of unprecedented opportunities beyond conventional rigid electronics. Despite their vast promise, a significant bottleneck lies in the availability of a transfer printing technique to manufacture such devices in a highly controllable and scalable manner. Current technologies usually rely on manual stick-and-place and do not offer feasible mechanisms for precise and quantitative process control, especially when scalability is taken into account. Here, we demonstrate a spatioselective and programmable transfer strategy to print electronic microelements onto a soft substrate. The method takes advantage of automated direct laser writing to trigger localized heating of a micropatterned shape memory polymer adhesive stamp, allowing highly controlled and spatioselective switching of the interfacial adhesion. This, coupled to the proper tuning of the stamp properties, enables printing with perfect yield. The wide range adhesion switchability further allows printing of hybrid electronic elements, which is otherwise challenging given the complex interfacial manipulation involved. Our temperature-controlled transfer printing technique shows its critical importance and obvious advantages in the potential scale-up of device manufacturing. Our strategy opens a route to manufacturing flexible electronics with exceptional versatility and potential scalability.

Wrinkles formation and evolution of nanoribbons with finite length on elastomeric substrate

We report in situ observation of wrinkles formation and evolution of Si nanoribbons with finite length on elastomeric substrate via white light interferometer. The wrinkle originates from the middle of the nanoribbon, propagates symmetrically to the two ends, and finally reaches the stable configuration. The wavelength and amplitude will increase abruptly when the released strain exceeds the critical value. The interface interaction between Si nanoribbons and elastomeric substrate plays the key role for wrinkles formation.

Ultra-thin and ultra-flexible temperature/strain sensor with CNT nanostrips

Flexible electronic devices have attracted much attention nowadays. Flexible sensors are required to be thin and flexible in order to better attach to the human being for health care. However, connecting parts of many flexible electronic sensors and the outer circuit are not flexible at all. Severe stress concentration exists and often results in cracks and device failure. In this paper, we present an ultra-thin and ultra-flexible temperature/strain sensor with CNT (Carbon Nanotubes) nanostrip connections. The sensor has good linearity (r2Temperature =0.997, r2strain=0.976). The connecting part is completely flexible and highly robust.