Thin, Soft, Miniaturized Instrumentation for Biomedical Sensing and Stimulation


Student thesis: Doctoral Thesis

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Award date3 Jan 2023


Recently, wearable electronics has drawn more and more attention in various fields including biomedical instrumentations for disease diagnosis, therapy and healthcare monitoring (e.g. Apple watch), neuroscience research such as brain-computer interfaces (BCI) and electroencephalography (EEG) recordings, energy harvesting for self-powering devices, and personal entertainments such as virtual reality (VR) headsets, etc. However, typically the commercial wearable instrumentations are still in rigid and bulky form, having poor skin-contact interface due to the mismatching between the high mechanical stiffness of the printed circuit board (PCB) or plastic shell and the softness of the skin, which seriously hinders the faithful signal acquisition ability. Rigid electronics, especially those need external power cords or signal cable connections, also limit the user’s movements and actions, and introduce uncomfortable feelings to the user, which is not suitable for long-term use in daily life. The critical problem is that wearable electronics should be as soft and thin as skin, and meanwhile, stretchable so that it could adapt to the deforming of the skin and won’t be fractured during actions. Furthermore, the wearable electronics system for sensing and stimulations in biomedical applications should also be wirelessly operable, for the purpose of getting the user or patient free of being tethered by bunch of cables, which helps with the long-term use such as continuous healthcare monitoring or chronic therapy. To realize this type of instrumentation in a thin, soft, miniaturized and stretchable form that could intimately attach to the skin, we developed a series of ultrathin, soft electronic devices and systems with elaborated materials, advanced fabrication methods, largely shrank size, and optimized mechanics design that makes it light-weighted, stable, comfortable for wearing and using and be able to sense signals accurately or provide efficient and personalized stimulations.

In this thesis, we mainly focus our wearable instrumentation on the mechanical contact sensing, energy harvesting and electrical stimulation on the body. As we aim to make the device comfortable and unobtrusive enough to minimize the feeling of foreign objects, the device should also be wireless to get rid of the tethering signal or powering cords, and even more ideally to be battery-free so as to lighten the weight. This requires the device to harvest energy needed for working from ambient environment or human body, i.e. self-powering. Triboelectric nanogenerators (TENG) is one major category of the self-powering devices, which utilizes contact electrification and electrostatic induction to transform mechanical energy into electrical energy when materials with different electron affinity contact and separate or sliding against each other. We fabricated a thin, soft and skin-integrated TENG, which is also called “epidermal TENG” (e-TENG), for collecting mechanical tactile signals and also turning everyday motions energy into electrical energy. However, one critical issue of the e-TENG is that higher energy output and higher sensitivity to tactile signals requires a larger area of the electrode, which is typically unstretchable metal thin film, and will limit the stretchability of the device. To address that, we investigated the possibility of reducing the area of the electrode using a cobweb-like serpentine pattern, so as to obtain better stretchability. However, since the reducing of effective area for collecting charges leads to a decrease of output voltage, a strategy of trading-off between effective working area of electrodes and the stretchability is studied. With an optimized area ratio of ~51.34%, the e-TENG could maintain a high voltage and current output of over 86% of the ones with intact electrode, and could stay stable after being stretched to ~30% strain. With this mechanics-performance relationship studied, we can apply this trading-off strategy in optimizing the design of stretchable epidermal TENGs. Here we also used the e-TENG for self-powered tactile sensing. For example, we could distinguish contact manners of a hand with different pressure, like touching, poking, tapping and hitting. The energy it harvested could also be used for lighting up to over 20 LED bulbs.

Aside from sensing, we also investigated the potential of applying stimulations to the body by instrumentation in the similar style. As we tried to keep it very thin that could comformally stick on the skin, we applied electrical current for stimulating, which only needs thin skin-contacting electrodes. We developed a complete on-skin, flexible, and wireless system for delivering electrotactile stimulations to the user’s hand, which aims to make them actually feel the tactile information when touching virtual objects in VR or AR and without any tethering restrictions. We could precisely control the current intensity, pulse width and frequency so as to finely tune the electrotactile perceptions by a compact driver unit with elaborated electrical design. By a thin, soft, hydrogel-based electrodes hand patch, we can efficiently deliver the stimulation signal to 32 different sites distributing across the whole hand, since the hydrogel keeps the skin wet and significantly lowers the impedance of the skin-electrode interface. With the strong ability to stimulate everywhere of the hand, we still need to know the sensitivity of the hand and how it distributes, since the sensitivity among different individuals and different parts of the hand varies widely. Through volunteer investigation, we found that the current threshold maps of people are strongly related to gender, age and even jobs, and the fingertips are way more sensitive than the palm. As a result, we need to personalize the stimulation parameters according to each user’s threshold feature, so as to precisely induce the intended sensation level and avoid invalid perception or causing pain. With this thin, soft and wireless electrotactile platform as the haptic interface, we can integrate tactile feedback into VR/AR systems by synchronized Bluetooth Low Energy communication. Vivid touching in the virtual world such as grasping a virtual tennis ball, tingling at touching a cactus, or a cartoon mouse running on your hand could be felt with the assistance of our instrumentation.

Above all, we have developed a series of thin, soft and miniaturized instrumentation for various purpose of biomedical sensing and stimulation. We did detailed research and tackled some limitations in stretchability, electrical output performances, powering method, cable tethering and also universal applicability to all users. We believe that in the future, soft electronics technology could really come into practical and clinical use, and become the next generation platform of personal equipment and biomedical instrumentations.

    Research areas

  • Biomedical instrumentation, stretchable electronics, skin electronics, self-powering technology, triboelectric nanogenerator, electrotactile feedback, haptic interface, human-machine interface