1. Introduction

Humans are considered as the smartest creatures on earth, but actually animals and plants have more superpower far surpassing humans in some abilities, one of which is the sense of touch.[1,2] Some animals possess an incredible tactile sensation which covers more than the sensation of pressure, heat, texture, and even pain between an object and the skin.[3] Like the extra-sensitive skin of crocodile with many protruding black spots on their scales called the dome pressure receptor or the outer skin sensory organ, which is as sensitive as a finger.[4] In addition, plants also have a sense of touch, and they can even “perceive” when their leaves are picked or attacked, Like mimosa will droop petioles and close its small leaves as “shyness” when touched by the outside world.[5] Each life entity has similar sensory organs which can be interpreted as perceiving physical/chemical information about their surroundings, noted as tactile.[3] The previous biology studies have found that the skin of humans and higher animals consists of three layers: epidermis, dermis, and subcutaneous tissue, where the intromission with “interlocked” configurations are existed between epidermis-dermis and dermis-subcutaneous tissue.[6,7] Benefiting from these interleaved interlocked layer, the weak stimuli could be perceived and amplified to obtain esthesia through this tactile sensor quickly.[8-10] From the above-mentioned relationship between sensing properties and interlocked structure, real skin provides a great imaginary space for us to imitate and develop artificial perception organ such as electronic skin (e-skin),[11] ionic skin(i-skin),[12] artificial limb,[13] soft robots[14] or brain computer interface (BCI),[15] which are called for potentializing numerous internet of things (IoT) applications involving the artificial intelligence (AI) -motivated sapiential healthcare internet (SHI),[16] intelligent cognition systems (ICS),[17] and widely distributed human-machine interaction (HMI).[18]
Tactile-like wearable electronics advanced rapidly in recent years with unique characteristics of perception-to-cognition-to-feedback external stimulus to build intelligent cognition systems which could mimic humans and higher animals-like specific tactile cognition. Advances in wearable sensors, flexible circuit integration techniques now allow e-skin and i-skin to detect key physiological indicators such as pulse, heartbeat, body motion, and certain inappreciable signals. Among diverse sensors, the capacitive pressure sensors are very attractive for its superior performance of expeditious responding and amplifying the local press stimuli and thus enable to feedback outside stimulus information instantaneously. In recent years, aiming to improve the compressibility and sensitivity of sensors to detect very weak signals, most of capacitive pressure sensors are focused on introducing micro-nano structures like pyramids,[19]microspheres,[20]nanowires,[21] layered structures,[22] and interlocked structures.[23] But the further research on multi-stage interlocked microstructures related to the tactile perception performance is rare. So, for clearly illuminating the correlative dependence of sensing properties on microstructure, researchers try to explore another new microstructure derived from nature to artificial staff, not only for enhanced sensitivity but also for exploiting simple preparation technology. Besides artificial templates, plants like lotus were also selected as templates for multi-level microstructures of the flexible electrode layer or dielectric layer.[24-26]
However, it still remains a huge space to improve the degree of sensitivity, response capacity, and device stability to satisfy the monitoring of subtle physiological signals especially. And simultaneously it needs address another challenge how to drive and sustainably operate the device with wireless and low consume power supply?
Currently, using substantial extra rigid energy-driving with limited-service lifetime, and inadequate flexible circuit entangled it into dilemma of actual application. So, it calls for wearable energy harvesters for resolving the wearable power supply issue where various types of surrounding or tight-fitting energy sources could be converted into electricity. The transformed electricity not only could directly act as active sensing signals of physiological parameters but also could power additional sensors as well as signal processing and data delivery. Emerging self-powered sensor systems integrating sensors with self energy-power management module could operate sustainably without an external power supply which absolutely are advantageous candidates in the next generation of intelligent wearable flexible electronics. What’s more, researchers have proposed the concept of in-situ utilization of low-order thermal and mechanical energy of body to convert it into electrical energy such as piezoelectricity,[27]triboelectric,[28]thermoelectricity,[29] hydrovoltaic power,[30] and humidity power.[31] The surroundings is full of low-order energy and heat variation,[32] which makes it possible to convert it into electricity using thermoelectric material to power pressure sensors, but there has been uncared for it.
Traditional thermoelectric materials are mainly inorganic semiconductor materials composed of covalent/ionic bonds, which have intrinsic rigidity and are not prone to flexible deformation.[33,34] In practical applications, it is difficult to fit closely with the heat source surface with complex curvature changes, which is easy to cause poor thermal contact, resulting in the reduction of conversion efficiency or temperature control accuracy/sensitivity. In contrast, ionogel, as a new type of thermoelectric material, became a new hot spot due to superior electrical conductivity and thermoelectricity, as well as excellent self-healing and shape memory properties, which makes ionogel an ideal ion skin sensing candidate.[35-37] However, compared with a large amount of work conducted on the strength, self-healing, and high tensile properties of ionogel by many researchers, research on the thermoelectric properties of ionogel is relatively limited, and the development of thermoelectric ionogel that collecting body heat to enable self-powered ionic skin sensing for real-time monitoring of human-related activities has been relatively rare so far. In addition to thermoelectric properties, the piezoelectric properties of ionogel have been vigorously investigated. Shen et al. prepared an electrical double layer (EDL) capacitive ion pressure sensor by exploiting the outstanding ionic conductivity of the ionogel, and the pressure sensor sensitivity was about 44 times higher than that of a traditional parallel-plate capacitor.[38] However, much of the research on ionogel has focused on heating/cooling outdoor operation or single functions.
Here, we constructed a multi-stage micro-structured multifunctional ionic skin (MM i-skin) with high sensitivity to address the problems of low sensitivity and dependence on exogenous drive of traditional sensors, which integrated nanogenerators and sensors in one body and achieved two working modes of piezoelectricity / thermoelectricity with the same material (Figure 1 ). And the microstructure template of the taro leaves and sandpaper were replicated by the bio-template method to build a multi-stage microstructured sensor. Then the obtained ion pressure sensor with papillary microstructured PDMS gold electrode and rough [EMIM]DCA ionogel dielectric layer, which featured a sensitivity of up to 77.4 kPa-1 below 1 kPa, a fast response time (46 ms), a wide pressure detection limit (1 kPa to 50 kPa), and a high cycle durability (over 1500 cycles), and it also can monitor the human physiological signals like breathing and pulse anytime. The prepared thermoelectric [EMIM]DCA ionogel with high Seebeck coefficient of 11.435 mV-1, excellent ionic conductivity of 8.55*10-2 S cm-1 and low thermal conductivity of 0.2 W m-1K-1, as well as an ionic power of 0.12 µW, and the output voltage reached 58.83 mV at temperature gradient of 5 K. To demonstrate the piezoelectric properties of ionogel, piezoresponse force microscopy (PFM) was adopted to demonstrate the corresponding voltage signals when the ionogel was pressed, and the ionic gradient would be created owing to the varying mobility of anions and cations, which can be explained by the ionic piezoelectric effect in the ionogel. By this way, the P-iskin was produced and applied for real-time monitoring of human limb activity signals and robot motion recognition. Further, integrating thermoelectric nanogenerator and pressure sensor converts body temperature gap into voltage for self-powered wearable health monitoring. In summary, with its significant advantages of high sensitivity, multifunction, and self-power, the MM i-skin is expected to be adopted in smart medical system to achieve home-style self-health monitoring in real time (Figure 1).