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).