10.3.1 Mechanical sensor devices
Human skin can feel complex external mechanical stimuli，including pressure，stretching，shearing，bending，vibration，sliding，and others.For e-skin devices，it is particularly important to design the materials and device structures to realize the sensing of mechanical stimuli.
When the skin is in contact with the external environment，pressure and strain are generated.According to the difference in pressure and strain，the human body can judge the basic properties of the environment.The sensing of pressure and strain is important for the e-skin.In e-skin devices，the sensing of pressure and strain is mainly realized through the changes of the device's resistance，capacitance，and triboelectric effect responding to the external forces.Among them，the e-skin devices based on the principle of capacitance and resistance change are not only simple in structure design，but also able to detect dynamic and static stimulus signals.Therefore，they have received the most extensive and in-depth research.
The e-skin devices based on the resistance change can realize the sensing of pressure or strain through three ways：①the change of the conductive percolation network in the polymer composite material；②the change of the contact resistance between the conductive material and the electrode；③the conductivity changes in the resistance of the materials.This kind of e-skin device transforms the applied force into the change of the device resistance，which is a very simple and commonly used strategy.
For example，by arranging single-walled carbon nanotubes on an elastic substrate（e.g.，PDMS）to form film，a strain sensor could be prepared.When the elastic substrate was stretched，the carbon nanotube film was broken.Therefore，gaps formed in the film，resulting in an increase in the resistance.The greater the degree of deformation，the greater the gap produced in the film and the greater the resistance.Therefore，strain sensing can be achieved by measuring resistance changes.The device could be integrated into the surface of clothing，band-aids，gloves，etc.，to achieve the sensing of different human movements，such as breathing，speaking，walking，and gestures（Figure 10-3）.This kind of e-skin device is expected to be widely used in health monitoring，virtual reality，robots and other fields.In addition，metal nanowires can also be used as conductive materials to construct stress sensors.The pressure causes a different degree of contact between the nanowire and the electrode，which further changes the resistance of the device.By fixing the voltage，the change of current with the stress is measured to realize the stress sensing.
E-skin devices based on changes in capacitance mainly realize pressure or strain sensing by changing the distance，area，and dielectric constant of the dielectric layer between the device electrodes.For example，by using a microstructured PDMS film as a dielectric layer，a pressure sensor based on capacitor can be prepared.As shown in Figure 10-4，the surface of the PDMS film in the device has an array of pyramid-shaped microstructure.When exposed to external pressure，the film can still maintain good elastic deformation ability，giving the PDMS film sensitive responsiveness.The external pressure causes the change of thickness of the dielectric layer，which further changes the capacitance of the capacitor and the output current of the field effect transistor，thereby realizing pressure sensing.In addition，single-walled carbon nanotubes are introduced into the PDMS substrate.After the substrate is stretched and relaxed，the nanotubes are bent，and the pressure and strain sensors based on parallel plate capacitors can be constructed（Figure 10-5）.The device has very sensitive pressure sensing performance and fast response speed，and has spatial resolution for the sensing of pressure and strain，which further improves the applicability of the device in e-skin.
Sensitivity is an important performance parameter of pressure sensors.Sensitivity can be defined as the slope of the curve of electrical signals（such as capacitance，resistance，current，and voltage）with external pressure.Generally，the sensitivity of a pressure sensor is related to the modulus of compression of the material.At a given pressure，the greater the change in the electrical signal，the higher the sensitivity of the device.For the aforementioned pressure sensors based on changes in capacitance and piezoresistance，a variety of methods have been developed to improve the sensitivity of the device.(https://www.daowen.com)
For the e-skin devices based on capacitor，the microstructure of the dielectric layer can improve the sensitivity.For example，the above-mentioned pyramid-shaped array is prepared on the surface of the PDMS.The pressure at the tip of the pyramid is very large，which causes a large deformation of the PDMS，leading to an increase in capacitance change.As a result，the sensitivity is significantly enhanced.The porous structure can also be used to increase the sensitivity of the pressure sensor.The holes can concentrate stress on the frame structure.When a small pressure is applied，the conductivity of the device changes significantly，and the sensitivity can be improved.
For piezoresistive devices，the contact resistance can be changed by changing the contact point or contact area between conductive materials，thereby improving the sensitivity of the device.For example，carbon materials（carbon nanotubes，graphene，etc.）can be integrated with cross-electrode arrays.Under the action of external force，the contact points or contact areas between the internal electrodes of the device and the conductive materials vary greatly.The sensitivity of the device is significantly improved，which can be used in the measurements of small pressures such as blood pressure and wrist pulse.
The increase in sensitivity usually results in a decrease in the measurable range of the device.The sensitivity of a sensor with high sensitivity will be decreased when the pressure increases.Thus，it is necessary to increase its sensing range while increasing the sensitivity.Cho et al.prepared a hemispherical array on the surface of the functional layer in the device，and prepared a piezoresistive pressure sensor with a sensitivity of 8.5 kPa-1 in the working range of 0-12 kPa.In addition，a capacitive pressure sensor can be prepared with electrodes with wrinkles on the surface，and the electrode area can be changed by pressure to achieve an increase in sensitivity and sensing range.
In addition to sensing stress and strain，human skin also has precise sensing capabilities for detecting the direction of force.For example，the skin can distinguish the direction of force，and it can also sense the sliding of objects on the surface of the skin.For e-skin devices，only by achieving their precise perception of different mechanical stimuli，can they be widely used in prostheses，robots，and others.From the perspective of devices，the sensing of sliding and force directions requires the devices to detect dynamic mechanical stimuli.Therefore，e-skin devices are mainly designed based on piezoelectric and triboelectric materials.Poly（vinylidene fluoride）is a typical piezoelectric material.Different protrusions can be prepared on its surface.The vibration caused by the sliding of different materials on the surface of these protrusions is used to detect the sliding state of the material on the surface of the device and the surface roughness of the material（Figure 10-6（a））.The nanogenerator based on triboelectricity can also be used to detect the sliding state of the surface of the device.By inserting carbon nanotube/PDMS composite electrodes in the device，these electrodes can respond to the moving objects and detect the moving directions of the objects on the surface of the device.
Through the device structure design，a variety of electrical signals（e.g.，resistance，capacitance，current，and voltage）can be measured on the same device，so that the normal and tangential forces can be distinguished.The mechanical sensor shown in Figure 10-6（b）is composed of a top electrode and four bottom electrodes，which are separated by a dielectric material，forming four capacitors.By detecting the changes of the four capacitances，the normal and tangential forces can be distinguished.Bergbreiter et al.put three pieces of conductive material around one piece of conductive material and designed a mechanical sensing device.When forces in different directions act on the device，the relative distance and contact area between the middle area and the surrounding conductive materials change，triggering the changes in resistance and capacitance of the device.By analyzing the output signal，the device can identify the force size and direction.Devices that can simultaneously detect the magnitude and direction of the force can be integrated into the robotic arm，to detect the pressure and shear force on the robotic arm when grasping and moving objects.