Hydrogel artificial skin: 77-fold stretchability and five-second self-healing ability

  The skin is the largest human organ. It is not only the physical protection barrier of the human body, but also an important medium for the human body to communicate with the outside world. Its surface and interior are rich in various receptors, such as touch, pressure, vibration, pain and temperature. Feel and so on.
  Human skin combines excellent mechanical properties and multi-functional sensing capabilities. This characteristic has inspired researchers to develop skin-like materials and sensors. Related research can not only provide artificial skin for patients who have lost their ability to sense due to burns, amputations, etc. Restoring related perception capabilities can also provide skin-like sensors for soft robots, human-machine interfaces, and wearable electronic devices.

Ideal artificial skin: malleable, good sensory, self-healing

  First, an ideal artificial skin should not only have good sensory abilities similar to human skin, but also have similar mechanical properties to human skin, ie, softness and extensibility. Taking artificial skin with mechanosensing properties as an example, traditional hard sensors such as resistive strain sensors, piezoresistive sensors, and capacitive sensors are no longer applicable, and are replaced by flexible electronic devices that have flourished in recent years.
  However, malleability, good sensing and self-healing are not easy to achieve, and researchers first encountered the contradiction between malleability and electrical performance.

  The first generation of mechanosensing artificial skins were based on electronic conductors and consisted of two parts, including a malleable elastomer and electronic conductor fillers (such as carbon black, metals, conducting polymers, carbon nanotubes, graphene, etc.).
  In recent years, ionic conductor-based hydrogels have attracted extensive attention due to their excellent properties such as good biocompatibility and mechanical properties similar to skin softness. But whether it is an elastomer composite based on electronic conductors or a hydrogel-based artificial skin based on ionic conductors, most of its mechanical sensing properties are based on changes in electrical conductivity, and the principle is similar to that of resistance strain sensors.
  The disadvantage of mechanosensing artificial skin based on altered conductivity is that its electrical properties and extensibility are strongly dependent on the density of the conductive filler (conductive network). Unfortunately, increasing the density of the conductive filler (conductive network) can improve However, at the same time, it will greatly reduce the ductility of the material, that is, it is impossible to achieve both good electrical properties and ductility.

  Compared with flexible resistive sensing devices, flexible capacitive sensing devices have unique advantages. Traditional capacitive sensors have a “sandwich” structure, i.e. conductor/dielectric/conductor.
  The capacitance C of the parallel plate capacitor can be expressed as: C=εгS/4πkd, where εг is the relative permittivity, k is the electrostatic force constant, S is the area of ​​the two plates facing each other, and d is the distance between the two plates. Therefore, when the sensor is deformed by stretching, bending, torsion, etc., both S and d may change, resulting in changes in capacitance properties, thereby converting mechanical deformation signals into electrical signals to achieve mechanical perception.
  Second, the ideal artificial skin should also have a self-healing ability similar to that of human skin. However, despite a great deal of research devoted to capacitive sensing of hydrogel-based artificial skins, conferring their self-healing ability still faces challenges.

  There is a certain contradiction between the ductility and self-healing ability of hydrogels. Most of the hydrogels with high ductility are composed of chemically cross-linked network structures and lack the possibility of self-healing; water with self-healing ability Gels are mostly formed by reversible physical cross-linking, but cannot withstand large strains.
  In addition, most of the existing hydrogel-based capacitive sensors use “hydrogel/elastomer/hydrogel” as a sandwich structure, and the interfacial bonding problem between hydrogel and elastomer has not been well resolved. And limited by the area of ​​the three-layer structure, its sensitivity also needs to be improved.

  In view of the limitations of the existing hydrogel-based artificial skin, the team of Professor Wang Wei and Professor Cao Yi from the School of Physics of Nanjing University proposed a single-layer hydrogel-based artificial skin with numerous surface peptides dispersed in the hydrogel matrix. Coated graphene sheet, in which the micro-graphene sheet is used as the conductive electrode plate, and the polypeptide coating and hydrogel are used as the dielectric, so the single-layer hydrogel-based artificial skin can be considered to be composed of countless dispersed microcapacitors in series and parallel. Body capacitance junctions, similar to the sensing mechanism of human skin’s discrete but interconnected receptors.
  Compared with the traditional hydrogel-based artificial skin, the single-layer hydrogel-based artificial skin has ultra-high ductility of 77 times the stretching ratio, good self-healing ability, and ultra-sensitive pressure and strain sensing ability. The single-layer hydrogel-based artificial skin also has good rheological properties, can be 3D printed into any shape, and has broad application prospects in the field of a new generation of flexible artificial skin. A related paper, titled “Stretchable and self-healable hydrogel artificial skin,” was published in the National Science Review.

Design and Sensing Principles

  Different from traditional sandwich-shaped capacitive sensors, adjacent graphene sheets dispersed in a polyacrylamide hydrogel network serve as conductive electrode plates for a single-layer hydrogel-based artificial skin, with peptides and graphene covered on the surface of the graphene sheets. The hydrogel between the sheets acts as a dielectric, forming numerous microcapacitors in the hydrogel network, and they form a bulk capacitance junction in the hydrogel in a series-parallel manner.
  Because of this, the single-layer hydrogel-based artificial skin has a larger equivalent electric double layer area, which means higher sensitivity than planar hydrogel sensors. After being mechanically deformed, the microscopic distribution of the microcapacitors in the single-layer hydrogel-based artificial skin will change, and the overall capacitive properties will be changed, and the mechanical deformation signal will be converted into an electrical signal to realize the perception of pressure or deformation.
  The single-layer hydrogel-based artificial skin contains three main components, namely, peptides, graphene, and hydrogel. The key scientific issue related to it is the interface design, including the interface between peptide and graphene, and between graphene and graphene. interface between hydrogels.
Selection, Preparation and Bonding of Polypeptides to Graphite Surfaces

  The peptide sequence was selected as GAGAY (G: glycine, A: alanine, Y: tyrosine), which is derived from silk protein and can form a β-sheet structure. At the same time, a pyrene group (Py) is connected to the N-terminus of the polypeptide chain, and is connected to graphene through the hydrophobic interaction between Py and graphene and the form of π-π stacking to prepare peptide-coated graphene ( Peptide-coatedgraphene, referred to as PCG).
  The Py-linked peptide Py-GAGAGY self-assembles directly on the graphite surface to form a fibrous peptide network. The single-molecule force spectrometer experimental measurement based on atomic force microscope AFM shows that the dissociation force between the peptide network Py-GAGAGY and the graphite surface has two characteristics. The dissociation force between Py-GAGAGY and graphite surface at high loading rate (400 nm s-1) is more than double that between Py and graphite surface. .
  These two characteristics indicate that the peptide network P y -GAGAGY can effectively self-assemble on the graphite surface, and can achieve strong dynamic interfacial bonding with the graphene surface; the high dissociation force not only helps graphene It can improve the overall fracture toughness of the monolayer hydrogel-based artificial skin, and the dynamic interfacial bonding characteristics provide the possibility for the self-healing ability of the monolayer hydrogel-based artificial skin.
Preparation of peptide-coated graphene

  In order to improve the exfoliation efficiency of graphene sheets, the research team mixed a small amount of polyethylene glycol PEG into the Py-GAGAGY solution. The two act together as a biodispersant, and under the action of ultrasonic dispersion, the graphite is mechanically dissociated into graphene sheets.
  Under the optimal ratio, Py-GAGAGY:Py-GAGAGY-mPEG=10:1, the efficiency of the produced peptide-coated graphene reaches 64%, and it has high long-term stability, which can ensure that SHARK has better performance. Mechanical properties and electrical stability. Experiments showed that the concentration of peptide-coated graphene did not decrease significantly within 1 month, and the concentration decreased by 30% within 3 months. The characterization by AFM, TEM, X-ray photoelectron spectroscopy, XPS and Raman spectroscopy showed that the average number of layers of the prepared graphene was 1.9±0.3, and the performance was good without obvious defects.
Preparation of peptide-coated graphene-hydrogel interface and monolayer hydrogel-based artificial skin

  In order to improve the connection between the peptide-coated graphene and the hydrogel network, the research team added a lysine to the end of the original polypeptide chain P y -GAGAGY to introduce a C=C double bond (this modification affects the preparation has no effect). The preparation of monolayer hydrogel-based artificial skin can be directly photoinitiated the polymerization of acrylamide containing peptide-coated graphene.
  Scanning electron microscope SEM characterization showed that the single-layer hydrogel-based artificial skin exhibited a porous hydrogel network structure, and adjacent peptide-coated graphene units were connected by polymer and peptide networks. The prepared single-layer hydrogel-based artificial skin has a water content of 70%, and has good mechanical properties, which can withstand torsion, bending and expansion deformation.
Mechanical property test

  The prepared monolayer hydrogel-based artificial skin is highly extensible, and it can be stretched 77 times its original length without breaking. Standard mechanical tensile tests show that the mechanical properties of the single-layer hydrogel-based artificial skin have the following characteristics:

Capacitive Mechanical Sensing Performance Testing

  The single-layer hydrogel-based artificial skin can be regarded as a bulk capacitance node composed of numerous parallel-plate microcapacitors. The sensing mechanism of the classic sandwich-shaped capacitive mechanical sensor lies in the deformation of the elastic dielectric layer caused by the deformation, which leads to the change of capacitance. shrink, according to the capacitance expression, its capacitance will increase accordingly.
  On the contrary, the single-layer hydrogel-based artificial skin is the opposite. Under the action of stretching, the plate spacing (the distance between adjacent peptide-coated graphenes) of the microcapacitors in the single-layer hydrogel-based artificial skin increases, and the capacitance increases. will decrease; on the contrary, when the single-layer hydrogel-based artificial skin is compressed and deformed, its capacitance increases.
  The test shows that the sensor based on the single-layer hydrogel-based artificial skin has a good linear correlation, and the capacitance change is approximately linearly related to the strain in the range of 2600% strain; ultra-fast response time, the response time is only a few seconds; Good stability and anti-fatigue properties with no significant change in performance after 1000 bending cycles and 5000 stretching cycles.

Scenario Test – Complex Motion Modality Perception

  1. Finger joint motion perception, attach a single-layer hydrogel-based artificial skin to the finger joint, during the bending and stretching process of the finger joint, the single-layer hydrogel-based artificial skin will undergo a combination of bending, compression and stretching Deformation, which is dominated by bending and compression, increases the capacitance throughout the movement of the finger joint, and the single-layer hydrogel-based artificial skin can sense the large-scale movement of the finger joint well.
  2. Sound perception. The research team also tested the ability of the single-layer hydrogel-based artificial skin to perceive high-frequency and low-amplitude sound. Studies have shown that proper pre-straining can improve the sensing ability of single-layer hydrogel-based artificial skin to sound waves, and the reason may be that pre-stretching leads to a better arrangement of peptide-coated graphene arrays in single-layer hydrogel-based artificial skin. Consistent. When the single-layer hydrogel-based artificial skin is stimulated by a sound of 32 dB, its capacitance will rapidly decrease by about 40%.
  3. Motion perception in the water environment. In order to further verify that the single-layer hydrogel-based artificial skin can still perform well in the presence of large background noise, the research team first demonstrated the single-layer hydrogel-based artificial skin in the water environment. The artificial skin was still able to sense knuckle movements well, and demonstrated that a single-layer hydrogel-based artificial skin can directly sense micro-flows in water.
Self-healing and remodeling ability of monolayer hydrogel-based artificial skin

  Due to the single-layer structure and the non-specific and reversible interaction between the polypeptides and graphene in the single-layer hydrogel-based artificial skin, it can not only achieve rapid self-healing of mechanical and electrical properties, but also It can realize the function of remodeling, and it also provides the possibility of 3D printing.
  Experiments show that the maximum tensile strain of the single-layer hydrogel-based artificial skin is reduced to 3500% after four fractures and remodeling, and the electrical properties are not significantly reduced.
  A single-layer hydrogel-based artificial skin can be used as a raw material for 3D printing, with a minimum line width of nearly 200 microns.