The emerging intelligence of materials and structures for electro-thermal conductivity have prompted the next generation of flexible electronics. Underlying mechanics studies support these physical systems for stretching, bending, and twisting motions while staying conductive. The critical link of engineered materials and mechanics can meet the demand of bio-conformable and deployable usage of diverse flexible electronics fields.
Self-healing and recyclable conductive composite
Soft electronics that needs to maintain extreme compliance are prone to wear, tear, and unexpected damages. When damage is expected during operation, self-healing electronics can prolong the longevity. In addition, if a device is needed for another purpose, a new life cycle of the soft electronic devices should be regenerated. Robust soft electronics can benefit from being self-healing, damage tolerant, reconfigurable, and ultimately recyclable.
We have developed a liquid metal (LM)-elastomer-plasticizer composite as a regenerative soft electronic platform. Thermoplastic styrene-isoprene-styrene (SIS) block copolymer is cross-linked with polybutadiene (PBD) plasticizer to make a reprocessable matrix for LM dispersion. A scalable embossing technique is introduced to coalesce discrete LM particles into connected liquid metal networks. This composite composite can advance the flexible electronics fields by the following standout features:
(i) Flexible wiring: Selectively conductive lines can be created by embossing through a patterned mold. These traces can be stretched with nearly constant resistance over numerous cycles, giving conductivities as high as 45400 S.cm-1 at 1200\% strain. This versatile approach can rapidly create multiple intricate conductive traces such as the letters ‘LM’ using a positive stamp.
(ii) Self Healing: Self-healing ability is a critical property for soft materials in rugged environments. During extreme damage, the liquid metal droplets reconfigure to autonomously heal the electrical conductivity to tolerate multiple damage events.
(iii) Re-wiring: The polymer network used here is reprocessible by local solvent treatment. The solvent locally separates the liquid metal network to effectively erase that specific region of the trace. For instance, the LED 1 is connected in a circuit by two pristine embossed traces. One of the traces is erased partially and two new traces are embossed to turn on the second LED. This imparts soft electronics the ability to reconfigure circuit elements in a nondestructive manner.
(iv) Recycling: The thermoplastic co-polymer matrix coupled with the liquid nature of liquid metal enables us to recycle and reuse the composites. We use a solution-process where the composite sheets are rapidly dissolved by solvent through shear mixing. When this emulsion is cured the LM droplets become isolated in the matrix which can embossed again for a new circuit. This process can be repeated multiple times and composites are recycled to generate new devices at the end of life.
Wearable motion monitoring sensors
These capacitive sensors are integrated with a glove to quantify the gesture of the proximal interphalangeal (PIP) joint of fingers. To demonstrate the sensitive motion detection ability of the sensors, we perform finger grip tests for two sizes of styrofoam balls with diameters of 47 mm and 19 mm. Due to a difference in the ball size, the PIP joints bend at different angles to grip a ball leading to distinct capacitive response. We alternatively hold and release the big and small balls for 10 seconds for three repeated cycles. The capacitance of the unbent sensor is 48 pF which rises to a stable value of 65 pF and 72 pF for picking big and small ball, respectively. The distinct signals corresponding to different sized balls indicate the strain sensitivity and repeatability of the sensors.
We have shown the procedures to control LM particle sizes using combinations of shear mixing and ultrasonication techniques. Uniform dispersion of the same sized LM particles can achieve linear dielectric response for increasing strain. We leverage this electro-mechanical coupling of dielectric composites to develop flexible sensors. The sensors consists of the soft LM embedded composite as a dielectric layer and two layers of LM coating as electrodes.
Resistive sensors from stretchable thin films
The stretchable LM elastomeric thin films (LETs) developed as multifunctional material is characterized by high compliance (modulus < 500 kPa), stretchability (> 700%) as well as electrical conductivity (Conductivity, G > 1 S). Due this composition, the LETs can withstand stretching, twisting, and bending deformation modes applied by an unevenly shaped object. An exemplary demonstration is shown by a LET circuit to power a LED during during stretching, bending, and twisting deformations. The film conformally wraps around the complex positive and negative curvatures of the object while a LED is consistently powered.
The resistance-strain relation of LEts is found to be linear even at 700 % strain. This linear sensitivity make LETs suitable to detect large strain of flexible electronics. We can sense the resistance change due to tensile strain of a substrate. The strain detection by the thin films is investigated for different deformation modes. In a strain and hold experiment, the substrate is sequentially stretched by 100%, then hold it in static condition for 100 s, then continue stretching to reach a maximum value of 500% strain. The resistance and strain measurements as a function of time indicate that the resistance output closely overlays on the applied strain. Further, the hold steps are not visible in the plot of normalized resistance and strain for this stepped experiment. In a cyclic strain sensing experiment, we increase the cyclic strain by 25% up to mid-range level of 100%. The subsequent cycles are increased by 100% to reach a final strain of 500%. The time-resistance relation exhibits a uniform rate of resistance change over time during loading and unloading of each cycle, overcoming the typical nonlinear response of resistive sensors at large strain. The high compliance of the films provides minimal interference with the underlying substrate motion which makes them suitable for strain monitoring of stretchable electronics, smart clothing, and robotics.
Stretchable electronic interfaces using metamaterials
The stiffness graded kirigami systems developed by data-driven mechanics study can program strain distributions across a stretchable sheet. The multilayered graded kirigami composites are capable of globally distributing strain across a planar flexible electronic device.
The efficiency of kirigami-gradients are demonstrated by a device composed of two parallel LED circuits. We have created kirigami patterned and unpatterned interfaces in series to place rigid electronics (LEDs). Specifically, the LEDs are connected by an electrically parallel circuit while the platforms are mechanically in series configuration so that failure of one interface would not interrupt functionality of the other. Upon stretching the graded kirigami interface survives while the traditional soft-rigid interface suffers an early failure. This device explains that incorporation of graded kirigami composite can effectively mitigate the interface failure and enhance longevity of stretchable electronics.
