Highly conductive,stretchable,durable,skin-conformal dry electrodes based on thermoplastic elastomer-embedded 3D porous graphene for multifunctional wearable bioelectronics

Long-term bioelectric potential recording requires highly reliable wearable dry electrodes.Laser-induced graphene(LIG)dry electrodes on polyimide(PI)films are difficult to conform to the skin due to the non-stretchability and low flexibility of PI films.As a result,high interface impedance and motion artifacts can occur during body movements.Transferring LIG to flexible substrates such as polydimethylsiloxane(PDMS)and Ecoflex allows for stretchability and flexibility.However,the transfer process produces a significant loss of conductivity destroying the structural function and electron conduction properties of the LIG.We found robust physical and chemical bonding effects between LIG and styrene-ethylene-butylene-styrene(SEBS)thermoplastic elastomer substrates and proposed a simple and robust low-conductivity loss transfer technique.Successfully embedded LIG onto SEBS to obtain high stretchability,high flexibility,low conductivity losses.Electrophoretic deposition(EPD)of poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid(PEDOT:PSS)on LIG forms an ultrathin polymer conductive coating.The deposition thickness of the conductive polymer is adjusted by controlling the EPD deposition time to achieve optimal conductivity and chemical stability.SEBS/LIG/PEDOT:PSS(SLPP)dry electrodes have high conductivity(114Ω/Sq),stretchability(300%)and reliability(30%stretch,15,000 cycles),low electrode-skin impedance(14.39 kΩ,10 Hz).The detected biopotential signal has a high signal-to-noise ratio(SNR)of 35.78 dB.Finally,the feasibility of SLPP dry electrodes for long-term biopotential monitoring and biopotential-based human-machine interface control of household appliances was verified.

Highly Stretchable and Self-healing Hydrogels Based on Poly(acrylic acid) and Functional POSS

Herein, we present a novel way for the production of self-healing hydrogels with stretch beyond 4200% than their initial length and relatively high tensile strength （0.1-0.25 MPa）. Furthermore, the hydrogel was insensitive to notch. Even for the samples containing V-notches, a stretch of 2300% was demonstrated. The hydrogels were developed by in situ crosslinking of the self-assembled colloidal poly（acrylic acid） （PAA）/functionalized polyhedral oligomeric silsesquioxane （POSS） micelles. This was achieved by the addition of functionalized polyhedral oligomeric silsesquioxane with tertiary amines and hydroxyls （POSS-AH） into the PAA reaction solution. The POSS-AH led to micellar growth, then the dual- crosslinked network was constructed. One type of crosslink was formed by hydrogen-bonding and ionic interactions between PAA chains and POSS-AH, the other type of crosslink was formed by covalent bonds between PAA and bis（N,N＇-methylene- bis-acrylamide）.

Tetradic double-network physical crosslinking hydrogels with synergistic high stretchable, self-healing, adhesive, and strain-sensitive properties

Herein,we demonstrate a tetradic double-network physical cross-linking hydrogel comprising of gelatin,polyacrylic acid,tannic acid,and aluminum chloride as wearable hydrogel sensors.Based on the coordination bonds,hydrogen bonds,and chain entanglements of the two networks,the acquired hydrogel possesses excellent tensile properties,self-healing performance,and adhesiveness to many substrates.Mechanical properties can be tuned with fracture strain ranging from 900 to 2200%and tensile strength ranging from 24 to 216 kPa,respectively.Besides,the hydrogel also exhibits good strain-sensitivity when monitoring the motions of humans,such as bending of fingers,bending of elbows.Hence,we can believe that the GATA hydrogel has numerous applications in soft robots,intelligent wearable devices,and human health supervision.

MXene-composited highly stretchable,sensitive and durable hydrogel for flexible strain sensors

The flourishing development in flexible electronics has provoked intensive research in flexible strain sensors to realize accurate perception acquisition under different external stimuli.However,building hydrogel-based strain sensors with high stretchability and sensitivity remains a great challenge.Herein,MXene nanosheets were composited into polyacrylamide-sodium alginate matrix to construct mechanical robust and sensitive double networked hydrogel strain sensor.The hydrophilic MXene nanosheets formed strong interactions with the polymer matrix and endowed the hydrogel with excellent tensile properties(3150%),compliant mechanical strength(2.03 kPa^(-1)in Young’s Module)and long-lasting stability and fatigue resistance(1000 dynamic cycles under 1,600%strain).Due to the highly oriented MXene-based three dimensional conductive networks,the hydrogel sensor achieved extremely high tensile sensitivity(18.15 in gauge factor)and compression sensitivity(0.38 kPa^(-1)below 3 kPa).MXene hydrogel-based strain sensors also displayed negligible hysteresis in electromechanical performance,typical frequent-independent feature and rapid response time to external stimuli.Moreover,the sensor exhibited accurate response to different scales of human movements,providing potential application in speech recognition,expression recognition and handwriting verification.

"Solid-Liquid" Vitrimers Based on Dynamic Boronic Ester Networks

The"solid-liquid"behavior of vitrimers have not been systematically investigated.Herein,a series of"solid-liquid"vitrimers bearing varying contents of dynamic boronic ester bonds were synthesized via thiol-ene click reactions.These vitrimers allow for flexibile modulation of their network structures and thus show a range of intriguing properties including high stretchability,flexible transition from elasticity to plasticity,strong strain rate dependence,and solid-liquid performance.The dynamic association rate of boronic ester bonds within these vitrimers could be apparently accelerated via increasing the content of boronic ester,which could be used to shape-program the flat vitrimer films into various complex 3D structures just with external force.Materials with such versatile dynamic behavior may open up a range of new applications.

Modified commercial UV curable elastomers for passive 4D printing

Conventional 4D printing technologies are realized by combining 3D printing with soft active materials such as shape memory polymers(SMPs)and hydrogels.However,the intrinsic material property limitations make the SMP or hydrogel-based 4D printing unsuitable to fabricate the actuators that need to exhibit fast-response,reversible actuations.Instead,pneumatic actuations have been widely adopted by the soft robotics community to achieve fast-response,reversible actuations,and many efforts have been made to apply the pneumatic actuation to 3D printed structures to realize passive 4D printing with fast-response,reversible actuation.However,the 3D printing of soft actuators/robots heavily relies on the commercially available UV curable elastomers the break strains of which are not sufficient for certain applications which require larger elastic deformation.In this paper,we present two simple approaches to tune the mechanical properties such as stretchability,stiffness,and durability of the commercially available UV curable elastomers by adding:(i)mono-acrylate based linear chain builder;(ii)urethane diacrylate-based crosslinker.Material property characterizations have been performed to investigate the effects of adding the two additives on the stretchability,stiffness,mechanical repeatability as well as viscosity.Demonstrations of fully printed robotic finger,grippers,and highly deformable 3D lattice structure are also presented.