Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Transient thermo-mechanical analysis for bimorph soft robot based on thermally responsive liquid crystal elastomers

Thermally responsive liquid crystal elastomers (LCEs) hold great promise in applications of soft robots and actuators because of the induced size and shape change with temperature. Experiments have successfully demonstrated that the LCE based bimorphs can be effective soft robots once integrated with soft sensors and thermal actuators. Here, we present an analytical transient thermo-mechanical model for a bimorph structure based soft robot, which consists of a strip of LCE and a thermal inert polymer actuated by an ultra-thin stretchable open-mesh shaped heater to mimic the unique locomotion behaviors of an inchworm. The coupled mechanical and thermal analysis based on the thermo-mechanical theory is carried out to underpin the transient bending behavior, and a systematic understanding is therefore achieved. The key analytical results reveal that the thickness and the modulus ratio of the LCE and the inert polymer layer dominate the transient bending deformation. The analytical results will not only render fundamental understanding of the actuation of bimorph structures, but also facilitate the rational design of soft robotics.

A thermal actuated switchable dry adhesive with high reversibility for transfer printing

Transfer printing based on switchable adhesive that heterogeneously integrates materials is essential to develop novel electronic systems,such as flexible electronics and micro LED displays.Here,we report a robust design of a thermal actuated switchable dry adhesive,which features a stiff sphere embedded in a thermally responsive shape memory polymer(SMP)substrate and encapsulated by an elastomeric membrane.This construct bypasses the unfavorable micro-and nano-fabrication processes and yields an adhesion switchability of over1000 by combining the peel-rate dependent effect of the elastomeric membrane and the thermal actuation of the sub-surface embedded stiff sphere.Experimental and numerical studies reveal the underlying thermal actuated mechanism and provide insights into the design and operation of the switchable adhesive.Demonstrations of this concept in stamps for transfer printing of fragile objects,such as silicon wafers,silicon chips,and inorganic micro-LED chips,onto challenging non-adhesive surfaces illustrate its potential in heterogeneous material integration applications,such as flexible electronics manufacturing and deterministic assembly.

Vibration analysis of fluid-conveying nanotubes embedded in an elastic medium considering surface effects

An analytical model is developed to study the surface effects on the vibration behavior including the natural frequency and the critical flow velocity of fluid-conveying nanotubes embedded in an elastic medium.The effects of surface elasticity and residual surface stress are accounted through the surface elasticity model and the Young-Laplace equation.A Winkler-type foundation is employed to model the interaction of nanotubes and the surrounding medium.The results show that the surface effects have more prominent influences on the nature frequency with smaller nanotube thickness,larger aspect ratio and larger elastic medium constants.Both surface layers and the elastic medium enhance the stability of nanotubes.This study might be helpful for designing the fluid-conveying nanotube devices in NEMS and MEMS systems.

Editorial: Thermal Stresses

Thermomechanical properties of materials have significant influences on the normal operation and service life of devices and structures.It is therefore of crucial importance to analyze their thermomechanical responses in numerous application areas such as mechanical engineering,civil engineering,electronic technology,and machine manufacturing.Demands on thermomechanical analyses or thermal stress analysis of materials and structures subject to various thermal loads are growing with the expanding of novel materials(e.g.,graphene,phononic crystals),new technologies(e.g.,three-dimensional printing,transfer printing),and new devices(e.g.,flexible electronics,stretchable electronics).In the last few decades,thermal stress analyses have attracted much attention from academia and industry with not only Journal of Thermal Stresses for publishing novel and cutting edge researches,but also series of International Congress on Thermal Stresses to exchange ideas and extend further collaborations for scientists and engineers who are involved in the field of thermal stresses.

Buckling of thin gel strip under swelling

The buckling of thin gel film has attracted much attention due to its applications in the design of three- dimensional structure from two-dimensional template. We have established an analytical model to study the swelling-induced buckling of a thin gel strip with one edge clamped and the others free. The closed-form solutions for the amplitude and wavelength of the buckled shape are obtained by energy minimization of the total potential energy. The analytical results agree well with finite element analysis based on the inhomogeneous gel theory without any parameter fitting. The model provides a route to study complex postbuckling behaviors of thin gel films and guidelines to design the buckled configuration quantitatively by controlling the swelling ratio.

Editorial: Recent advances in mechanics of unconventional electronics

Conventional electronics is planar, hard, and rigid due to the intrinsic brittle nature of inorganic semiconductor materials （e.g., silicon and gallium arsenide）. The modern electronic technology has typically been concerned with large or small but durable and long-lasting electronics. Recently developed materials and mechanics concepts yield unconventional electronics with unique characteristics （e.g., deformable, degradable, etc.）.

Recent advances on thermal analysis of stretchable electronics

Stretchable electronics, which offers the performance of conventional wafer-based devices and mechan- ical properties of a rubber band, enables many novel applications that are not possible through conven- tional electronics due to its brittle nature. One effective strategy to realize stretchable electronics is to design the inorganic semiconductor material in a stretchable format on a compliant elastomeric substrate. Engineering thermal management is essential for the development of stretchable electronics to avoid adverse thermal effects on its performance as well as in applications involving human body and biological tissues where even 1-2℃ temperature increase is not allowed. This article reviews the recent advances in thermal management of stretchable inorganic electronics with focuses on the thermal models and their comparisons to experiments and finite element simulations.

Transfer printing techniques for flexible and stretchable inorganic electronics

Transfer printing is an emerging deterministic assembly technique for micro-fabrication and nano-fabrication,which enables the heterogeneous integration of classes of materials into desired functional layouts.It creates engineering opportunities in the area of flexible and stretchable inorganic electronics with equal performance to conventional wafer-based devices but the ability to be deformed like a rubber,where prefabricated inorganic semiconductor materials or devices on the donor wafer are required to be transfer-printed onto unconventional flexible substrates.This paper provides a brief review of recent advances on transfer printing techniques for flexible and stretchable inorganic electronics.The basic concept for each transfer printing technique is overviewed.The performances of these transfer printing techniques are summarized and compared followed by the discussions of perspectives and challenges for future developments and applications.

An atomistic model of silk protein network for studying the effect of pre-stretching on the mechanical performances of silks

Silk protein builds one of the strongest natural fibers based on its complex nanocomposite structures.However,the mechanical performance of silk protein,related to its molecular structure and packing is still elusive.In this study,we constructed an atomistic silk protein network model,which reproduces the extensive connection topology of silk protein with structure details of theβ-sheet crystallites and amorphous domains.With the silk protein network model,we investigated the structure evolution and stress distribution of silk protein under external loading.We found a pre-stretching treatment during the spinning process can improve the strength of silk protein.This treatment improves the properties of silk protein network,i.e.,increases the number of nodes and bridges,makes the nodes distributed homogeneously,and induces the bridges in the network well aligned to the loading direction,which is of great benefit to the mechanical performances of silk protein.Our study not only provides a realized atomistic model for silk protein network that well represents the structures and deformations of silk proteins under loading,but also gains deep insights into the mechanism how the pre-loading on silk proteins during spinning improves the mechanical properties of silk fibers.

Implantable Thermal Therapeutic Device with Precise Temperature Control Enabled by Foldable Electronics and Heat-Insulating Pads

Thermal therapy has continued to attract the attention of researchers and clinicians due to its important applications in tumor ablation,wound management,and drug release.The lack of precise temperature control capability in traditional thermal treatment may cause the decrease of therapeutic effect and thermal damage to normal tissues.Here,we report an implantable thermal therapeutic device(ITTD),which offers precise closed loop heating,in situ temperature monitoring,and thermal protection.The ITTD features a multifunctional foldable electronics device wrapped on a heat-insulating composite pad.Experimental and numerical studies reveal the fundamental aspects of the design,fabrication,and operation of the ITTD.In vivo experiments of the ITTD in thermal ablation for antitumor demonstrate that the proposed ITTD is capable of controlling the ablation temperature precisely in real time with a precision of at least 0.7℃ and providing effective thermal protection to normal tissues.This proof-of-concept research creates a promising route to develop ITTD with precise temperature control capability,which is highly desired in thermal therapy and other disease diagnosis and treatments.

Soft,stretchable thermal protective substrates for wearable electronics

Wearable electronics have continued to attract the attention of researchers and clinicians due to their great potential in medical applications.During their operations,the undesired heating may cause thermal discomfort or damage to skin.Seeking materials and structures for advanced thermal protection has become an urgent issue.Here,we report a soft,stretchable thermal protective substrate for wearable electronics with remarkable thermal insulating performance,mechanical compliance and stretchability.The thermal protective substrate features a composite design of the widely used polymeric material polydimethylsiloxane with embedded heat absorbing microspheres,consisting of phase change materials encapsulated inside the resin shell.Experimental and numerical studies show that the thermal protective substrate could be subjected to complex deformations over 150% and could reduce the peak skin temperature increase by 82% or higher under optimizations.In vivo demonstration of this concept on the mouse skin illustrates its unusual thermal protection capability for wearable thermal management.