Review of flexible microelectromechanical system sensors and devices

Xiaopeng Yang and Menglun Zhang

AFFILIATIONS State Key Laboratory of Precision Measuring Technology and Instruments,Tianjin University,Tianjin 300072,China

ABSTRACT Today,the vast majority of microelectromechanical system(MEMS)sensors are mechanically rigid and therefore suffer from disadvantages when used in intimately wearable or bio-integrated applications.By applying new engineering strategies,mechanically bendable and stretchable MEMS devices have been successfully demonstrated.This article reviews recent progress in this area,focusing on high-performance flexibl devices based on inorganic thin films We start with the common design and fabrication strategies for flexibilit and stretchability,summarize the recent application-oriented flexibl devices,and conclude with criteria and opportunities for the future development of flexibl MEMS sensors.

KEYWORDS MEMS sensor,Flexible electronics,Stretchable sensor

I.INTRODUCTION

Inorganic semiconductors such as silicon,gallium arsenide,and gallium nitride are by far the most well-established routes to high-performance electronics/optoelectronics.1However,their rigid and brittle format suffers from disadvantages when used in intimately wearable or bio-integrated applications ranging from representative applications such as electronic eyeball cameras2to advanced surgical devices and“epidermal”electronic monitoring systems.3There is significan interest in technologies that enable microelectromechanical system (MEMS) devices to sit directly on flexibl foils or plastic sheets.4–9In addition,particular attention has been paid to electronic devices based on elastomeric substrates that are not only bendable but also stretchable and can thereby provide both versatility and functionality.10–20Such devices allow intimate,conformal,and seamless contact with complex structures and nonplanar surfaces,such as those found in health-monitoring systems,21–25that are subject to high strain levels.These characteristics cannot be realized using conventional rigid structures formed on silicon wafers or glass panels.Flexible electronic devices offer great potential for applications such as flexibl displays,electronic textiles,sensory skin,artificia muscles,radio-frequency(RF)tags,optical MEMS,conformal X-ray imagers,and electronic-eye cameras.26–35

Various types of MEMS devices with small size and low power consumption have been developed to satisfy the requirements of current applications.However,with regard to flexibility a number of problems arise,such as how to achieve sufficien flexibilit and seamless integration into a complete flexibl system.The main difficult here lies in how high performance can be retained in flexibl devices compared with traditional rigid devices,while also realizing new capabilities.

Investigations of flexibl and stretchable electronics were originally motivated by their use in flexibl displays,36–41but more recently attention has focused on applications to biomedical devices that are able to intimately integrate with the human body and on devices that exploit curvilinear,ergonomic,or bio-inspired layouts.42Various options have been considered to provide flexibil ity for MEMS devices in such applications.One approach involves the use of new electronic materials,such as nanomaterials and organic materials that are intrinsically flexibl43–47or composite elastomer conductors that are stretchable.16,48However,organic materials pose challenges such as degradation in device performance and incompatibility with previous fabrication processes such as deposition,crystallization,and doping.On the other hand,recent studies10,12,42,49have demonstrated that with suitable material selection,mechanical design,and integration strategies,traditional inorganic-material-based structures can be made mechanically compliant,avoiding the limitations of incompatibility and performance degradation.In this way,the advantages of MEMS devices in terms of their small size,high performance,and low energy consumption can be preserved in a flexibl format for new applications in biomedical and wearable electronics.50–52

Recent advances in materials science and manufacturing have provided opportunities for the development of flexibl electronics with unique potential for use in bio-integrated applications.Kim53reviewed flexibl and stretchable electronics that could be integrated with the brain,skin,and heart to study disease states or establish human–machine interfaces.Kimet al.54described the manipulation and use of semiconductor nanomaterials for various components of mechanical devices and the integration of these devices into flexibl plastic foils and stretchable rubber sheets.Suitable materials and their processing for stretchable electronics in bio-integrated surgical tools and bio-inspired cameras were discussed by Kimet al.1Yuet al.55presented some inorganic semiconductor nanomaterials,together with corresponding design strategies,for use in flexibl electronics for biomedical applications.The mechanical properties and electronic performance of inorganic materials for flexibl electronics,together with appropriate design strategies,were outlined by Kimet al.42Maet al.56reviewed the underlying ideas,fabrication/assembly routes,and microstructure–property relationships of stretchable electronics.However,to date,there have been no reviews focusing on flexibl MEMS devices.

The present paper describes a variety of flexibl MEMS devices based on inorganic materials.Section II highlights the key design and fabrication strategies of MEMS devices aimed at realizing flex ibility,including transfer printing,device integration,and various interconnections according to optimized mechanical configurations Section III describes typical flexibl MEMS devices and their corresponding applications.Generally,the performance of these flexibl MEMS devices can reach levels comparable to those of conventional wafer-based technologies,which exceeds the range accessible to known organic-material alternatives.Section IV highlights the key requirements for the further development of flexibl MEMS devices.As pointed out in the concluding Sec.V,flexibl MEMS sensors have a promising future for application across a range of fields particularly in wearable devices,health-monitoring systems,and biomedical engineering.

II.ENGINEERING STRATEGIES FOR FLEXIBLE MEMS

A.Transfer printing

MEMS thin-fil structures are mechanically bendable.However,the substrate,which only plays a supporting role,is too rigid to bend.One direct method for achieving flexibilit is to replace the rigid substrate with a flexibl one.Transfer printing is a technology that transfers high-performance inorganic materials grown on primary substrates to other heterogeneous substrates,57,58thereby solving the problem of incompatible substrates.The materials or structures that are transferred in this manner are generally thin,ranging from a few hundred nanometers to a few micrometers,and thus are flexible The transfer operation is generally accomplished using additional transfer stamps,which are generally made of a silica-gel material with small Young’s modulus,such as polydimethylsiloxane (PDMS).Under van der Waals interactions,the stamp picks up the released devices from the donor wafer and then transfers them to the receiver substrate to achieve device flexibilit[Fig.1(a)].59

Acoustic MEMS devices such as fil bulk acoustic resonators(FBARs)60–63and piezoelectric micromachined ultrasonic transducers (PMUTs),64–72require the space provided by a cavity to allow free vibration of the resonant body.A misalignment in the cavity or inadequate boundary constraints will result in poor performance or even failure to work properly.To facilitate transfer printing with adequate cavity space for high-performance flexibl acoustic MEMS devices,Sunet al.73,74proposed the FlexMEMS technique.The key part of this technique is the transfer printing of the MEMS structure onto a flexibl substrate [Fig.1(b)]58with a predefine cavity,and accurate alignment of the structure and cavity is essential.75,76Experimental implementation of the FlexMEMS technique has shown that the performance of the flexibl device is comparable to that of its counterparts on rigid substrates.For different substrates,both flexi ble Lamb wave resonators(LWRs)on polyimide(PI)and polyethylene terephthalate(PET)substrates[Fig.1(c)]have been realized.74FlexMEMS-enabled devices have demonstrated excellent mechanical flexibilit and electrical performance during and after bending trials.

Special release techniques have also been usedin lieuof stamps to fabricate flexibl devices.For example,the thermal-release transfer-printing technique was used to directly pick up a prepared three-dimensional (3D) PMUT and transfer-print it onto a mica substrate,thereby avoiding the problems encountered when picking up using a thermal-release tape without additional complex treatment.The PMUT device was used for curved surface imaging because of its excellent flexibilit and high transparency[Fig.1(d)].77

Transfer printing can be used to fabricate a flexible-devic array without additional processing steps.In a batch-fabrication method,the production efficienc can be greatly enhanced,and the cost of fabrication can be accordingly reduced.78,79Flexible fil bulk acoustic-wave filter consisting of several FBARs were fabricated using this method.78In another example,a high-aspect-ratio ultrathin strain sensor was integrated with a flexibl printed-circuit substrate[Fig.1(e)].79Jianget al.80,81successfully demonstrated largearea transfer printing of 3D high-aspect-ratio MEMS structures using the difference in adhesion between the donor and receptor[Fig.1(f)].Transfer printing of a device array can also be done in a step-by-step and repetitive manner.Using a transfer machine with high resolution and multidimensional position control,micrometerlevel transfer accuracy and a large throughput per hour can also be achieved.

B.Direct integration into a flexible substrate

FIG.1.Processing approaches,devices,and arrays realized by transfer printing.(a)Schematic of the transfer-printing procedure:contacting a stamp with a donor substrate that supports prefabricated micro-or nanostructures,peeling the stamp away,which leads to the removal of selected structures from the donor substrate,and printing onto a receiving substrate to complete the process.Reproduced with permission from Carlson et al.,Adv.Mater.24,5284–5318(2012).Copyright 2012 WILEY-VCH Verlag GmbH&Co.(b)Fabrication process of the FlexMEMS technique.Reproduced with permission from Yang et al.,IEEE Electron Device Lett.38,1125–1127(2017).Copyright 2017 IEEE.(c)Photographs of a flexibl LWR on a thin PI substrate.Reproduced with permission from Sun et al.,Appl.Phys.Lett.115,093501(2019).Copyright 2019 AIP Publishing LLC.(d)Schematic of the fabrication process for a PMUT on a flexibl and transparent mica substrate using the thermal-release transfer technique.From Li et al.,in 2009 International Conference on Measuring Technology and Mechatronics Automation,ICMTMA 2009,Vol.2,pp.45–49.Copyright 2009 IEEE.Reproduced with permission from IEEE.(e)Optical and scanning electron microscopy(SEM)images after the transfer printing of the piezoelectric strain sensor array.From Yamashita et al.,in Proceedings of 2015 IEEE Sensors,Busan,South Korea.Copyright 2015 IEEE.Reproduced with permission from IEEE.(f)Transfer of a pillar array to a flexibl polymer membrane.After transfer printing and removal of the plastic sheet,the pillar array,which is integrated with a polymer membrane that withstands bending,is formed.Reproduced with permission from Zeng and Jiang,J.Microelectromech.Syst.20,6–8(2011).Copyright 2011 IEEE.

Integrating MEMS devices and sensors with flexibl substrates82can provide new capabilities and greatly enhance functionality.Direct integration of devices and flexibl substrates is another effective method for realizing flexibilit and stretchability of a system,offering the advantage of process compatibility between MEMS and a complementary metal oxide semiconductor (CMOS)or a polymer substrate.With this approach,commercially available MEMS devices can be used directly.83For example,the components of a system can be realized by micromachining,microelectronic technology,bio-/chemotechnology,or MEMS technology.Each technology provides different devices with specifi functionality.Through integration,it is possible to combine components and create a micro/nanosystem with multiple functions.84The most commonly used method is heterogeneous integration,84in which MEMS devices are directly bonded to a flexibl substrate or flexibl wires to achieve overall system flexibility

Yamadaet al.83proposed a pseudo-system-on-chip (pseudo-SoC) integration by incorporating an electrostatic MEMS and its driver,which was a CMOS-level system,for mobile electronics applications [Fig.2(a)].This integrated system offers the complementary advantages of a system-in-a-package and a systemon-a-chip.85–87Jentoftet al.88demonstrated a sensor array that could grasp sensing events and detect object shapes.The sensor array was formed by direct integration of commercial barometers and commercially compatible flexibl printed-circuit boards[Fig.2(b)].In addition,IC-integrated MEMS devices are highly desirable owing to their simultaneous elimination of the bias board and interconnection cables,which simplifie implementation procedures and improves system reliability.To avoid the need for a high-temperature deposition process in the CMOS post-treatment process,low-temperature deposition processes such as sputtering,spin coating,and electroplating were employed by Xuet al.32to fabricate an IC-integrated flexibl shear-stress sensor skin [Fig.2(c)].Through-hole vias and multilayer electrical routings were used to realize flexibilit of the system proposed by Hsuet al.89Xiaoet al.90used the flip-chip-on-fl technique to attach sensor and actuator chips to the same flexibl substrate within a small space[Fig.2(d)].Overall system-level flexibilit can be achieved using the above methods.

FIG.2. (a) Micrograph of a pseudo-SoC incorporating an electrostatic MEMS and a high-speed PWM driver CMOS-LSI:(i) driver MOS-LSI with dimensions 2.9 × 2.9 mm2;(ii) electrostatic MEMS with dimensions 2.8 × 2.8 mm2;(iii) pseudo-SoC with thickness 100 μm.From Yamada et al.,in 2010 IEEE CPMT Symposium Japan,Tokyo,Japan,pp.1–4.Copyright 2010 IEEE.Reproduced with permission from IEEE.(b) Sensitive,flexible and stretchable tactile arrays.From Jentoft et al.,in 2013 16th International Conference on Advanced Robotics (ICAR),Montevideo,Uruguay,pp.1–6.Copyright 2013 IEEE.Reproduced with permission from IEEE.(c) Simplifie post-CMOS fabrication process:(i) cross section of wafer;(ii) patterned metal and dielectric layers;(iii) deposition and patterning of 10-μm parylene C at the front side;(iv) deposition and patterning of 1.5-μm parylene N;(v) thinning-down of the wafer from the back side,forming silicon islands using deep reactive-ion etching (DRIE) and removal of the dielectric layer;(vi) deposition of another 10-μm parylene C at the back side;(vii) releasing of the shear-stress sensor using BrF.Reproduced with permission from Xu et al.,J.Microelectromech.Syst. 12,740–747 (2003).Copyright 2003 IEEE.(d) Process flo of solder-bump fabrication.From Xiao et al.,in 51st Electronic Components and Technology Conference 2001,Orlando,FL,USA,pp.750–754.Copyright 2001 IEEE.Reproduced with permission from IEEE.

FIG.3. (a) Fabrication of a flexibl and stretchable electronic device using a bottom-up process:(i) schematic of PI-FBAR with a PI support layer and Ni electrodes;(ii) microscope images of the fabricated PI-FBAR;(iii) surface roughness of ZnO/PI;(iv) flexibl magnetic sensor based on the PI-FBAR.From Wu et al.,in 2016 IEEE International Nanoelectronics Conference(INEC),Chengdu,China,pp.1–2.Copyright 2016 IEEE.Reproduced with permission from IEEE.(b)Optical image of a flexibl AlN/PI module after its release.From Sun et al.,in 2018 IEEE Micro Electro Mechanical Systems(MEMS),Belfast,UK,pp.455–458.Copyright IEEE.Reproduced with permission from IEEE.(c) Cross-sectional view of a spherically shaped ultrasonic transducer with an acoustic-impedance gradient-matching layer.From Zhang et al.,in 2006 7th International Conference on Electronics Packaging Technology,ICEPT’06,Shanghai,China,pp.1–4.Copyright 2006 IEEE.Reproduced with permission from IEEE.(d)Flexible strip of a CMUT device after peeling off from a silicon wafer.Reproduced with permission from Wang et al.,Micromachines 7,226(2016).Copyright 2016 Elsevier.(e)Schematic cross sections of(i)two flexibl CMUT array elements fabricated using trench refillin with PDMS and(ii)a flexibl CMUT array assembly based on a reverse-fabrication process.(f)Photographs of(i)the flexibl CMUT array prototype obtained by reverse fabrication and(ii)the prototype bent around a cylindrical bar with a radius of 8.5 mm.(e)and(f)From Caronti A et al.,in 2008 IEEE International Ultrasonics Symposium,Beijing,China,pp.2092–2095.Copyright 2008 IEEE.Reproduced with permission from IEEE.

A number of studies have investigated the fabrication of flexi ble and stretchable devices by direct deposition of inorganic materials on a polymer substrate in a bottom–up process.For example,following the initial spinning of a polymer substrate on a silicon or glass substrate,active material was deposited on this polymer,and the sacrificia layer was finall released.91,92To fabricate a flexibl magnetic sensor,Wuet al.93spun PI on a silicon substrate,onto which they deposited two layers of Ni electrodes by electron-beam evaporation and a 2-μm piezoelectric fil of ZnO by direct current(DC)reaction magnetron sputtering[Fig.3(a)].The characteristics of this magnetic sensor were investigated both experimentally and theoretically.Sunet al.94deposited Mo (0.2μm)/AlN(1.5μm)/Mo(0.2μm)stack layers onto an oxide layer and then used a lithographic process to pattern the device layer.After the PI layer had been coated,the AlN/PI module was released from the Si test wafer.This method avoided the problems that would have been caused by the high temperatures involved in direct deposition of an AlN fil on the PI,such as stress-induced crimp.The AlN/PI fil had a similar elastic modulus to polyvinylidene fluorid(PVDF),but greater fabrication-process compatibility [Fig.3(b)].Feng and Liu95fabricated microcavities on a flexibl copper sheet by micromachining and then deposited onto this a lead zirconate titanate (PZT) slurry to produce a PZT micro-pillar array,which,after development,was released as a curved thin PZT layer.The transducer thus fabricated achieved a center frequency of 26 MHz and a −6 dB bandwidth of approximately 65%.Zhanget al.96used an HF/HCl escharotics system to pattern a 500×200×1μm3PZT cantilever beam.The backside of the wafer was dry-etched.Subsequently,each sensor formed a silicon island.The sensor array was made flexibl by coating it with a polymer [Fig.3(c)].A flexibl capacitive MUT(CMUT)was demonstrated on an ultrathin PI substrate that was spun on a silicon wafer from which it was then peeled off[Fig.3(d)].97Zhouet al.98explored the possibility of directly integrating piezoelectric ZnO and AlN thin film on a spin-coated PI substrate to realize flexibl acoustic-wave resonators.A variety of flexibl MEMS devices have been fabricated using bottom-up processes.99–101In addition to the above-mentioned methods,flexibilit can be realized by fillin trenches between unit cells.In a demonstration by Carontiet al.,102CMUT dies were embedded using a flexibl backing process and coating layers,and the resulting array assembly could withstand a bending radius of less than 10 mm[Figs.3(e)and 3(f)].

FIG.4.(a)Island–bridge structure:(i)fabricated MEMS FSS on a rigid-fle substrate with the MEMS devices supported over rigid islands while the substrate is curved;(ii)process flo for fabrication of rigid-fle MEMS.From Coutts et al.,in 2007 International Microwave Symposium(IMS 2007),Honolulu,HI,USA,pp.497–500.Copyright 2007 IEEE.Reproduced with permission from IEEE.(b)Microfabrication process of a flexibl CMUT:(i)definitio of the seed layer(Cr/Au);(ii)patterning of the fence;(iii)nickel electroplating over the seed layer;(iv)holes formed by patterning using EPON 1002F;(v)nickel electroplating through the holes to defin the individual arrays;(vi)bonding of the two layers.From Chong et al.,in The 8th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems,Suzhou,China,pp.923–926.Copyright 2013 IEEE.Reproduced with permission from IEEE.(c)Flexible-array design in which each PZT element is separated by flexibl PI joints.Reproduced with permission from Dann et al.,Proc.SPIE 7629,76290R(2010).Copyright 2010 Society of Photo-Optical Instrumentation Engineers(SPIE).

Couttset al.103demonstrated an island–bridge structure by fabricating a rigid-fle Kapton (PI film substrate using a wetetching process.A MEMS-tunable conformal frequency-selective surface (FSS) was integrated onto this substrate [Fig.4(a)].The use of Kapton instead of silicon for the islands offered the advantage of easier processing.A method for fabricating CMUT arrays was proposed by Chonget al.,104who used a novel rivet structure to fasten isolated metal islands to a flexibl polymer film The rivet structure securing the metal islands was formed by nickel overplating,which solved the problem of achieving adhesion between the metal and polymer.A photoresist was patterned to form via holes and served as the polymer fil [Fig.4(b)].Dannet al.105also used an island–bridge technique to realize a flexibl device,with a 12-mm-diameter ring-annular array mounted on top of an array of islands that withstood 10 000 bending cycles at an angle of 60° [Fig.4(c)].These types of integration methods offer advantages for the fabrication of flexibl device systems.Multilayer functional systems such as those described here can be crucially important for cost-sensitive applications.

FIG.5. (a) Schematic and design of a stretchable ultrasonic transducer array:(i) schematic of the device structure;(ii) exploded view illustrating each component of an element;(iii)optical image(bottom view)of the four elements,showing the morphology of the piezoelectric material and bottom electrodes;(iv)tilted SEM image of a 1–3 piezoelectric composite;(v)optical image(top view)of the four elements,showing the morphology of the backing layer and top electrodes;(vi)–(viii)optical images of the stretchable device when(vi)bent around a developable surface,(vii)wrapped on a surface that cannot be developed,and(viii)in a mixed mode of folding,stretching,and twisting,which show its mechanical robustness.Reproduced with permission from Hu et al.,Sci.Adv. 4,eaar3979 (2018).Copyright 2018 American Association for the Advancement of Science.(b)Schematic and working principle of a flexibl device for monitoring central blood pressure:(i)Schematic of stretchable ultrasonic device with key components marked.A high-performance 1–3 composite with periodic piezoelectric rods embedded in an epoxy matrix suppresses shear vibration modes and enhances longitudinal ultrasonic penetration into the skin.The device is mounted on the human neck and monitors central blood pressure by capturing the pulsating vessel diameter of the carotid artery,internal jugular vein,and external jugular vein,using a pulse–echo method as illustrated as the bottom left graph.The device can locate the dynamic anterior and posterior walls of the vessel using a high-directivity ultrasonic beam,as bottom middle graph shows.Signals reflecte from the anterior and posterior walls appear in the bottom right.(ii)-(iv)Demonstration of the mechanical compliance and robustness of the device.Reproduced with permission from Wang et al.,Nat.Biomed.Eng.2,687–695(2018).Copyright 2018 Nature Publishing Group.(c)Illustration of a fully integrated device that is slightly deformed.Reproduced with permission from Xu et al.,Science 344,70–74.Copyright 2014 American Association for the Advancement of Science.(d)Serpentine shaped structure:(i)island–bridge structure with serpentine geometry;(ii)geometric parameters for serpentine interconnection.Reproduced with permission from Fu et al.,Appl.Phys.Lett.106,091902(2015).Copyright 2015 AIP Publishing LLC.

C.Shaped electrical connections

In brittle inorganic materials,one important route to flex ibility and stretchability involves the use of specialized structural layouts and mechanical designs.42One of the most intuitive approaches is to adopt special electrical connections to transform in-plane stresses into out-of-plane electrode changes.The use of special electrode interconnections combined with inorganic materials based on elastomeric substrates has shown good strain tolerance.Because the strain and stress caused by bending and stretching are borne mainly by the flexibl interconnections on the substrate,a wide range of tensile properties becomes possible.

FIG.6.(a)Folded MEMS IMU:(i)device side of fla structure;(ii) handle side of fla structure;(iii) folded structure.The double-sided fabrication process and through-wafer interconnects enable effi cient utilization of the IMU internal volume.Reproduced with permission from Efimovskay et al.,J.Microelectromech.Syst. 26,1030–1039.Copyright 2017 IEEE.(b)Wafer-level fabrication process for a double-sided IMU with throughwafer interconnects.From Efimovskay et al.,in 2015 IEEE International Symposium on Inertial Sensors and Systems (ISISS), Hapuna Beach,HI,USA,pp.1–4.Copyright 2015 IEEE.Reproduced with permission from IEEE.(c)Flexible substrates with (i) origami and(ii) kirigami structures,and (iii) origamistructure substrate with parallel faces in its folded state.From Yasuga et al.,in 2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS),Vancouver,Canada,pp.909–912.Copyright 2020 IEEE.Reproduced with permission from IEEE.(d) Multifunction in-plane flexibl patch.From Yamamoto et al.,in 2017 IEEE 30th International Conference on Micro-Electro-Mechanical Systems (MEMS),Las Vegas,NV,USA,pp.239–242.Copyright 2017 IEEE.Reproduced with permission from IEEE.

Figure 5 (a) shows a device system that employs this strategy.Thin wavy metal lines directly deposited and patterned on the PI provide interconnections between the separately fabricated and assembled cells.By initially coating PI onto Cu sheets and then baking them on a hotplate,fiv separate pieces of Cu sheets were then patterned in an island–bridge-structured geometry using pulsed-laser ablation.PDMS served as a temporary substrate,onto which the PI/Cu sheet was laminated for laser ablation.The fiv layers were aligned to the previous layer of electrodes after assembly of the transducer arrays.For relatively small or negligible strain in the system,it could be designed to withstand different stretchability and twisting actions using a related strategy.106In a similar manner,high-performance composite material and flexibl interconnectors were used to form a conformal ultrasonic device that could monitor a central blood-pressure waveform [Fig.5(b)].107Figure 5(c) shows a soft microfluidi assembly of sensors,circuits,and radios for the skin fabricated using this strategy.108

The devices described in the preceding paragraph were based an“S”shaped geometry.In another approach,a serpentine geometry has been adopted.Figure 5(d) shows an example of the use of serpentine-shaped interconnectors to provide device flexi bility.The manufacturing process was compatible with the MEMS technology,and the serpentine interconnection carried most of the strain during the stretching process to ensure normal operation of the whole system.109In addition to the stretchable system shown Fig.5(d),other flexibl devices were also fabricated using serpentine structures.110In addition to the above examples,there have been a number of other studies of interconnection structure,for example,mechanical structure and optimization,107,111–122aimed at realizing flexibilit and stretchability of inorganic devices such as MEMS.

In addition to the methods described so far,origami and kirigami structures123–125have been extensively explored to provide flexibilit and stretchability of devices.Figure 6(a) shows a miniature 50-mm3folded inertial measurement unit (IMU) fabricated using a MEMS process.First,wafer-level and high-aspectratio single-axis sensors were fabricated and then interconnected using flexibl hinges,forming a 3D configuratio similar to a silicon origami.The process for a double-sided folded IMU was initiated using deep reactive-ion etching(DRIE)of 500-μm-deep blind via holes patterned on the handle side of a silicon-on-insulator(SOI)wafer.Then,flexibl hinges and interconnects were fabricated.Finally,the sensor was define through top-side processing and wafer etching using SiO2as a hard mask[Figs.6(a)and 6(b)].126,127Figure 6(c)shows an origami-structured flexibl electronic substrate whose faces are parallel to the target of the attachment surface.The parallel faces are important from an engineering point of view in that they make direct contact with the surfaces of the target of attachment and mounted electronic elements such as sensors or light emitters.These characteristics allow the realization of flexibl devices that can detect a shear force or flo velocity parallel to the object surfaces.128Using a kirigami structure,Yamamotoet al.129designed an allprinted planar-type multifunctional health-monitoring patch based on an in-plane PET fil integrated with an acceleration sensor for motion detection and with skin temperature and electrocardiogram sensors[Fig.6(d)].

FIG.7. (a) Response of buckled GaAs ribbons to stretching and compression.(b) SEM image of a sample formed using these procedures.Reproduced with permission from Sun et al.,Nat.Nanotechnol.1,201–207(2006).Copyright 2006 Nature Publishing Group.

Controlled buckling of semiconductor nanoribbons is another general approach for obtaining stretchable electronic devices.First,an elastomeric substrate is mechanically prestretched.Then,nanoribbons are define on it.Release induces compressive forces on the ribbons,which leads to“wavy”deformation (Fig.7).The buckling amplitudes can be controlled using the prestrain level.The tensile properties of the device are positively correlated with the amplitude of fluctuation130There have been a number of theoretical analyses of this strategy and investigations of the appropriate materials and their responses to applied strain.42,131–133More sophisticated systems that contain integrated circuits,MEMS devices,and microfluidic could be built on the basis of this approach.Since this strategy has been summarized elsewhere,130it will not be considered further in this paper.

III.FLEXIBLE MEMS DEVICES AND APPLICATIONS

A.Micromachined ultrasonic transducer

Among the most powerful MEMS devices that can be constructed using the approaches described above are micromachined ultrasonic transducers.134–144Figure 8(a)144shows a flexibl PMUT(FPMUT)array integrated on PDMS for brain ultrasound stimulation.Thinned bulk PZT,of thickness 55μm,was bonded to a silicon wafer.Then,a flexibl ultrasonic transducer array was fabricated by integrating it with a flexibl substrate,as described in Sec.II.The bending radius was less than 5 mm,thus allowing the device to be in contra-shape contact with a mouse brain.Good ferroelectric properties could be achieved using PZT thinning.By measuring the output sound pressure,the sound intensity was found to reach 44 mW/cm2at 80 V,which meets the requirements for ultrasonic neural stimulation.Researchers from KU Leuven designed and fabricated a bendable SOI-based PMUT array[Fig.8(b)].145,146PMUT array islands were connected to one another by silicon springs using a MEMS procedure without any additional processing.The PMUT performance was not compromised by the fabrication on silicon islands.Each bendable array contained six islands with dimensions 3×3 mm2,including 3×3 PMUT cells,in which each PMUT had a diameter of 410μm and a thickness of 6μm,with a 1-μm PZT layer as piezoelectric material.The PMUT array could withstand a bending radius of 1 mm.The PMUT elements in the array provided a displacement response of 2.1μm/V and aQfactor of 104 at 426 kHz.Another novel PMUT was fabricated in which preetched PZT blocks were introduced into holes in a PI substrate[Fig.8(c)].147,148Good conformal contact with skin surfaces and a suitable resonant frequency made this device suitable for heart imaging.The resonant frequency remained unchanged when the device was wrapped around a 1-cm diameter cylinder.This flexibl packed ultrasonic transducer also demonstrated good waterproof performance after hundreds of ultrasonic electric tests in water.Figure 8(d)149shows a conformable and stretchable PMUT interconnector that was combined with a flexibl substrate.The sandwiched structure of the bulk PZT array provided good ultrasonic penetration when tested on different pork tissues.The device had a resonant frequency of 321.15 kHz and a maximum sound pressure level (SPL) of 180.19 dB at a depth of 5 cm in water.The device was considered very promising for adjuvant treatment of bone injury.Figure 8(e)shows a typical example of an FPMUT fabricated using transfer printing of PMUT from a silicon donor wafer to a PI substrate,in a technique developed at Tianjin University.150The measured resonant frequency,effective coupling coefficient and displacement sensitivity of this device were 2.58 MHz,1.42%,and 30 nm/V,respectively.

FIG.8.(a)FPMUT array integrated on PDMS for brain ultrasound stimulation:(i)schematic of FPMUT integrated into a mouse brain array;(ii)photograph of fabricated FPMUT array with 16 elements;(iii)front-side optical images of fabricated FPMUT elements.Reproduced with permission from Lee et al.,Microsyst.Technol.23,2321–2328(2016).Copyright 2016 Nature Publishing Group.(b)Graphical illustration of an SOI-based PMUT array,including six silicon islands,and a 3×3 PMUT array connected by silicon springs.Reproduced with permission from Sadeghpour et al.,J.Microelectromech.Syst.29,378–386(2020).Copyright 2020 IEEE.(c)FPMUT based on preetched PZT blocks introduced into holes in a PI substrate:(i)schematic;(ii)the device wrapped around a finge.Reproduced with permission from Yang et al.,RSC Adv.3,24900–24905 (2013).Copyright 2013 The Royal Society of Chemistry.(d) Conformable and stretchable PMUT interconnector combined with a flexibl substrate:(i)schematic of FPMUT array;(ii)photograph of FPMUT array;(iii)FPMUT array attached to the skin of the arm;(iv)received signal at a frequency of 350 kHz.Reproduced with permission from Liu et al.,Sensors 20,86.Copyright 2019 Authors,licensed under a Creative Commons Attribution 4.0 License.(e)FPMUT fabricated by transfer printing of PMUT from silicon donor wafer to PI substrate:(i)schematic of FPMUT array;(ii)fabricated FPMUT array on thin polymer substrate.(iii)–(x)Schematic of the FPMUT fabrication process using transfer printing:(iii)and(iv)PMUT structure fabrication on a silicon wafer;(vi)and(viii)receiver-substrate preparation;(v),(vii),and(ix)PMUT transfer using a PDMS stamp;(x)peeling off from the silicon substrate.From Sun et al.,in 2018 IEEE International Ultrasonics Symposium(IUS),Kobe,Japan,pp.1–4.Copyright 2018 IEEE.Reproduced with permission from IEEE.

FIG.9.(a)Progress in research on CMUTs at the National Kaohsiung University of Applied Sciences.Reproduced with permission from Pang and Chang,Sensors 17,1443(2017).Copyright 2017 Authors,licensed under a Creative Commons Attribution 4.0 License.(b)Flexible CMUT array for side-looking IVUS imaging:(i)refillin throughwafer trenches in a flexibl silicon substrate with PDMS;(ii)schematic of catheter-based side-looking IVUS imaging probe.Reproduced with permission from Zhuang et al.,J Microelectromech.Syst.17,446–452(2008).Copyright 2008 IEEE.

Another important flexibl MUT is the capacitive micromachined ultrasonic transducer(CMUT).151–153Figure 9(a)154summarizes the progress made in research on these devices by the group at the National Kaohsiung University of Applied Sciences,culminating in the fabrication of a transparent flexibl CMUT using rolllamination technology.The fabrication temperature of this device was below 100° C,which is much lower than that for a conventional silicon-based CMUT and reduces the stress unevenness caused by high temperature.The high visible-light transmittance (>80%) of this transducer and its ability to operate on 40-mm-radius curved surfaces make it of critical importance for use in curved displays.Indium tin oxide–PET (ITO–PET) served as substrate,with SU-8 photoresist forming a sidewall,and vibrating membranes and a silver nanowire serving as a transparent electrode.As shown in Fig.9(b),Zhuanget al.155fabricated flexibl CMUT arrays for side-looking intravascular ultrasound(IVUS)imaging in which,to achieve flexibility deep trenches in a silicon substrate were refille by PDMS.The trench width varied between 6 μm and 20 μm,and the trench depth was 150 μm.The transducer array could be attached to a 450-μm-diameter tip,which illustrated the potential for catheter applications (this is less than the catheter size).The 250 × 250 μm2transducer elements had a resonant frequency of 5.0–4.3 MHz and a capacitance of 2.29–2.67 pF.It was experimentally verifie that the DC bias voltage ranged from 70 V to 100 V.Using a similar technique,Chenet al.156fabricated a curved row–column-structured CMUT array.The fina flexibl device exhibited a 4.5-MHz centre frequency with a −3 dB fractional bandwidth of 82%.

B.Resonators

Surface acoustic-wave resonators,157bulk acoustic-wave resonators,93,158–162electrostatic actuators,163and MEMS LWRs58,74have been demonstrated.Jianget al.75fabricated a 2.6-GHz air-gaptype thin-fil piezoelectric MEMS resonator on a flexibl PET fil using the FlexMEMS technique.At a quality factorQof 946,the performance was comparable to that of FBAR on a silicon substrate while being bent to 1 cm.As shown in Fig.10(a),164after transfer of an ultraflexibl FBAR from silicon onto a PI thin film another 4.6-μm-thick PI thin fil was installed as an encapsulation layer at the mechanical neutral plane.The flexibl resonator exhibited aQof 1108,an effective coupling coefficien of 5%,and a minimum bending radius of 0.5 mm.

Jianget al.78constructed a flexibl radio-frequency (RF) filte with a central frequency of 2.4 GHz based on fil bulk acousticwave resonators [Fig.10(b)].The active material for each FBAR,which was a sandwiched structure with thin-fil electrodes,was aluminum nitride thin film The filte had a peak insertion loss of −1.14 dB,a −3 dB bandwidth of 107 MHz and a temperature coefficien of frequency of −27 ppm/° C.At the test state of 2.5-mm bending radius,the filte showed no significan performance degradation.This filte shows promise as an important component of future flexibl wireless-communication systems.

LWRs73,74have also been demonstrated on a flexibl substrate (PET) [Fig.10(c)].58Using a PDMS stamp,three different LWR topologies were simultaneously transferred to a flexibl substrate with air cavities using the FlexMEMS technique.The resonators exhibited similar performance to their counterparts on a silicon substrate and demonstrated excellent mechanical stability and flexibilit under deformation.They had a quality factor of 1294 and a coupling coefficien of 2.2%.Jinet al.165deposited ZnO nanocrystals on a cheap and bendable plastic fil[Fig.10(d)].The flexibl surface acoustic wave(SAW)devices thus fabricated exhibited two wave modes:Rayleigh and Lamb waves with resonant frequencies of 198.1 MHz and 447.0 MHz,respectively.Particle concentration using SAWs was achieved with these devices.

C.Tactile and strain sensors

Conventional force and strain sensors can be classifie mainly as piezoresistive,capacitive,piezoelectric,and resonant types.166–168Flexible tactile and strain sensors have great potential for application in health monitoring and medicine,where they are required to make conformal contact with curved or nonconformal surfaces.169Shiet al.170constructed a flexibl pressure sensor based on electroplated-Ni film in which mechanical cracking is induced by stress concentration when the sensor is subjected to pressure,and this cracking in turn causes changes to the sensor resistance.This pressure sensor has a small response time,which is promising for wearable-device application such as pulse detection.Figure 11(a)shows a MEMS–CMOS integrated tactile sensor on a flexibl and stretchable bus covering a social robot body.120In this configuration the CMOS substrate was flip-bonde to a low-temperature co-fire ceramic(LTCC)substrate,which simplifie the fabrication process.A flexibl and stretchable wire was fabricated using metal etching and PI laser cutting.These tactile sensors could send coded digital signals under different forces.

FIG.10.(a)Flexible FBAR with a total thickness of 11.6 μm:(i)schematic of flexibl FBAR encapsulated in PI thin film (ii)cross-sectional schematic of the thin-fil layers of flexibl FBAR embedded at the neutral plane;(iii)photograph of the flexibl FBAR wrapped around a finge.From Zhang et al.,in 2018 IEEE Micro Electro Mechanical Systems(MEMS),Belfast,UK,pp.170–173.Copyright 2018 IEEE.Reproduced with permission from IEEE.(b)Flexible RF filter (i)circuit topology diagram with three series and two shunt resonators;(ii)and(iii)cross-sectional schematics of the donor device;(iv)schematic of the main steps of the receiver-substrate preparation and fina assembly;(v)front and(vi)backside microscopic images of the flexibl filter (vii)photograph of flexibl filte bent on a glass stirring rod.Reproduced with permission from Jiang et al.,Small 14,1703644(2018).Copyright 2018 WILEY-VCH Verlag GmbH &Co.(c)LWRs on a flexibl substrate:(i)photograph;(ii)microscope image;(iii)SEM image of flexibl LWR with interdigital transducers(IDTs)at the top;(iv)zoomed-in view of LWR with IDTs at the top.Reproduced with permission from Yang et al.,IEEE Electron Device Lett.38,1125–1127(2017).Copyright 2017 IEEE.(d)Flexible thin-fil ZnO/PI SAW devices.Reproduced with permission from Jin et al.,Sci.Rep.3,2140(2013).Copyright 2013 Nature Publishing Group.

FIG.11.(a)Structure of a CMOS-on-LTCC differential capacitive tactile sensor:(i)bird’s eye view;(ii)cross-sectional view.From Asano et al.,in 2015 Transducers—18th International Conference on Solid-State Sensors,Actuators and Microsystems(TRANSDUCERS),Anchorage,AK,USA,pp.97–100.Copyright 2015 IEEE.Reproduced with permission from IEEE.(b)Schematics of a differential piezoresistive wind-speed sensor:(i)substrate under no-wind condition;(ii)substrate bending owing to wind;(iii)cross section of sensor under no-wind condition;(iv)cross section of the sensor affected by wind.Reproduced with permission from Yi et al.,Electron.Lett.56,201–203(2020).Copyright 2020 The Institution of Engineering and Technology.(c)3D model of a cross-sectional view of a force sensor with exaggerated z axis.Reproduced with permission from Ahmed et al.,IEEE Sensors J.13,4081–4089(2013).Copyright 2013 IEEE.(d)Photograph of an ultrathin microsensor.174 From Lee et al.,in 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems(NEMS)2012,Kyoto,Japan.Copyright 2012 IEEE.Reproduced with permission from IEEE.(e)Schematic of the fragmentation of SWCNT paper in elastic substrates.The inset is an x–z cross section of the stretched sample.Reproduced with permission from Zhou et al.,ACS Appl.Mater.Interfaces 9,4835–4842(2017).Copyright 2017 American Chemical Society.(f)Schematic of nanocracking strain sensor fabrication and encapsulation.Reproduced with permission from Park et al.,Sensors 18,2673(2018).Copyright 2018 Authors,licensed under a Creative Commons Attribution 4.0 License.

FIG.12.(a)Wafer transfer-technology process flo for RF MEMS switch transfer:(i)reversed process sequence in fabricating an RF switch on a silicon wafer;(ii)thermal bonding to an FR-4 substrate;(iii)grinding and removal of silicon;(iv)RF switch-release etch using HF.From Zhang et al.,in 56th Electronic Components and Technology Conference 2006,San Diego,CA,USA.Copyright 2006 IEEE.Reproduced with permission from IEEE.(b)Schematic of a MEMS ultrasonic actuator:(i)MEMS actuator array on a flexibl PEN substrate;(ii)cross section of the sandwiched MEMS actuator on PEN,which is made of aluminum and 4-μm-thick parylene;(iii)specification of the MEMS actuator.Reproduced with permission from Kim et al.,IEEE Electron Device Lett.33,1072–1074(2012).Copyright 2012 IEEE.(c)MEMS micro-actuator array for tactile display.Reproduced with permission from Streque et al.,IEEE Trans Haptics 3,88–97(2010).Copyright 2010 IEEE.(d)MEMS accelerometers on a peeled-off flexibl substrate.Reproduced with permission from Gonenli et al.,IEEE Sensors J.11,2318–2326(2011).Copyright 2011 IEEE.(e)Single-electrode-mode textile-TENG made up from three different functional fabrics based on PET fabric.Reproduced with permission from Xiong et al.,Nat.Commun.9,4280(2018).Copyright 2018 Nature Publishing Group.

A differential piezoresistive wind-speed sensor has been fabricated by mounting two piezoresistors with the same size on both surfaces of a flexibl substrate [Fig.11(b)].171The substrate bends under the action of wind,resulting in changes in electrical resistance that depend on the wind speed.Experiments have shown that the sensor can measure wind speeds of up to approximately 20 m/s.Motivated by the need to measure the force and pressure on nonplanar surfaces for structural health monitoring,MEMS force sensors embedded in flexibl PI substrates have been constructed,with an average piezoresistive gauge factor of 1.75 [Fig.11(c)].172The sensors have an average sensitivity of 1.25 V/N,with values ranging from 0.266 to 2.248 V/N.By using transfer printing,a fragile strain sensor has been integrated onto a flexibl printed-circuit substrate.The output voltage of the manufactured sensor exhibited a linear dependence on strain.79High-performance wearable strain sensors based on a graphene/polymer composite fil have been demonstrated for human-motion detection.173Other pressure sensors100,174[Fig.11(d)]and tactile sensors175have also been explored for applications in health care,176–179in intracortical probe insertion mechanics,180and as shock sensors.91

With the use of metallic nanoparticles and nanowires,carbon nanotubes (CNTs),or graphene-based materials,181the challenges posed by the fabrication of flexibl high-stretchability strain sensors can be partially overcome.Channel nanocrack-based strain sensors(nCBSSs)and network nCBSSs have shown great potential.182Zhouet al.183demonstrated strain sensors based on fragmented singlewalled carbon nanotube(SWCNT)paper embedded in PDMS that retained sensitivity even at very high strain levels [Fig.11(e)].The sensing layers remained unaffected when a novel one-step laser encapsulation(OLE)method was used.Subsequently,nanocracking strain sensors were developed to detect motions around eyes,where oil and sweat are generated.Figure 11(f)184presents a schematic of nanocracking strain sensor fabrication and encapsulation.Nanocracking strain sensors also show great potential as audio analog-todigital converters for acoustic signature recognition and for other large motions of the human body.

D.Other devices

In addition to the flexibl MEMS devices described above,a number of other flexibl MEMS devices have been presented.These include RF MEMS switches.185–187Researchers from Nanyang Technological University transferred RF MEMS switches onto flexibl printed-circuit boards using thermal compressive bonding,mechanical grinding,and wet removal of silicon [Fig.12(a)].185These switches demonstrated low insertion loss(≤0.15 dB at 20 GHz)and high>isolation (∼21 dB at 20 GHz),which illustrates the potential for integration of RF MEMS components with other RF devices on organic substrates.185,187

In addition to RF MEMS switches,electrostatic actuators on a flexibl substrate were demonstrated by Kimet al.,188who presented a MEMS system on a flexibl substrate interfaced with integrated driver circuitry[Fig.12(b)].A suspended diaphragm,made of parylene,served as a passivation layer to generate an acoustic pressure wave perpendicular to the substrate.A single actuator had an SPL of 27 dB at 25 kHz,and a 1×2 array emitted up to 34.6 dB SPL at 1-cm distance.A 4×4 flexibl micro-actuator array with a resolution of 2 mm was presented by Strequeet al.189[Fig.12(c)].Each actuator was made of a PDMS elastomeric membrane,which was magnetically actuated by coil–magnet interaction.This technique has potential for application in tactile displays and offers great advantages in terms of comfort in use.

Microcantilevers could be used as flexibl stethoscope transducers.96A PZT thin fil was deposited and patterned on silicon nitride,which served as the supporting layer,using the sol–gel technique.Polymer was coated on both sides of a 500 × 200 × 1 μm3PZT cantilever beam to ensure flexibilit of the sensor array.A PDMS-based electrostatically actuated MEMS cantilever beam was presented by Singh and Patrikar.190

Using surface micromachining,three-axis accelerometers were realized on Si and flexibl PI substrates [Fig.12(d)].101Testing revealed high sensitivities ranging from 22 fF/g to 27 fF/g in thezaxis and from 12 fF/g to 18 fF/g in the lateral axes.Wafer-level packaged MEMS accelerometers between two PI layers were fabricated by Mahmood and Çelik-Butler.191The devices could be bent up to a 1.0-cm radius and were compatible with robotic or prosthetic fingertips In addition to these devices,flexibl MEMS accelerometers for failure assessment in aerospace systems99and packaged flexibl and bendable micro-accelerometers187have been demonstrated.

In the area of power generation,the availability of MEMS has promoted the development of flexibl triboelectric nanogenerators(TENGs)that convert mechanical energy into electricity.192A TENG based on contact electrificatio coupled with electrostatic induction between two media for energy conversion from dynamic stimuli was reported by Fanet al.193A wearable and washable textile-TENG was demonstrated using a synergetic triboelectric trapping layer of black phosphorus protected by cellulose-derived hydrophobic nanoparticles[Fig.12(e)].194To widen the range of applications,attempts have been made to improve the performance of TENGs in various ways,such as modificatio of surface materials,introduction of micro/nanostructures,and fabrication of novel device structures.

IV.DISCUSSION:CRITERIA AND CHALLENGES

The following criteria should be satisfie by a flexibl MEMS device and its fabrication method:

1.To provide intimate contact with a complex surface,the device should be as bendable as possible and should preferably also have a certain level of stretchability.

2.The device should have stable and reliable performance under bending or stretching conditions.

3.Its fabrication process should preferably be compatible with the MEMS process to reduce cost,achieve high yield,and maintain high production throughput.

The transfer-printing technique transfers a MEMS structure from a donor wafer to a target polymer substrate.Thin-fil MEMS structures thus produced can achieve high bending capability.In addition,transfer printing does not have strict requirements on the thermal budget during the manufacturing process,and therefore is compatible with the MEMS process.The use of van der Waals forces and a high-precision alignment machine allows low-cost production.However,a large-area flexibl array must satisfy stringent requirements in terms of the alignment precision of the transfer machine.The disadvantage is that transfer printing suffers from difficultie in satisfying robust boundary constraints.In the case of bending and stretching,device performance is significantl affected by the strength of the boundary constraints,and inappropriate constraints can lead to device failure.

Rigid devices can be integrated directly into flexibl substrates.Although the minimum bending radius is limited by the size of the rigid component,and the conformal contact with a complex surface is poorer,the device performance usually remains good.Simultaneous integration of different devices is also possible for a wide range of applications.By contrast,when MEMS structures are directly constructed on a flexibl substrate,the quality of the film and MEMS structures on the substrate is limited because of process incompatibility,and therefore the device performance is degraded.

In the strategy of shaped electrical connection,specially shaped electrodes with functional layers are connected to a device,with system deformation being provided by a patterned electrode.This strategy facilitates integration of devices in 3D space.The resulting systems can bear large deformation and provide conformal contact with complex surfaces.However,the fabrication process is complex,and therefore achieving high process yield and throughput is difficult

The crucial challenges for realizing flexibl and stretchable MEMS devices are as follows:

1.MEMS structures must be able to move or be coupled with the external environment during operation.Therefore,appropriate boundary-constraint conditions should be met.For example,cavities in a flexibl substrate and seamless bonding between the MEMS structure and substrate are very important.In particular,in the case of a large-area device array,alignment between structures and cavities seriously affects the device performance.A large misalignment can lead to serious degradation or even device failure.

2.In a flexibl MEMS array,cross talk among elements cannot be neglected.For example,in imaging applications,there is significan coupling among the ultrasonic transducers in the array,whereas this does not usually need to be considered in conventional silicon-based PMUT arrays.Strong coupling significantl affects key resolution specifications195,196MUTs based on a polymer substrate typically have a much lower output pressure than those based on a silicon substrate.For operation at a given resonance,a polymer membrane must be much smaller than a silicon membrane,because of its low flexura rigidity.

3.MEMS devices cannot operate properly without suitable and reliable packaging.MEMS devices contain movable and fragile structures,and therefore their packaging is different from that of conventional integrated circuits.197Wafer-level capping and hermetic/vacuum sealing are often required in MEMS devices.Reliable packaging of MEMS devices while maintaining mechanical flexibilit is challenging.In addition,connection of MEMS devices to external circuits is often necessary.For flexibl MEMS devices,achieving connections with low electrical loss and high mechanical robustness is a challenging task.

V.CONCLUSIONS AND OUTLOOK

The work summarized in this paper has provided convincing proof that high-performance flexibl and stretchable MEMS devices can be constructed using appropriate strategies.Fabrication processes include transfer printing,shaped electrical connection,and heterogeneous integration using diverse classes of inorganic material-based MEMS structures and flexibl substrates (plastics,elastomers,and others).The flexibl MEMS devices reviewed here include,among others,MUTs,pressure sensors,strain sensors,and resonators.Various disciplines,ranging from mechanical engineering,electrical engineering,and chemistry to materials science,have been involved in efforts to realize flexibl MEMS sensors and systems.

From an engineering perspective,future flexibl systems will be greatly influence by the availability of flexibl and stretchable MEMS devices.In biomedical applications,the low mechanical modulus of flexibl MEMS devices will allow them to match shape with biological tissues.Moreover,system integration of flex ible MEMS devices will contribute to future consumer electronics and the rapid development of the Internet of Things.In our view,the fundamental challenges posed by the development of flexibl systems,and their wide scope of application,together with the considerable social implications of their use,provide strong motivation for continued and expanded research efforts in this field42

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.62001322),the Tianjin Municipal Science and Technology Project (No.20JCQNJC011200),the National Key Research and Development Program of China (No.2020YFB2008801),and the Nanchang Institute for Microtechnology of Tianjin University.

The authors declare that they have no known competing finan cial interests or personal relationships that could have appeared to influenc the work reported in this paper.