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Journal Club Theme of May 2016: Recent Progress in Curvilinear Electronics and Mechanics

Jianliang Xiao's picture

Recent Progress in Curvilinear Electronics and Mechanics

Jianliang Xiao

Department of Mechanical Engineering, University of Colorado Boulder

1. Introduction

Stretchable electronics has attracted a lot of attention from both academia and industry due its enormous application in many areas. It brings rubber-like mechanical properties to the otherwise rigid and brittle electronics systems, while retains the high electrical performances of conventional wafer based electronics. Stretchable electronics is usually soft, elastic, and can be freely deformed into complex shapes. Therefore, it can intimately wrap onto 3D complex, arbitrary curvy surfaces, for applications ranging from biointegrated electronics for diagnosis, treatment and human-machine interfaces, to bio-inspired devices that utilizes curvilinear configurations to achieve superior performances. In the past decade, tremendous amount of work has been done on stretchable and flexible electronics, it’s impossible to cover all of them. In this discussion, I’ll specifically focus on recent efforts and achievements in developing curvilinear electronics, and the associated mechanics issues.

2. Bio-integrated Curvilinear Electronics

Because of its soft, elastic nature, curvilinear electronics can be designed to match with the complex, curvy surfaces of human body, to realize bio-integrated electronics for applications such as health monitoring, diagnosis, therapy and drug delivery. Figure 1a is a flexible electronic sensor integrated on the surface of a porcine heart [1]. Because of its ultrathin thickness and extreme mechanical flexibility, the sensor can be strongly bonded on the surface by utilizing capillary force provided by bio fluid even when the heart is beating. This sensor monitors in real time the high density cardiac electrophysiological signals, and therefore can provide important information for diagnosis. Figure 1b shows a 3D multifunctional integumentary membrane wrapped around a rabbit heart [2]. It integrates temperature sensors, pH sensors, ECG sensors, strain gauges and LEDs, and offers capability of measuring different signals simultaneously.  Figure 1c exhibits a flexible mechanical energy harvester based on PZT, integrated on the surface of a bovine heart [3]. During heart beating, it can generate energy that can be stored in a rechargeable battery to support a pacemaker or other devices that need power supply. Figure 1d is a meshed array of extremely flexible electrodes, intimately integrated with the very complex surface features of a feline brain [4]. It can provide accurate, high-density electrocorticography (ECoG) mapping of the brain.

 

Figure 1. Bio-integrated curvilinear electronics. (a) flexible cardiac sensor mounted on a porcine heart, (b) 3D multifunctional integumentary membrane wrapped around a rabbit heart, (c) flexible mechanical energy harvester integrated on a bovine heart, (d) Flexible mesh array of electrodes on a feline brain, (e) epidermal electronics, (f) wearable drug delivery system, (g) stretchable electronics integrated with a glove, (h) electronic devices mounted on different regions of the ear for EEG monitoring.

The above devices can provide direct and accurate measurement of signals from different organs, but they requite invasive surgery to integrate devices, which is not feasible for daily application. Therefore, electronic devices that can be integrated with human skin are desired. To be suitable for daily use, such devices need to be very stretchable, soft, shape conformal, biocompatible, and breathable, so that these devices can function during daily activity and don’t make people feel uncomfortable. Figure 1e demonstrates epidermal electronics that can be bonded very well onto human skin, even during stretching, compression and torsion [5]. Multifunctional sensors are integrated to provide monitoring of strain, ECG, EMG, and temperature information. Antenna, and wireless power coils, RF coils and diodes are also integrated to provide wireless charging and communication capabilities. Figure 1f is a wearable drug delivery system that can be integrated with human skin [6]. Release of drug in this device can be controlled by stretching applied to the system, to provide anticancer and antibacterial treatment. Figure 1g illustrates stretchable electronics integrated with a glove [7]. Figure 1h shows electronic devices mounted on different regions of the ear, to conveniently monitor EEG signals [8]. Such devices can also be extended to provide brain computer interface.

Mechanics issues associated with these systems include but are not limited to: (1) how to design the electronics to be stretchable, flexible, and shape conformal? (2) how to ensure good contact between the electronics and the surface human skin/organs? (3) how to design the electronics to ensure human comfort when they are integrated with human body? (4) how to ensure reliability of the electronics during complex dynamic loading cycles?

3. Bio-inspired Curvilinear Optoelectronics

Natural imaging systems demonstrate many attractive attributes that cannot be found in commercially available cameras. For example, human eyes form very sharp images with very simple optics. On the contrary, each commercial camera needs a complex set of optical lenses to adjust optical paths of the light rays to achieve comparable sharpness of imaging. Another type of natural imaging system is compound eyes that are commonly found in arthropods. A compound eye is usually composed of hundreds or thousands of individual imaging units, i.e. ommatidia, on a hemispherical surface. Such hierarchical structures of compound eyes, although cause reduced resolutions, can provide very wide field of view angle, low aberration, high sensitivity to motion and infinite depth of field. One common feature in these natural imaging systems, including human eyes and compound eyes, is that the light sensing elements are distributed on curved surfaces. In the human eyes, the sensing elements are located on concave, hemispherical surfaces to accommodate the hemispherical imaging planes formed by the simple optics. In compound eyes, the sensing elements are located on convex, hemispherical surfaces to coordinate with the 3D microlens array to achieve their unique imaging characteristics. Therefore, to achieve the imaging characteristics found in natural eyes in cameras, photodetectors have to be placed on similar curvilinear surfaces.

Figure 2. Bio-inspired curvilinear optoelectronics. (a) Electronic eyeball camera, (b) Tunable electronic eye with zoom, (c) Artificial apposition compound eye camera.

Figure 2a shows a fully functional hemispherical electronic eyeball camera that mimics the structure and function of human eye [9]. This layout of photodetectors was first fabricated in planar geometry and then transformed into hemispherical shape to resemble the geometry of retina in human eyes. Good imaging capability has been demonstrated with a simple plano-convex lens. Figure 2b exhibits a tunable hemispherical eyeball camera system that combines the advantages of human eyes and commercial cameras [10]. In this system, the curvatures of both the optical lens and the hemispherical imaging plane can be adjusted coordinately to realize zoom, a capability can be found in commercial cameras but not in human eyes. Figure 2c is an artificial apposition compound eye camera, mimicking the compound eyes of insects [11]. Arrays of elastomeric microlenses and stretchable photodetectors were separately fabricated and integrated in their planar geometries, which were then transformed into a hemispherical shape to realize the artificial compound eye camera. This digital camera exhibits an extremely wide field of view angle (160 degrees) and infinite depth of filed.   

Mechanics issues in these systems include: (1) how to design the optoelectronic system to sustain large deformation during shape transformation? (2) how to predict and track the positions of photodetectors to ensure accurate postprocessing of images? (3) how to predict the deformation of the system to guide operation and adjustment of different elements to comply with optics? (4) how to ensure reliability during long term operation?

4. Discussion

Curvilinear electronics has shown incredible advancement in the past decade, and many interesting applications have been demonstrated. Mechanics of different systems has also been studied [12-16]. Some more recent studies have been trying to combine curvilinear electronics with soft active materials to realize smart electronics that can respond to environment, or can change shapes in a programmable manner [17-18]. Despite all these exciting achievements in curvilinear electronics, one challenge to be resolved before mass production and commercialization is how to manufacture stretchable electronics and systems in an economical way. This work demonstrated an interesting way to achieve this goal by mechanically cutting laminated thin films into stretchable layouts [19].

References:

[1] J. Viventi et al., A Conformal, Bio-interfaced Class of Silicon Electronics for Mapping Cardiac Electrophysiology, Science Translational Medicine 2, 24ra22 (2010).

[2] L. Xu et al., 3D multifunctional integumentary membranes for spatiotemporal cardiac measurements and stimulation across the entire epicardium, Nature Communications 5, 3329 (2014)

[3] C. Dagdeviren et al., Conformal piezoelectric energy harvesting and storage from motions of the heart, lung, and diaphragm, Proc. Natl. Acad. Sci. USA 111, 1927 (2014)

[4] D.-H. Kim et al., Dissolvable Films of Silk Fibroin for Ultrathin Conformal Bio-Integrated Electronics, Nature Materials 9, 511-517 (2010)

[5] D.-H. Kim et al., Epidermal Electronics, Science 333, 838 (2011)

[6] J. Di et al., Stretch-Triggered Drug Delivery from Wearable Elastomer Films Containing Therapeutic Depots, ACS Nano 9, 9407-9415 (2015)

[7] H.C. Ko et al. Curvilinear electronics formed using silicon membrane circuits and elastomeric transfer elements. Small 5, 2703 (2009).

[8] J.J.S. Norton et al., Soft, curved electrode systems capable of integration on the auricle as a persistent brain–computer interface, Proc. Natl. Acad. Sci. USA 112, 3920-3925 (2015)

[9] H. C. Ko et al., A Hemispherical Electronic Eye Camera Based on Compressible Silicon Optoelectronics. Nature 454, 748-753 (2008).

[10] I. Jung, J. Xiao et al., Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability, Proc. Natl. Acad. Sci. USA 108, 1788-1793 (2011)

[11] Y. M. Song, Y. Xie, V. Malyarchuk, J. Xiao et al., Digital Cameras With Designs Inspired By the Arthropod Eye, Nature 497, 95–99 (2013)

[12] S. Wang, J. Xiao, I. Jung, J. Song, H. C. Ko, M. P. Stoykovich, Y. Huang, K.-C. Hwang and J. A. Rogers, Mechanics of Hemispherical Electronics, Appl. Phys. Lett. 95, 181912 (2009).

[13] S. Wang, J. Xiao, J. Song, H. C. Ko, K.-C. Hwang, Y. Huang, and J. A. Rogers, Mechanics of curvilinear electronics, Soft Matter 6, 5757–5763 (2010)

[14] C. Lü, M. Li, J. Xiao, I. Jung, J. Wu, Y. Huang, K.-C. Hwang, and J.A. Rogers, Mechanics of tunable hemispherical electronic eye camera systems that combine rigid device elements with soft elastomers, Journal of Applied Mechanics-Transactions of the ASME 80, 061022 (2013)

[15] Z. Li, and J. Xiao, Mechanics and Optics of Stretchable Elastomeric Microlens Array for Artificial Compound Eye Camera, J. Appl. Phys. 117, 014904 (2015)

[16] Z. Li, and J. Xiao, Strain tunable optics of elastomeric microlens array, Extreme Mechanics Letters 4, 118-123 (2015) 

[17] C. Yu et al., Electronically Programmable, Reversible Shape Change in Two- and Three-Dimensional Hydrogel Structures, Adv. Mater. 25, 1541-1546 (2013)

[18] S. Lin et al., Stretchable Hydrogel Electronics and Devices, Adv. Mater., DOI: 10.1002/adma.201504152 (2015)

[19] S. Yang et al.,  “Cut-and-Paste” Manufacture of Multiparametric Epidermal Sensor Systems, Adv. Mater. DOI: 10.1002/adma.201502386 (2015)

 

Comments

Hi Jianliang,

Thank you for sharing! But I have a question: when the electronics working, will it heating up the body tissues? 

Jianliang Xiao's picture

Hi Yuhang,

Thanks for the comments! Yes, heating will definitely be an issue, especially for processing units and LED devices. Not only heating the tissue could be an issue, the heating in devices could also cause failures due to  high temperature or heat induced mechanical breakdown. In other words, packaging and heat management are also essential. There are some studies on packaging related heat management issues:

Flat flexible polymer heat pipes, J. Micromech. Microeng. 23 (2013) 015001;

Design, fabrication and performance tests for a polymer-based flexible flat heat pipe, Energy Conversion and Management, Volume 70, June 2013, Pages 10–19;

Flexible and conformal thermal ground planes,  Proc. 37th Annual Government Microcircuit Applications and Critical Technology Conference (GOMACTech), March 19-22, 2012, Las Vegas, NV

Flexible Thermal Ground Plane Enabled by ALD/MLD-Based Barrier Coatings, IMAPS/ACerS 5th International Conference and Exhibition on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT 2009), April 20-23, 2009, Denver, CO

<p>Jianliang,</p>
<p>Thanks for such an interesting brief review on curvilinear electronics and related mechanics. It is very useful for those working on the issues.</p>
<p>Rui</p>

Jianliang Xiao's picture

For the nice comments!

YongAn Huang's picture

Jianliang, thank you for sharing such a great topic. I am focusing on the manufacturing of large-area, flexible curvilinear electronics. The future is flexible, but also curviliear. Currently, the transfer printing technique is adopted to bridge the gap between Si-based microelectronics and flexible electronics. However, how to address the curviliear transfer printing for large flexible (not stretchable) electronics. After all, the stretchable electronics integrated with PDMS or Ecoflex are suitable for human body, rather than fixed curviliear surface in high-temperature environment. 

Jianliang Xiao's picture

Yongan,

You are working on a very important issue. How to manufacture flexible, curvilinear electronics so that they can reach the market with reasonable price is very critical. Both US and China governments have put down a lot of resources to address manufacturing challenges. For example, US government put down $75 million dollars to establish a flexible electronics manufacturing institute (NextFlex) last year. Lots of challenges, but also opportunities.

YongAn Huang's picture

Jianliang,

The flexible electronics become very important research field in government, industry community and academic community in US. However, in China, the research group of flexible electronics plays relatively small role in the country’s national program planning, relative to other fields such as microelectronics, nanotechnology, 3D printing, laser fabrication, and so on.

Last year, our Flexible Electronics Research Center, combining the NSFC, hosted an high-level forum for flexible electronics. Prof. Yonggang Huang was invited to attend this forum, and gave a very important speech. It's very gratifying to see that the NSFC released General Project Group for flexible electronics this year. Meanwhiel the flexible display has attracted national attention. Even so, the manufacturing falls behind the design of flexible electronics, and becomes the bottleneck for flexible electronics into the market.

Jianliang Xiao's picture

Yongan,

Thanks for the comments!

ShuodaoWang's picture

Jianliang,

Thanks a lot for sharing the nice review!  I agree that achieving conformability and good interfacial coupling between electronics and bio-tissues remain the most critical mechanics challenges in applications similar to those in Section 2.  Strategies to accommodate the continuous efflux of dead cells from the surface of the tissues and the processes of transpiration will also be needed in these bio-integrated applications, especially when used for extended amount of time.

Jianliang Xiao's picture

Shuodao,

Thanks for the comments! You are absolutely right. There could also be biological issues associated with long-term integration of electronics, and these have not been looked into yet. 

Jianliang Xiao's picture

Shuodao,

Thanks for the comments! You are absolutely right. There could also be biological issues associated with long-term integration of electronics, and these have not been looked into yet. 

Yonggang Huang's picture

Jianliang,

Thanks for the very nice overview on curvilinear electronics.  It is very clear that mechanics plays a critical role in this area.  In addition, I would like to add that research in this area has already led to opportunities for commercializations, such as the first head-mounted impact monitor CHECKLIGHT for sports developed by Reebok and MC10, Inc. (already on the market), the skin-patched UV dosimeters by L'Oreal, and the skin-mounted health monitor BioStamp (to measure ECG, EMG, ...) by MC10, Inc. (already on the market).      

Jianliang Xiao's picture

Prof. Huang,

Thanks for the comments! It's exciting to learn these products are already on the market. There are a lot more opportunities for flexible electronics to change and improve people's lives. Wearable electronics will the next big milestone for electronics industry.

chaofenglu's picture

Jianliang,

Thanks for sharing your latest review on curvilinear electronics. For micro/nano scaled LED or photodetector elements made of wirtzite crystals, the existence of piezoelectricity may pose significant influences on the transport of electron in the funcitonal elements.  This means that the photonic-electronic coupling behavior, or the luminace efficiency of LED or the efficiency of photodetector, may be enhanced or restrained by mechanical deformation. We may use this feature to design environment adaptable optoelectronic devices. For example, tuning the luminance intensity of ILED by exerting mechanical deformation.

Jianliang Xiao's picture

Chaofeng,

Thanks for the comments! Yes, in many ways, deformation can also be useful to tune electrical, photonic and optical properties. You pointed out a very good example.

Jianliang,

Thank you for sharing the nice review on curvilinear electronics.  Stretchable electronics have many important medical applications.  I completely agree with you about the mechanics issues associated with these systems, which are important for the applications of the stretchable electronics.  These are both challenges and opportunities for mechanics.

Jianliang Xiao's picture

Jian,

Thanks for the comments! Yes, there are still many challenges that need to be addressed in the development of not only curvilinear electronics, but also generally stretchable electronics. I have listed a few, but certainly there many more that I didn't cover. I'd welcome any comments and discussion on possible challenges and problems in this area.

Zhigang Suo's picture

Here is a new review article on stretchable electronics by Hussain and co-workers.

Jianliang Xiao's picture

Zhigang,

Thanks for sharing this article! A very good review on stretchable electronics.

Yonggang Huang's picture

Jianliang,

Your overview focuses mainly on curvilinear INORGANIC electronics.  There exists a large amount of work on ORGANIC electronics.  Any comments?  Particularly the advantages of each?

Jianliang Xiao's picture

Yes, there are a lot of research focuses on organic electronics as well. The most important drawback of organic electronics is the charge mobility of organic semiconductors, which is far lower than typical inorganic semiconductors. This limits the application of organic electronics only in low speed electronics. 

On the other hand, organic electronics is intrinsically stretchable, but the stretchability is only a few percent, as far as I know. It's certainly better than inorganic semiconductors (1%), but to make really stretchable devices, design in mechanics, similar to stretchable inorganic electronics, is still needed. 

Lihua Jin's picture

Thanks Jianliang for the nice review. Thanks Yonggang for examples of wearable devices on the market. I am interested to get the UV dosimeters by L'Oreal :)

Organic materials have a lot of choices of side chains, copolymers, chain interaction, and chain morphology, which makes it more feasible to design materials to realize stretchability. One example is shown here

http://pubs.acs.org/doi/abs/10.1021/ma500286d

Jianliang Xiao's picture

Thanks Lihua for sharing this nice reference on organic electronics. 

Dear Jianliang,

Thanks for providing the insight for recent developments of curvilinear electroics.  Stretchable inorgnic devices are definitely attractive, but there  other unique benefits from organic devices.  For example, the conducting polymer has the potential to transport the neurotransmitters in their ionic form.  As electrical signaling occurs with ions and protons rather than electrons in biological systems, the device that can monitor and control ionic/protonic currents provides an ideal interface (http://www.nature.com/articles/ncomms1489).  Therefore, I think integration from both these two types of materials would be interesting.

In addition, what's your perspective of the future development for stretchable devices?

mingli's picture

Hi, Jianliang, very good review on the flexible electronics. And I agree with Shuodao and you that achieveing comformability is important and the most critical challenges in the further. But how to define this as a mechanical problem? It is easy to understand that to design the electronics to be stretchable, to ensure good contact, to ensure reliability are mechanical problem, since we can have some mechanical indice, such as maximum strain, contact pressure, fatigue life as objective function when seeking the optimal design. but to ensure human comfort, can we find some mechanical parameter to characterize the conformability?

Jianliang Xiao's picture

Ming,

Good point! Yes, I think these mechanical properties can be used to guide the design to ensure human comfortness:

1. effective modulus, if it's comparable or even smaller than human skin/tissue, then human can hardly feel the electronics, as studied in the epidermal electronics discussed above

2. stretchability, if the electronics can be stretched more than the skin, it's also hard for people to notice its existence

3. breathability, to make the device permeable to air and sweat.

There could be other parameters that can be used to quantify human comfortness, any thoughts?

mingli's picture

Hi, Jianliang,

good suggestions.  However, the definition of  effective modulus and stretchability are actually responding to the first two questions at the end of section II in your review, even without consideration of human conformatness, we also need to consider the effective modulus and stretchability to ensure its performance and longvity, is this right?

breathability might be good choice, but how to reflect this in mechanical analysis? such as stress, strain which represent stretchability as in epidermal electronics or electronic eye case.

also, to improve breathability, is porous material useful? if so, we can design the pore size to achieve good breathability.

Jianliang Xiao's picture

Yes, stretchability can be important to both the performance and comfortness. 

To characterize breathability, I think one needs to think of vapor transport or fluid transport through the device/substrate. In this regard, porous materials could be a way to improve breathability, and fabrics are a excellent example. 

Matt Pharr's picture

Hi Jianliang,

Thanks for your very nice review!  In addition to (and related to) effective modulus, we can also use the interfacial stresses that develop between the organ and substrate/device as a key metric, e.g. interfacial stresses that develop during natural stretching of the skin during daily activities.  Under stretching of the organ of interest, both shear and normal stresses can result at this interface due to the mismatch in mechanical properties between the organ and the substrate/device.  Organs such as the skin have a minimum threshold level of somatosensory perception, i.e., below a certain level of interfacial stress between the substrate/organ, the body cannot feel the device at all.  In practice, these interfacial strains can be measured by putting an array of markers at the interface between the organ/skin and can also be calculated via simulation to converge toward optimal designs that minimize stresses/strains.

Yonggang Huang's picture

Matt,

An excellent point!  In fact, typically 20KPa is a representative value for normal skin sensitivity, below which the interface does not feel the existence of the device.  For extremely sensitive skins, 2KPa is a representative value of skin sensitivity.  It is quite a challenge to make the interfacial normal and shear stresses below 2KPa, but this has been achieved in some recent work from Profs. John Rogers and Yonggang Huang's groups.

Jianliang Xiao's picture

Thanks Matt and Prof. Huang for making such an excellent point! 

Zhigang Suo's picture

Thank you, Jianliang, for hosting this timely thread of discussion.  Some of us are now developing stretchable and transparent devices using hydrogels.  These hydrogels are ionic conductors, like tissues of plants and animals.  Typical devices involve both ionic and electronic conductors.  We call these devices ionotronics.  Here are the slides of a recent talk.  In addition to describing devices, the talk describes challenges in mechanics and materials.  

Because living tissues are ionic, and because most engineered devices are electronic, ionics and electroncis are always integrated at some level. 

The September 2013 iMech jClub focussed on stretchable ionics.

Yonggang Huang's picture

Dear Zhigang,

I enjoyed your great talk at the symposium in honor of Fong in Cambridge last week.  Thanks for posting its slides.  

Ionotronics is indeed a very interesting and promising field, led by your 2013 Science paper.  

Is short circuit a concern for electronics in hydrogel since the latter is full of water?

Zhigang Suo's picture

Thank you, Yonggang, for your kind words.  Your groundbreaking work on stretchable electronics has been an inspiration for us (slide 7 in the talk).  

In a hydrogel, water molecules and a polymer network aggregate by weak bonds (slide 8).  The mesh size of the polymer network is on the nanoscale.  For example, jello is a hydrogel.  At a macroscopic scale, the hydrogel is solid-like, and water does not flow.  At a molecular scale, the hydrogel is liquid-like:  water molecules diffuse, the polymer network changes conformation, and ions diffuse in water.  Thus, hydrogel is a stretchable ionic conductor.

In using hydrogels as an conductor, we prevent short by design.

For example, we separate  two layers of hydrogel by a dielectric (slide 32).  This setup is analogous to two metals separated by a dielectric.

We also make direct contact between a metallic electrode (electronic conductor) and a hydrogel (ionic conductor) (slide 32).  This setup is analogous to that in a supercapacitor.  So long as the voltage across the interface is lower than about 1 V, the interface behaves like a capacitor, and no electrochemical reaction will occur.

Nanshu Lu's picture

Dear Jianliang,

Thank you very much for the timely and thoughtful review on bio-integrated and bio-inspired electronics. Although the mechanics of freestanding soft electronics are widely studied, the mechanics at the interface between bio-tissues and electronics still remains to be explored. They physics is very rich at such interface because there is coupled mechanics, thermal, electrical, and chemical interactions along that interface. For example, we know that the conformability between epidermal electronics and microscopically rough human skin governs the interface impedance and motion artifacts. Prof. John Rogers and Prof. Younggang huang first established the conformable vs. non-conformable criterion, and Prof. Shuodao Wang studied the critical stress that may cause slippage between skin and electronics. My group recently built a model which can fully capture the non-conformable, partially-conformable, and fully-conformable conditions between epidermal electronics and skin as well as neuroelectronics and brain. To measure the adhesion between extremely soft materials like bio-tissues, my group has also provided the complete experimental and theoretical procedures for the JKR measurement of the adhesion between extremely soft materials using PDMS and Ecoflex as examples.

Yonggang Huang's picture

Nanshu,

Very interesting and insightful comments.  I noticed that your paper on non-, partially-, and fully-conformable conditions was submitted to JAM on 12/8/2015 and published on 1/27/2016 -- very fast.

Zhigang Suo's picture

Dear Nanshu:  Thank you so much for articulating this important consideration of being conformal.  Your comments remind me of the technology of wafer bonding, developed in semiconductor industry some decades ago.  Even hard materials can form bonds without any glue, if the hard materials are flat enough.  There, also, some elastic deformation is needed, because the surface may not be atomistically flat.  Here is a 1998 paper by Yu and Suo:  a model of wafer bonding by elastic accommodation.  The paper cites the literature of wafer bonding technology.  

For soft materials, the low modulus makes such "wafer bonding" easier.

Incidentally, Yonggang and I have to compete to get your papers.  I list your paper in Extreme Mechanics Letters on stretchable tin oxides.  Under Yonggang, JAM no longer jams papers any more.  We've got a tough competition.

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