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?

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.

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.

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.

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.

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?

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.

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

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.

Thanks Lihua for sharing this nice reference on organic electronics.

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.

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.

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

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.

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?

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.

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,

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?

Zhigang,

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

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

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.

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.

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.

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,

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,

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).

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.

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.

Yongan,

Thanks for the comments!

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.