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In reply to Dear Zhigang,
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.
In reply to Ionotronics
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.
In reply to breathability
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.
In reply to Inorganic versus Organic
Thanks Lihua for sharing this nice reference on organic electronics.
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For an intro to subroutines get the file
http://imechanica.org/files/Writing User Subroutines with ABAQUS.pdf
I managed to apply a traction load (shear) using a UTRACLOAD but wondering how I can have two subroiutines (DLOAD, UTRACLOAD) one for pressure load and one the resultant shear traction load, in one job?
Can we get a video link of the lectures? The slides are really informative and the talks would be informative too to listen!
In reply to Comfort
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.
In reply to coupled heat transfer and mass diffusion
Thank you for your answers
yes, it is a possibility .Also, it can be done with a uel? no?
In reply to Human comfortness
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.
In reply to Inorganic versus Organic
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
Really happy to know you are starting your group. All my bes
In reply to Human comfortness
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.
In reply to To ensure human comfort
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?
In reply to Jianliang,
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.
Congratulations for starting your group, Bas!
I send my best wishes for finding outstanding students to work with you.
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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?