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Stretchable, Transparent, Ionic Conductors

C. Keplinger, J­Y. Sun, C.C. Foo, P. Rothemund, G.M. Whitesides and Z. Suo, Science, 30 Vol 341 no. 6149, 984­987 (2013) 

Fig 1: (A) shows the initial set up of the ionic conductor model before any voltage has been applied. The Dielectric elastomer rests in a thick sandwich between the two electrodes. (B) shows the model after voltage has been applied and the dielectric elastomer has expanded outward, increasing the area. (C) Shows the elastomer shaped as a heart before voltage is applied and (D) shows it expanded after the voltage has been applied.

An issue with existing electronic stretchable transparent conductors is their limitation when applied to soft machines and materials. To meet the requirements of the material similar to skin, heart and brain, Zhigang Suo of Harvard School of Engineering and Applied Sciences and his team assembled a stretchable, transparent, ionic conductor. This allows the conductor to mimic the elastic properties of cells and tissues while still maintaining higher resistivity, in a way that existing electronic conductor cannot.

The conductor was designed with two electrodes acting as the conductors, electrolyte elastomers, acting as ionic conductors and forming a double layer with the electrode, as well as a dielectric acting as an insulator. Arrangement can be seen in Fig. 1. Dielectric elastomer was set in between two membranes of electrolytic elastomer, allowing for the desired stretchiness and transparency. A 100-\uc0\u956 m-thick polyacrylamide hydrogel with NaCl and a 1-mm-thick very high bond (VHB) tape were used as the electrolyte and dielectric, respectively. Voltage applied between two electrodes would collect on the two faces of the dielectric with opposite charges, pinching the center and causing it to reduce in thickness and enlarging the area (Fig. 1 Part A,B). A 167% increase to the area was achieved (Fig. 1 Part C,D).

A transparent loudspeaker was built to demonstrate 20 Hz to 20 kHZ sound range. Piezoelectic polymers are commonly used in loudspeakers as the conductors, but replacing them with ionic conductors allows the loudspeaker to be operated with lower electric fields. One drawback to this ionic design is that ionic conductors have lower sheet resistance than some loudspeaker conductors due to the increase in stretching. However, the ionic conductors'92 higher resistivity and high stretchability still give a positive outlook for its use in cases that require biocompatibility.

Read more about this research or read the full paper at Some Journal Here

-Marcus Rice

Figure 2, (a) Rod packing composed of straight rods. One unit cell of the structure with slightly helical filaments and a single helix of the structure shown with unit cell length L and radius R. (b)Total free energy as a function of unit cell size L helix radius R. A path of low total free energy configurations is followed by corneocytes as they swell and dry. The lower images show the two states.

Shaping the Skin: The Interplay of Mesoscale Geometry and Corneocyte Swelling

Myfanwy E. Evans and Roland Roth, Phys. Rev. Lett. 112, 038102 (2014)

Corneocytes are skin cells comprised of keratin intermediate filaments. This outer layer of our skin reacts to prolonged exposure to water by swelling, forming the wrinkles familiar to most who have spent too long in the water. The expansion required to form these wrinkles exceeds what would be allowed by the elastic extension of the filaments alone. Another mechanism must be at work to account for the phenomenon. It is thought that the filaments are chiral, allowing them to cooperatively unwind and lever off each other. Their arrangement may also be ordered, allowing for the process to be reversible.

Researchers in Germany have devised a model that replicates these observed macroscopic effects. The model consists of helical rods that are varied in unit cell size and in radius (figure 2) suspended in a water-like solvent. They examined the free energy balance for the various configurations.

As seen in in figure 2, the arrangement of the rods follow an ideal path of low total free energy. The researchers show that the elastic energy of the keratin filaments is in near balance with the grand potential energy. Should the elasticity be too great, they would never expand. Likewise, if it was too weak, the corneocytes would always be in their swollen form. The two opposing forces are comparable enough in value that only a small external change is needed to prompt a shift from one state to the other. The ease of the transition is further helped by the hydrophilic nature of the helical rods. The rods are coated with a thin layer of water, preventing direct contact with each other. This layer also prevents cross-linking, which is often the dominant configuration in similar systems.

Through this model, the authors have demonstrated how small scale structures and patterns can have macro-scale effects on the morphology and behavior of materials. These data may give insights into conditions that affect the skin. For example, the authors suggest dermatitis may be caused by excessive drying and hydrating of the skin, allowing some cross-linking of the keratin intermediate filaments in skin to occur.

-Michael Lane


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