# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 2 Materials and Structural Design
### Structural Design
#### 2.2.2 Ultrathin Structure  
Considering that the induced strains are linearly proportional to the overall thickness, flexible electronics can be achieved by reducing the thickness; therefore the induced strains are decreased [82, 83]. [PERSON] et al. introduced plastic electronics with nanometers-thick organic transistors on 1-um-thick ultrathin polymer (polyethylene naphthalate (PEN)), as shown in Figure 3b [72]. This ultrathin polymer can be crumpled and stretched up to 230%, which enables conformal attachment to arbitrary and curvilinear surfaces, such as the 1-um-thick ultrathin polymer (polyethylene naphthalate (PEN)), as shown in Figure 3b [72].  
Figure 3: (**a**) (Left) Schematic image of the hybrid substrate where the reinforced islands are embedded in the elastic substrate. (Right) SEM images before (top) and during (bottom) 30% stretching. The arrow indicates the direction of stretching direction. Scale bars, 500 μm SEM images before (top) and during (bottom) 30% stretching. The arrow indicates the direction of stretching direction. Scale bars, 500 μm. SEM images before (top) and during (bottom) 30% stretching. The arrow indicates the direction of stretching direction. Scale bars, 500 μm. (reprinted with permission from [15]. Copyright 2018 American Association for the Advancement of Science); (**b**) at only 2 μm thickness, devices are ultraflexible and can be crumpled like a sheet of paper. Scale bar, 1 cm (reprinted with permission from [72]. Copyright 2013 Nature Publishing Group); (**c**) A 4 \(\times\) 4 stretchable organic electrochemical transistors (OECT) array when intrinsically stretched. Scale bar, 3 mm (reprinted with permission from [73]. Copyright 2018 American Association for the Advancement of Science); (**d**) Si ribbon structures formed on a PDMS substrate pre-strained to 50% and patterned with \(W_{\text{act}}\) = 15 μm and \(W_{\text{in}}\) = 250, 300, and 350 μm (from left to right). The image was taken by tilting the sample at an angle of 45\({}^{\circ}\) (reprinted with permission from [74]. Copyright 2006 Nature Publishing Group); (**e**) Corresponding optical micrographs of the wrinkled Au film electrode at 0%. Scale bar, 100 μm (reprinted with permission from [75]. Copyright 2018 American Chemical Society); (**f**) scanning electron microscopy images of third-order Peano-based wires on skin and a skin-replica (colorized metal wires), showing the conformal contact of the wires on the substrate. Scale bar in (**f**), 500 μm (reprinted with permission from [76]. Copyright 2014 Nature Publishing Group); (**g**) Image of a 4 \(\times\) 4 TCR sensor array after application to the skin using a water-soluble adhesive tape based on poly(vinyl alcohol) (reprinted with permission from [77]. Copyright 2013 Nature Publishing Group); (**h**) Optical image of a final mesh heater after biaxial stretching up to \(\sim\)100% (reprinted with permission from [78]. Copyright 2015 American Chemical Society); (**i**) SEM images of Ag mesh, demonstrating a space length among Ag lines of 45 μm (reprinted with permission from [79]. Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany). (**j**) High-magnification SEM image of the CNT loops (reprinted with permission from [80]. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany). (**k**) SEM images of PUS-AgNW-PDMS composites (reprinted with permission from [81]. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).  
as human skin. [PERSON] et al. developed active organic electrochemical transistors (OECTs), which were encapsulated with a 1.2-um-thick parylene layer, as shown in Figure 3c [73]. Fabricating electronic devices with a total thickness of 2.6 um enabled the devices to have high flexibility and stretchability. Based on these properties, a cyclic test was conducted by applying a strain of 15%. After the cyclic test, the transconductance and drain current were measured and only had a 7% difference, which showed sufficient mechanical stability.