(Source: Medical Technology Network)
At present, the so-called body surface Electronic equipment-also known as “electronic tattoos”-is basically still in the experimental stage. The idea is to directly attach the electronic devices we know-composed of chips, antennas, memory and other components. On the surface of the human body, functions such as monitoring the temperature of the body surface and displaying the user’s identity are completed. Monitoring the body surface temperature can make the work of doctors, polygraphs, etc. easier; and the identity verification function allows ID cards, credit cards, and even car keys to be gone forever. You can take them away with a wave of your sleeves. What you want to take away is safer and more convenient than a QR code.
The prospect of this technology is very attractive. It is likely to redefine future wearable electronic devices. With the further development of technology, it may not be a dream to move the entire computer to the surface of human skin. Since all of this looks beautiful, why is the research still only in the experimental stage? Because electronic tattoos have a big bottleneck in energy supply: they cannot produce or store energy by themselves, and can only be powered by microwaves emitted by third-party equipment, but the external energy supply is cumbersome and unstable-this obviously limits the electronics The evolution of tattoos is destined to require stable and sufficient energy supply to support complex functions.
People have always hoped that energy conversion and storage can be realized directly on the body surface, and a variety of ideas have appeared-generating energy through piezoelectric materials, triboelectricity, thermoelectricity, etc.; the corresponding energy storage medium is energy density Higher batteries and super capacitors, etc. These devices are always too large and hard for human skin. Can you imagine a small dumbbell tied to your arm just to monitor your body temperature at any time? So the first step of a good electronic tattoo is a power source that can generate electricity, is light and comfortable to wear, and best fits the human skin.
The electronic tattoo has fulfilled the various requirements mentioned above very well: it is composed of a double pn junction solar cell and a lithium battery, both of which are made into tiny particles about 3×3mm, connected by a retractable circuit, and are The management chip works in collaboration, and finally, through the multi-layer ultra-thin elastic material package, the electronic components and the skin surface are bonded and fixed, and play a protective role. It can be said that the sparrow is small but has all the internal organs.
Double pn junction solar cell: The so-called pn junction is the semiconductor material unit we often say. For example, LED lights are one of them. Apply a certain voltage to both ends of such a pn junction, and it can emit light in a certain frequency spectrum. For example, red, blue, etc.; and this process is reversed in solar panels-solar units can absorb light in a certain frequency range to generate electricity. Ordinary solar panels have only one type of pn junction, which can only absorb sunlight in a certain frequency range, and the energy conversion efficiency is only 30%; the advantage of double pn junction is that its two pn junctions can absorb light in different frequency ranges. The energy conversion efficiency is increased to more than 40%. For applications that require high-efficiency absorption of light regardless of cost, such as spacecraft, the number of pn junctions can be further increased, and of course the conversion efficiency will be further improved.
Figure 1 below reveals the basic structure of this new electronic tattoo. The main two layers are a circuit composed of solar cells, lithium batteries and control chips, and an elastic protective film to protect the electronic structure.
Figure 1: An exploded view of the new electronic tattoo. The top one is the encapsulation layer that is close to the skin. The second layer is the power management circuit, which is composed of solar cells/lithium batteries/control chips. ) Connected; the bottom is the protective ultra-low modulus silicone interlayer and the stretchable substrate
The biggest mechanical challenge of this electronic tattoo is how to ensure the normal operation of its electronic components when the skin surface changes. The mechanical design of this electronic tattoo solves this problem, thanks to the upper and lower protective films that wrap the energy unit, and the zigzag connection lines, as shown in Figure 2.
Figure 2: The lower protective film structure of the electronic component and the zigzag circuit connected to the electronic component. Under this design, although the deformation of the electronic tattoo causes a large-scale strain on the main layer, it has almost no effect on the electronic component layer.
The idea of this design is to control the strain of the electronic component within the allowable range through the protective film and the connecting wire under the premise of matching the skin deformation (the critical value of the relative strain that causes the component to yield is about 0.3%). The protective film is divided into two layers. The elastic coefficient of the main layer (core) (~3 kPa) is much lower than that of the secondary layer (shell, ~60 kPa), just like a layer of elastic rubber (secondary layer). Soft cotton (main layer).