Technologies that combine electronic components with the human body are now able to restore function, monitor health conditions, and/or impart or enhance abilities. This field is driven by welfare and biomedical applications: Products such as prosthetic limbs, cochlear implants, and pacemakers have already been commercialized. With the recent progress in electronic, information, and communication technologies, devices that can be worn by people and even placed in contact with their skin are being functionalized electrically and promoted as “wearable electronics.” Examples include smart glasses and contact lenses with electronic functions such as video recording for life logging and/or glucose monitoring in addition to restoring vision. Powered exoskeletons enable elderly people to regain movement and healthy people to enhance their ability to lift heavy objects. When electronics are interfaced with the body, either by wearing or implantation, it is critical to minimize invasiveness.
Given this background, introducing electronic functions to the surfaces of organs, especially the skin, is at the forefront of multidisciplinary research efforts. A practical solution is to manufacture electronic devices on thin polymeric films and then laminate them onto curved surfaces. With this approach, displays and sensor arrays on foils are applied to curved surfaces such as robot bodies. Although the surface topologies of biological tissues are typically more complex than those of machines, the conformability of thin-film devices on foils can be improved by reducing their total thickness and/or Young’s modulus—parameters crucial in achieving the required low flexural rigidity. The feasibility of electronic functionalization of human skin with ultrathin devices has been demonstrated by the pioneering and inspiring work of Gao et al., Kim et al., and Huang et al., in which terms such as electronic tattoos and epidermal electronics were coined. In their work, ultrathin silicon devices and other electronic elements with thicknesses of a few micrometers were directly laminated onto the surface of the skin. Organic thin-film devices are expected to introduce more diverse functions with features that are complementary to those of inorganic devices, such as amenability to large areas and low-cost manufacturing, along with inherent mechanical softness. To make use of these functions, organic thin-film devices or quantum-dot light-emitting diodes (QLEDs) have been manufactured on 1-μm-thick foils, and these devices have demonstrated remarkable mechanical flexibility, withstanding a minimum bending radius of 10 μm or less.
The combination of different types of organic devices on an ultrathin film is crucial to realizing multiple electronic functions on the surface of the skin using organic ultrathin-film devices for smart wearable and medical systems. Optoelectronic devices are especially important in medicine because these devices can noninvasively detect vital signs and other clinical information. Recently, organic LEDs, polymer light-emitting diodes (PLEDs), and organic photodetectors (OPDs) were manufactured on glass or bulky (less than 1 mm) plastic substrates and then combined to form a transmission mode pulse oximeter and muscle contraction sensor. In other reports, organic LEDs and photovoltaics were fabricated on 1-μm-thick films but were driven in nitrogen atmosphere. Realizing ultraflexible optical sensors with extended stability in ambient air would allow their intimate and unobtrusive integration on the skin or in the body and enable a cornucopia of applications.
One of the largest barriers preventing the realization of air-stable ultraflexible organic optoelectronic devices is the ability to form a high-quality passivation layer on an ultraflexible substrate. For example, water vapor transmission rates (WVTRs) of 10−6 and 10−3 to 10−4 g/m2 per day should be met for practical applications of PLEDs and OPDs, respectively. Substrates with a thickness of a few micrometers or less are easily deformed by thermal expansion and are susceptible to damage during handling or by high-energy processes such as plasma deposition. Thus, the passivation layers must remain thin and have to be formed using low-temperature processes, with minimized usage of energy-intensive processes.