Overview
This critical review, co-authored by Jungmok Seo (then at the Nanobio Device Laboratory, School of Electrical and Electronic Engineering, Yonsei University), who now leads the BLISS Lab at Yonsei University, surveys how nanomaterials placed on flexible and elastomeric substrates were used to obtain functions not achievable with planar silicon technology. It covers piezoelectric energy harvesters, chemical and pressure sensors and flexible bioelectronics, with attention to fabrication techniques, materials, operation mechanisms and signal outputs.
Background
High-quality films and nanomaterials are difficult to deposit directly on elastomeric substrates, which tolerate only low processing temperatures. The review describes how dry transfer printing and the use of stretching to create uniform nanogaps and nanowrinkles in elastomeric substrates made it possible to combine high-performance inorganic nanomaterials with compliant supports, and how research in flexible electronics was moving from materials development toward new device functions.
How the review is organized
The review is organized by function. Section 2 covers piezoelectricity and piezoelectric nanogenerators with unusual form factors, Section 3 covers chemical and pressure sensors built from nanomaterials on flexible substrates, and Section 4 covers visual prostheses, controlled drug release and bio-integrated circuits. For each example it describes the materials, the structure or transfer method, the operating mechanism and the reported output.
Scope of the review
Table 1 compares the piezoelectric charge coefficients and relative dielectric constants of common piezoelectric thin films and nanomaterials, and Table 2 lists the gases detected with Si, carbon nanotube (CNT), graphene, graphene oxide, metal oxide and Pd sensing materials. The figures reproduce examples from the cited studies, including buckled lead zirconate titanate (PZT) ribbons, a nanowire gas sensor, nanostructured polydimethylsiloxane (PDMS) pressure sensors, a nanowire e-skin, a single-walled carbon nanotube (SWCNT) strain sensor, retinal microelectrode arrays, strain-responsive microcapsules, silk microneedles and conformal cardiac, brain and epidermal electronics.
Main points of the review
For energy harvesting, the review emphasizes that the strain needed for wearable devices often exceeds the fracture limits of piezoelectric materials. It notes that the piezoelectric charge of ZnO increases from 9.93 pC N⁻¹ in bulk to 26.7 pC N⁻¹ in nanobelt form, that polyvinylidene fluoride (PVDF) nanofibers accommodate a maximum strain of about 2% but have weak coupling, and that PZT fractures at only 0.2% tensile strain. Buckled PZT nanoribbons on prestrained PDMS tolerated nearly 8% strain and gave a current density of about 2.5 μA/mm2, compared with about 0.1 μA/mm2 for PZT nanowire devices, and a ZnO nanowire/PVDF hybrid fiber on a human arm produced about 0.1 V and 10 nA cm⁻².
For chemical sensing, the review discusses dry transfer of high-quality semiconductors onto plastic and notes that transferred nanomaterials can lose their shape or be damaged, lowering performance and yield. Examples include transferred Si nanowire arrays responding to nitrogen dioxide (NO2) up to 200 ppb, a CNT/reduced-graphene film sensitive to 0.5 to 10 ppm NO2 from room temperature to 350 °C, and a lithography-free approach in which stretching a Pd film on PDMS produced nanogaps controlled to 300 nm at 25% strain that close when hydrogen is absorbed and reopen when it is released.
For pressure sensing, the review describes a microstructured PDMS dielectric that detected a 20 mg fly, equivalent to 3 Pa, with response times more than 100 times faster than earlier devices of similar materials; Ge/Si nanowire-array transistors with pressure-sensitive rubber that responded from 2.2 to 14.4 kPa, remained stable after 2000 bending cycles at a 2.5 mm radius and formed a 19×18 pixel e-skin with 84% functional pixels; and an aligned SWCNT film strain sensor that endured strain up to 280% with a 14 ms response time.
For bioelectronics, the review summarizes retinal prostheses motivated by the survival of up to about 80% of retinal neurons, and notes that nanocrystalline diamond microelectrodes carried charge injection of up to several hundred μC/cm2 compared with 10 to 100 μC/cm2 for Pt. It also covers buckled microcapsules on PDMS that flattened at 8.5% strain and released more molecules at larger strain, silk microneedles with sustained release over 2 h, a cardiac mapping device of 2016 transistors in 288 unit cells tested in Yorkshire pig hearts, electrodes on a 2.5 μm mesh polyimide film delivered on a dissolvable silk support that gave reliable recordings on a feline brain, and epidermal electronics on a silicone sheet with a modulus of about 60 kPa and a thickness of about 30 μm that adhered to skin by van der Waals forces alone.
Open questions
The authors identify four issues to address before fully integrated flexible systems can be commercialized: substrates that endure high strain, methods to keep material and electronic properties stable in the deformed state, fabrication of identical devices over large areas, which depends on controlling nanomaterial alignment, and residual stress that accumulates over repeated bending and folding.
Related research
Epidermal Electronics
DOI: 10.1126/science.1206157 ↗
The related paper’s abstract reports circuits with mechanical properties matched to skin and electrophysiological measurements. It connects one of the review’s examples to its original study.
Abstract-based introductionSources used in this summary
This summary is based on the publisher PDF of the review, including Tables 1 and 2 and the figure legends; the cited primary studies were not read, so their results are reported as the review states them.