Breathable Nanowood Biofilms as Guiding Layer for Green On‐Skin Electronics

Abstract

thermoelectric [3,4] and moisture-inducedelectric [5] effects also has gained great attention (skin and on-skin electronics (On-skinE) themselves are energy storehouse). However, several challenges are to be faced, "thin-film" On-skinE I) cannot install "bulky" heatsinks [6] or sweat transport channels, but the output power of the thermoelectric generator and moistureinduced-electric generator depends on the temperature difference (∆T) across generator [4] and the ambient humidity (AH), [5] respectively; II) also lack a routing and accumulation of sweat for biosensing technologies, [7,8] and lack a targeted delivery of drugs for precise feedback transdermal therapy technologies; [9,10] and III) need insulate between the heatgenerating unit and heat-sensitive unit.In these regards, a "routing and accumulation" of heat and sweat can "concurrently" enhance the ∆T/AH-dependent output power, enable a reliable sweatbased biosensing, and prove the ability of targeted delivery of saline-soluble drugs for precise feedback transdermal therapy. Therefore, it is in great demand for developing strong-guiding films, which can help insulate between units and guide the heat and sweat to another in-plane direction.Besides, breathability [3] and stretchability [11] are essential for the on-skin use of electronics with long-term comfort. Several strategies have been proposed to realize the "stretchability" and the strategy placing rigid electronic units on the Thin-film electronics are urged to be directly laminated onto human skin for reliable, sensitive biosensing together with feedback transdermal therapy, their self-power supply using the thermoelectric and moisture-induced-electric effects also has gained great attention (skin and on-skin electronics (On-skinE) themselves are energy storehouses). However, "thin-film" On-skinE 1) cannot install "bulky" heatsinks or sweat transport channels, but the output power of thermoelectric generator and moisture-induced-electric generator relies on the temperature difference (∆T ) across generator and the ambient humidity (AH), respectively; 2) lack a routing and accumulation of sweat for biosensing, lack targeted delivery of drugs for precise transdermal therapy; and 3) need insulation between the heat-generating unit and heat-sensitive unit. Here, two breathable nanowood biofilms are demonstrated, which can help insulate between units and guide the heat and sweat to another in-plane direction. The transparent biofilms achieve record-high transport // /transport ⊥ (//: along cellulose nanofiber alignment direction, ⊥: perpendicular direction) of heat (925%) and sweat (338%), winning applications emphasizing on ∆T/AH-dependent output power and "reliable" biosensing. The porous biofilms are competent in applications where "sensitive" biosensing (transporting // sweat up to 11.25 mm s −1 at the 1st second), "insulating" between units, and "targeted" delivery of saline-soluble drugs are of uppermost priority.

A New Class of Electronic Devices Based on Flexible Porous Substrates

“…Guiding layer for on-skin electronics [21] (Continued) Difficulty in the production of micropores (<100 nm) and hierarchical porous structure Pressure detection sensitivity (0.63 kPa −1 ), durability, and stability of the pressure sensor (>10 000 loading and unloading cycles, with a 0.5 n load)…”

“…[ 18 ] Therefore, both sides of the porous substrate can be functionalized and easily integrated. The capillarity of porous substrates can be used for fluid transport, [ 21 ] fluidic valves, [ 22 ] fluidic delay switch, [ 23 ] without the need for an external pump. In addition, capillarity benefits the rapid absorption and penetration of active material and alleviates uneven evaporation and nonuniform pattern caused by the coffee stain effect during printing or coating.…”

Nanocellulose and Its Interface: On the Road to the Design of Emerging Materials

“…Breathable porous thin films nanowood have been obtained for skin-interfaced electronics, where "sensitive" biosensing (transporting sweat), "insulation" between units, and "targeted" delivery of saline-soluble drugs are necessary. [219] The films obtained from wood require simple, environmentally friendly, and cost-effective fabrication methods. Depending on the method details, the film characteristics (i.e., anisotropy, transparency, flexibility) can be appropriately tuned.…”

“…Figure 17. Schematics of the fabrication of breathable nanowood biofilms: the well-aligned CNs in L2 layer (the main portion of the cell walls) of a) wood slice is transferred into the porous delignified biofilm (b) and transparent delignified biofilm (c).Adapted with permission [219]. Copyright 2019, Wiley-VCH.…”

Visualisation of H2O2 penetration through skin indicates importance to develop pathway-specific epidermal sensing

“…Robust, non-invasive, or minimally invasive sensing on skin, often referred to as epidermal sensing, is one of the tools required to realise personalised healthcare, at point-of-care units or home self-monitoring. Epidermal sensing is carried out using epidermal electronics [1][2][3], microneedle patches [4], attachable, stretchable, biodegradable sensors [5], etc. These developments have strongly advanced sensing of molecular biomarkers by collecting and analysing sweat right on the skin [6].…”