Fully rubbery logic gates enable stretchable pressure-mapping skin

Technology News | February 4, 2019

By eeNews Europe

Wearables Flexible Electronics Analog Sensing / Conditioning

So far, organic semiconductors always suffered from a comparatively low carrier mobility and most viable flexible or stretchable electronics applications relied on the design of rigid functional islands with deformable interconnects (pre-stretched and wrinkled or meandering electrodes).

Here the researchers boosted the carrier mobility of a P3HT–nanofibrils (NFs)/PDMS (polydimethylsiloxane) composite organic semiconductor using low weight-concentrations of highly conductive metallic carbon nanotubes (m-CNTs) at its surface. The dopant m-CNTs were dry-transferred through a lamination/delamination process of the polymeric semiconductor on a glass substrate bearing the dispersed m-CNTs.

Their paper “Fully rubbery integrated electronics from high effective mobility intrinsically stretchable semiconductors” published in the Science Advances journal explains how the dry-transferred m-CNTs substantially enhance the effective carrier mobility of the semiconductor compound by offering “fast lanes” for the charge that shorten the transport distance within the channel (while not creating full-length percolated shortcuts throughout the polymer). At a weight concentration of 0.05 wt %, the m-CNT does not form a percolated network, the authors verified.

The dry transfer doping preserves the crystallinity of the P3HTs-NFs, the authors report, characterizing carrier effective mobility to about 9.76 cm²/V·s, several folds that of non-doped organic semiconductor composites. By contrast, mixing the m-CNT and P3HT in a solution phase would not lead to a high mobility semiconductor because the crystallinity of the P3HT-NFs would sharply, the article explains.

A rubbery transistor showing the high effective mobility
intrinsically stretchable m-CNT–doped P3HT-NFs/PDMS
semiconductor (brown), the AuNPs-AgNWs embedded
PDMS electrodes, and the ion gel gate line.

The researchers then created transistors, using the m-CNT–doped P3HT-NFs/PDMS (polydimethylsiloxane) composite as the semiconductor channel, gold nanoparticles with conformally coated silver nanowires (AuNPs-AgNWs) embedded in the elastomer PDMS (AuNPs-AgNWs/PDMS) as the elastomeric source and drain electrodes, and ion gel as the gate dielectric. After proving the transistors would remain operational through multiple stretching cycles up to 50% of elongation, the researchers designed rubbery logic gates, including inverters NAND and NOR.

The rubbery transistors array and an exploded
schematic view.

Again, testing the logic gates under mechanical strains of 0, 10, 30, and 50% along and perpendicular to the channel length direction revealed normal operation, yielding the correct logic output states (only the voltage gain and switching threshold voltage changed slightly under mechanical strain).

Transistor arrays under different mechanical deformation modes.

The fully integrated pressure-sensing skin with
integrated readout matrix.

Next, the researchers designed a fully stretchable tactile sensing skin integrating an 8×8 active matrix readout together with a layer of pressure-sensitive rubber whose resistance sharply decreased from several hundred megohms to several ohms when the applied pressure exceeded the 100 kPa threshold.

Thanks to the 8×8 active matrix readout transistors, they were able to map pressure across the stretchable matrix, with no cross-talk between adjacent sensing pixels.

Even when the skin was stretched along both directions (to 30%) and released, measured output voltages remained stable. Its tactile mapping capabilities remained reliable even after more than 100 cycles of stretching and releasing, the paper reports, concluding that such stretchable integrated electronics with rubber-like mechanical properties could be used in bioelectronics, wearable systems, and for large-scale integrated circuits.

University of Houston – www.uh.edu

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