Graphene — hexagonally arranged carbon atoms in a single layer with superior pliability and high conductivity — could impact the development of future motion detection, tactile sensing, and health monitoring devices.

Flexible graphene devices can be made with lasers, like the wearable pressure sensor shown here. (Photo: Huanyu Cheng)

Several substances can be converted into carbon to create graphene through laser radiation. Called laser-induced graphene (LIG), the resulting product can have specific properties determined by the original material. Samples of polyimide, a type of plastic, were irradiated through laser scanning. Researchers varied the power, scanning speed, number of passes, and density of scanning lines to see how different parameters of the laser processing process create different nanostructures.

The researchers found that lower power levels, from 7.2 watts to roughly 9 watts, resulted in the formation of a porous foam with many ultrafine layers. This LIG foam exhibited electrical conductivity and a fair resistance to heat damage — both properties that are useful in components of electronic devices.

Increasing the power from approximately 9 watts to 12.6 watts changed the LIG formation pattern from foam to bundles of small fibers. These bundles grew larger in diameter with increased laser power, while higher power promoted the web-like growth of a fiber network. The fibrous structure showed better electrical conductivity than the foam. This increased performance, combined with the fiber's form, could open possibilities for sensing devices. As long as the fiber is conductive, it can be used as a scaffold; subsequent modifications on the surface could enable a number of sensors such as a glucose sensor on the skin or an infection detector for wounds.

Varying the laser scanning speed, density, and passes for the LIG formed at different powers also influenced conductivity and subsequent performance. More laser exposure resulted in higher conductivity but eventually dropped due to excess carbonization from burning.

The team designed, fabricated, and tested a flexible LIG pressure sensor. For the first design, they sandwiched a thin LIG foam layer between two polyimide layers containing copper electrodes. When pressure was applied, the LIG generated electricity. The voids in the foam reduced the number of pathways for electricity to travel, making it easier to localize the pressure source, and appeared to improve sensitivity to delicate touches.

This design, when attached to the back of the hand or the finger, detected bending and stretching hand movements as well as the characteristic percussion, tidal, and diastolic waves of the heartbeat. This pulse reading could be combined with an electrocardiogram reading to yield blood pressure measurements without a cuff.

In the second design, the researchers incorporated nanoparticles into the LIG foam. These tiny spheres of molybdenum disulfide, a semiconductor that can act as a conductor and an insulator, enhanced the foam's sensitivity and resistance to physical forces. This design was also resilient to repeated use, showing nearly identical performance before and after nearly 10,000 uses.

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