Wearable sensors created using laser-induced graphene

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A flexible, wearable pressure sensor made from laser-induced graphene. (Image: Penn State University/Huanyu Cheng)

Researchers have fabricated and tested flexible, wearable sensors made from laser-induced graphene (LIG) – graphene made via laser processing.

The sensors could be used for health monitoring or enhancing the performance of prosthetic limbs.

Graphene (hexagonally arranged carbon atoms in a single layer) offers superior pliability and high conductivity, making it an exciting material for enabling technology such as flexible, wearable electronics.

Several substances can be converted into carbon to create graphene using laser radiation. The resulting product, known as laser-induced graphene (LIG), will have specific properties determined by the original material.

Before creating the sensing devices, the researchers, from Penn State University, Fuzhou University, Huazhong University of Science and Technology, and Hebei University of Technology, experimented with LIG production by irradiating samples of polyimide (a type of plastic) via COlaser scanning. They varied the power, scanning speed, number of passes and density of scanning lines, publishing their findings in Science China Technological Sciences.

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

Increasing the power from approximately 9W to 12.6W 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 fibre network. The fibrous structure showed better electrical conductivity than the foam. According to the researchers, this increased performance combined with the fibre’s form could open possibilities for sensing devices.

‘In general, this is a conductive framework we can use to construct other components,’ confirmed Professor Huanyu Cheng of Penn State's Department of Engineering Science and Mechanics. ‘As long as the fibre is conductive, we can use it as a scaffold and do a lot of subsequent modifications on the surface to 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 carbonisation from burning.

Demonstrating a low-cost LIG sensor 

Cheng and the team set out to design, fabricate and test a flexible LIG pressure sensor.

‘Pressure sensors are very important,’ remarked Cheng. ‘We can use them not only in households and manufacturing but also on the skin surface to measure lots of signals from the human body, like the pulse. They can also be used at the human-machine interface to enhance performance of prosthetic limbs or monitor their attachment points.’

The team tested two designs. For the first, 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 localise the pressure source, and appeared to improve sensitivity to delicate touches.

This first 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. According to Cheng, 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.

Both designs were cost-effective and allowed for simple data acquisition, according to Cheng.

The researchers plan to continue exploring the designs as standalone devices for health monitoring or in tandem with other extant equipment.

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