Toward perdurable flexible electronics

(Nanowerk Spotlight) In wearable electronics, to acquire stability and simultaneously preserve stretchability, sensitivity, as well as scalability, are of high significance yet challenging for practical device applications.
Piezoresistive strain sensors are considered the most popular choice for this purpose due to their relatively facile designs as well as readout principles through transferring the displacement of sensors into resistance change. However, most of these strain sensors have been primarily focused on either sensitivity or stretchability.
In order to realize practical device applications of stretchable strain sensors, stability without sacrificing sensitivity, stretchability, and scalability is crucial.
To achieve a sensitive and stretchable strain sensor with simultaneously reliable and repeatable features, it is of paramount significance to rationally consider not only the active materials, but also the configuration of the device system.
The key to realizing high stability is to reduce the strain in the device under stretching conditions.
Researchers from Osaka Prefecture University reported a kirigami-structured graphene-polymer hybrid nanocomposite for strain sensors by a laser direct writing technique on a polyimide sheet (Journal of Materials Chemistry C, "Highly stable kirigami-structured stretchable strain sensors for perdurable wearable electronics").
The approach of laser direct writing not only contributes to the conversion of polyimide material into conductively porous carbonized film, but also gives rise to the formation of kirigami-shaped structures with highly stretchable capability in a fast and efficient manner.
Using such kirigami structures, strain in the sensor material can be drastically reduced in the circumstances of highly stretching conditions. To protect the device, ecoflex polymer is applied as the passivation layer.
Significantly, this sensor platform presents almost no performance degradation even after more than 60,000 stretching cycle tests due to less strain within the sensor.
teSchematic of an integrated stretchable strain sensor with printed electrodesxt
Schematic of an integrated stretchable strain sensor with printed electrodes (left). Long-time and multi cycle tests of the proposed two types of strain sensors (right). (Image courtesy of the researchers) (click on image to enlarge)
"Stretchable strain sensors usually suffer from performance degradation after several hundreds or thousands of cycles due to fatigue of structures or materials," said Prof. Hyunhyub Ko, an international leading expert on flexible electronics from Ulsan National Institute of Science and Technology (UNIST). "It is a highly effective way to employ kirigami-like structures in combination with active sensing materials using advanced laser manufacturing to achieve the long-term stability for flexible sensors."
Future work could be extended to investigate the kirigami shape and dimension-dependent flexible strain sensors to further enhance the stretchability as well as sensitivity for wearable applications.
Provided by Department of Physics and Electronics, Osaka Prefecture University as a Nanowerk exclusive


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