Printed electronics has acquired tremendous attention recently and its market size is expected to reach $300 billion over the next two decades. Printed electrochemical sensors, in particular, are an important sub-section of printed electronics that play a pivotal role in healthcare, energy and security domains. However, the fragile nature of these printed electrochemical devices greatly hampers the complete harnessing of their potential in many scenarios, such as, wearable applications, where harsh mechanical deformations are fairly common. External stress induced device failure is indeed the “Achilles heel” of the printed electronics field and yet not much has been done to develop printed devices that can withstand extreme stress and self-heal upon damage. We have thus attempted to fill this scientific vacuum by fabricating the first examples of all-printed, inexpensive, highly stretchable and self-healing electrochemical sensors. These novel devices have been realized by judiciously synthesizing screen printable inks that contain elastomeric binders and microcapsules loaded with self-healing agent.

PEDOT:PSS^[1] and CNT^[2] based stretchable inks have been developed for fabricating stretchable electrochemical sensors that can withstand extremely high levels of strains (upto 500%) with negligible effect on their structural integrity and performance. The sensors have been extensively characterized by electrochemical techniques to study the effect of repeated stretching (500%), torsional twisting (180⁰) and indenting (5mm) on their electrochemical properties. Finally, the wide-range applicability of this platform to realize highly stretchable electrochemical sensors and biofuel cells has been demonstrated by fabricating and characterizing potentiometric ammonium ion sensor, amperometric glucose sensor and enzymatic glucose biofuel cell and self-powered sensor.

The self-healing printed electrochemical sensors rely on the release of healing agent from microcapsules that loaded within the inks.^[3] Upon damage, the microcapsules release the healing agent that leads to local dissolution of the binder and allows redistribution of the conductive fillers to regain the sensor’s electrochemical properties. Thorough characterization of the self-healing sensors reveal that their electrochemical properties can be restored within a few seconds even when the device is completely damaged. Ultimately, we utilized this platform to demonstrate a self-healing sodium sensor that can self-restore its sensing ability even after complete damage.

The stretchable and self-healing printed electrochemical sensors have immense applications in the field of wearable electrochemical sensors.^[4] These devices can be easily mated with the human skin for continuous non-invasive monitoring of vital chemicals, such as, electrolytes (Na⁺, NH₄⁺, Cl^-etc) and metabolites (lactate, sodium, alcohol etc). The stretchable and self-healing ability of these devices enable them to endure extreme deformations commonly experienced by the human skin and yet perform without much impact of their sensing ability. Our work thus holds great promise in the wearable healthcare domain wherein defiance towards extreme mechanical deformations is crucial.

References:

1. “All-Printed Stretchable Electrochemical Devices”, Bandodkar, A. J. et al. Adv. Mat.2015, 27, 3060.

2. “Highly Stretchable Fully-Printed CNT-based Electrochemical Sensors and Biofuel Cells:  Combining Intrinsic and Design-induced Stretchability”, Bandodkar, A. J. et al. (under revision)

3. “Self-healing Inks for Autonomous Repair of Printable Electrochemical Devices”, Bandodkar, A. J. et al. Adv. Electron. Mat.2015 (doi: 10.1002/aelm.201500289).

4. “Non-invasive wearable electrochemical sensors: a review”, Bandodkar, A. J. and Wang, J. Trends Biotechnol., 2014, 32, 363.