Ion-sensing paper gives pressure sensors a new sense of touch

According to MEMS Consulting, paper, as an easily accessible, low-cost, insulating, soft and portable material, has long been used as a flexible platform for chemical and biological sensing. For example, pH test strips, blood glucose test strips, and early pregnancy test strips are all widely used biochemical sensors .

As this field continues to develop, paper-based sensors based on nanotechnology are expected to become simple, portable, disposable, low-power and low-cost sensing devices that can be widely used in medicine, explosive and toxic substance detection and environmental research. Due to the fibrous structure of paper, they can be modified with functional additives, such as carbon-derived materials (such as carbon nanotubes and graphene), conductive polymers and metal nanocomposites, resulting in new functions and sensing modes.

Previously reported pressure-sensitive papers and pressure sensors made from them are mainly based on three existing sensing mechanisms, namely resistive, capacitive and triboelectric.

"Recently, we introduced a completely new pressure sensing mechanism, called flexible iontronic sensing (FITS), which senses capacitance changes by exploiting the pressure between an electrode and an ionic surface," said Tingrui Pan, a professor at the University of California, Davis. "It is noteworthy that, unlike traditional resistive and capacitive sensing methods, the iontronic pressure sensor based on the unique FITS mechanism has extremely high sensitivity and resolution, while parasitic noise can be largely ignored due to its extremely high signal-to-noise ratio."

a) Pressure response mechanism of ionic pressure sensing paper (ISP) device, in which fibrous ions serve as ionic regions and conductive fibers serve as conductive regions; b) Scanning electron microscope (SEM) image of ISP ion region; c) Molecular structure description of ionic liquid and polymer matrix; d) Preparation of ISP device with "sandwich" structure: i) Printing top electrode on ionic paper; ii) Printing bottom electrode on the back of ionic paper; iii) Printing bonding pattern for integration; iv) Folding to form a "sandwich" structure; v) Finally, packaging and integration; e) Preparation of ISP device with double-layer structure: i) Printing interdigitated electrodes on ordinary paper; ii) Printing ionic ink on the designed area; iii) Printing bonding pattern for integration; iv) Folding to form a double-layer structure; v) Finally, packaging and integration.

In their latest paper, "All-in-one ionic electronic sensing paper," published in the journal Advanced Functional Materials, Professor Pan and his collaborators introduced a monolithic ionic electronic paper substrate that contains both ionic and electrode regions, which can serve as an integrated flexible sensing platform.

Professor Pan pointed out: "Our new sensing paper extends the principle of ion-sensing to a more adaptable material system that is directly printable, custom cuttable, and 3D foldable at low cost, just like regular paper."

Now we can build a pressure sensing platform through simple paper-specific operations such as printing, cutting, gluing and folding. Since pressure sensing devices are no longer limited to being constructed in a flat form, 3D pressure sensing origami can now also be used to detect spatial pressure.

Basically, everything and every surface around us can be used as a sensing structure, such as in smart toys, smart packaging, health wearable devices, disposable electronics, wallpapers, furniture, etc., and even as a flexible human-machine interface.

Professor Pan said: "This technology ushers in a new era where everything we touch has tactile capabilities, and people can communicate with any object by touching, pressing and tapping, providing a comprehensive human-computer interface through visual, auditory and tactile input."

a) The ISP origami bracelet is prepared by printing and folding; b) The ISP origami bracelet can be used for pulse monitoring; c) The pulse wave signal collected by the ISP origami bracelet; d) The ISP origami piano is prepared with a pressure-sensitive keyboard; e, f) The ISP origami piano with force sensing function can detect pressure and control the volume.

However, there are still some problems to be solved. For example, the interconnection problem: the current signal interconnection between paper electronic devices and signal processing systems through conductive glue is still unstable. However, Professor Pan's team believes that this problem can be solved through low-temperature welding and special paper FPC (flexible printed circuit) interconnection grooves.

"Research on sensor devices is no longer just about the sensing element. Power supply, signal processing and electronic circuits should all be taken into account. Such integrated devices should also be compatible with skin, fabric, etc., both in shape, modulus and function." Professor Pan concluded