Interacting With Printed Sensors

By Michael P.C. Watts
If there is one feature that distinguishes all our modern portable devices from the traditional PC (a wonderful concept—the “traditional PC”), it’s the way we interact. Separate keyboards are done. It’s all touchscreens on pretty much everything, along with other sensor opportunities.

There are many uses for the built-in cameras in cell phones from videoconferences to anything Messrs. Weiner and Rivera can imagine. A more useful approach is to deduce information about the user. For instance, cell phones can identify when you look away fron the screen and pause a video, presumably using the camera. I am not convinced how useful a feature this is, seeing as I do not watch long videos on my pad, but it is a great starting point for a new capability. The non-contact swipe is another interaction detected by the camera sensor. In the future, stereo cameras could be used to provide distance sensing and are obviously low enough cost these days.

Making the surface of a portable device into a multifunctional surface is a great application for flexible electronics, because large area, low cost, thin, and flexible are all desirable. Some of the ideas discussed in detail at the Printed Electronics Conference “ IDTech” in Berlin, were improved touchscreens, large area infrared photocells, and surfaces that provide feedback from both the front and back of the device.

The latest touchscreens are built into the front surface of the display. “In-Cell” is the Synaptics version. The sensor is a set of transparent crossbars, and the contact of a finger or other conductive object changes the capacitance of the sensor. The built-in sensor means fewer pointing errors when you touch a button on the screen as compared to a stuck-on overlay. An indication of some of the complexities was shown in a report from TI that heat from fingers can mislead a touchscreen.

Synaptics In-Cell touch screen strategy

Further improvements to touchscreens need higher conductivity bars, more flexible materials, and more transparency. The most popular materials strategy is to use composites with conductive nanofibers. Cambrios are focusing on silver fibers that produce a film with 5 ohm per square resistance. The narrow wires improve transparency.

Large area pressure sensors are being developed by Tactonic. They creating an array of small bending plates so that the complete shape of the pressure source can be deduced. They see an opportunity for all sorts of shaped sensor surfaces such as steering wheels, or cell phone backs.

The idea of large area transparent infra-red photocells to detect both contact and proximity touch is being developed by Isorg. They use an organic semiconductor so that the devices are transparent.

Providing mechanical feedback on a transparent surface would really improved interaction with the device. Numerous studies have shown that tactile feedback improves interaction. One approach uses piezo electrical materials that change shape under electrical voltage. The most effective materials are derivatives of poly(vinylidine chloride)—PVDF. Oriented PVDF molecules have a strong dipole, which creates changes in molecular shape under electric fields. Strategic Polymer Systems has built PVDF copolymers and composites that will generate 5% strain in a rigid film—enough to create a mechanical motion that can be easily detected by a fingertip.

—Mike Watts is a longtime semiconductor process guy/specialist/nerd. More info. at www.impattern.com.