Research out of the Soft Machines and Electronics Laboratory at Case Western Reserve University (CWRU) has produced a novel human-machine interface (HMI) device that allows users to manage smart devices using a fundamental human physiological function: breathing. Such a device could make home automation accessible to a portion of the population currently unable to use voice or touch commands typical of most devices today.
Demonstration of a user controlling appliances using the HMI system. Image used courtesy of Cao et al
Assistant professor Changyong “Case” Cao led the research group in developing the self-powered breathing HMI device based on triboelectric nanogenerator (TENG) technology. While TENG-based breathing HMI devices have been investigated before by other research groups, Cao and his team say their device’s careful configuration makes it comfortable, effective, and closer to market.
How Do TENGs Work?
In the simplest terms, TENGs work by converting mechanical input into electrical output. This makes it an appealing candidate for sensing or power generation.
The two main principles behind TENGs are the triboelectric effect and electrostatic induction. The triboelectric effect is a type of electrification from contact, where a charge is transferred from one surface to another after a period of contact. Electrostatic induction is the redistribution of electrical charge based on nearby influences—for example, all negative charges collecting on the point of an object close to something else with a high positive charge. These are both phenomena you might experience when rubbing a balloon against a rug and then having it “stick” against the wall.
Illustration of the working principle behind TENGs. Image used courtesy of ACS Nano
TENGs take advantage of these principles by layering two materials, making contact between them, and then creating distance between the two to cause a movement of charge. This flow of electrons can be stored or used as an output signal.
Researchers beyond Case Western Reserve University have found promise in TENGs in biomedical engineering and wearable devices. Other applications for TENGs include smart floors that generate energy from people walking, smart textiles embedded with TENGs, and electronic skin sensors.
The Challenges of TENG-based Devices
While the overall principles might seem straightforward and simple, TENG-based applications face several challenges, including:
- Tuning various types of materials with different electrostatic properties for the layers
- Finding optimal separation distance between the layers
- Identifying optimal surface contact size
- Selecting the type of surface contact (e.g. sliding, vertical)
- Determining the desired sensitivity to the input
- Controlling the strength of the output
A New Breath-controlled Smart Home Device
For the Case Western Reserve University prototype, the researchers developed a customizable polyurethane structure that affixes TENGs to the nose. The device consists of a soft clip that slides on the septum with enough pressure on the septum walls to remain in place with two small magnets on the upper part of the clip. At the bottom end of the clip outside the nose are two TENG sensors beneath the nostrils.
The TENG sensors consist of several material layers:
- Substrate: A 0.5 mm-thick polyethylene terephthalate layer at the very bottom, including 8 x 1.4 mm holes along the circumference for airflow
- Electrode: A 75 µm-thick copper film on the upper layer of the PET substrate
- Electrification material: A laminated natural latex layer on the copper
- Top electrification layer: A high-tensile EcoFlex layer with printed sandpaper microstructures, separated from the bottom electrification layer by a 3D-printed polylactic acid spacer
Cao’s research team investigated several different materials for the flexible layers. They also sought a comfortable structure for wear within the user’s nostrils. Image used courtesy of Cao et al
While the team investigated different materials for the bottom electrification layer, including copper and nylon, they ultimately concluded that natural latex could produce the strongest voltage output since it loses electrons the easiest among the material candidates. Overall, the device weighs approximately 1.48 grams.
A Comfortable Nose-clip Wearable Design
When designing the prototype, the researchers sought to determine the optimal distance from the nostrils to the TENGs, spacing between electrification layers, and the diameter of the TENGs. The team needed to simultaneously produce enough charge from breathing to generate an output signal while mitigating too much output noise from normal breathing. Some of these parameters would likely be customized to the user.
One of the things that make the CWRU device different from other TENG-based breathing HMIs is its design: the prototype is worn in a non-invasive, comfortable, and customizable wearable. Other prototypes from different teams have involved significantly more face gear to wear the sensor, such as a face mask.
Controlling Household Appliances With Breath Patterns
To test the sensors as an HMI device, the team performed several tests to turn household appliances on and off, including a lamp, a fan, and a computer monitor. The TENG outputs were fed to a processing circuit that computed the breathing pattern with a microcontroller that would then initiate the action via a relay switch.
Diagram of the TENG-based HMI system. Image used courtesy of Cao et al
In addition to using the device as an HMI, the sensor may also be used for respiration monitoring. The researchers claim the sensor’s design is robust against moisture build-up from breathing and movement, making it useful for monitoring sleep apnea. In this use case, the sensor could then trigger an alert when a user momentarily stops breathing.
Cao estimates that the team’s sensor could be available to the public within three to five years.
