In the last blog, we explored the system specification guidelines for neural input wristbands including reach, accuracy, latency and MAC. In this blog we continue to delve into specification guidelines for neural input wristband, this time focusing on the product's specifications.
The relevant discussion begins at 31:45
Number of Electrodes
Spatial resolution plays a crucial role in wrist-worn neural interfaces, enabling us to differentiate between various signal sources like the ulnar, median, and radial nerves. Getting this right is key because it helps us accurately understand subtle finger movements.
However, there's a balance to strike. If we have too few sensors, the spatial resolution might not be enough, and we can't accurately interpret finger movements. But, having too many sensors can make the device uncomfortable to wear for a long time and might not look nice, which could affect how many people want to use it. After all, appearance matters when it comes to user adoption.
The number of electrodes depends on a few things:
User Physiology: We have to see how much skin area we can cover comfortably.
How We Place the Electrodes: How we arrange them on the device.
Types of Gestures: Different actions we want to capture.
Number of Gestures: How many different actions we need to detect.
We propose that having three electrodes is good for basic actions like pointing, clicking, and moving things around.
We need at least two sensors to tell the difference between actual finger movements and random signals. This is important, especially when one sensor isn't touching the skin properly. If one sensor isn't working well, the system can still figure out what's happening based on the other sensors.
Several other factors need to be considered in sensor-electrode design, such as electrode materials, inter electrode distance, and electrode properties. These aspects are crucial as electrodes come in direct contact with the user's skin and determine the number of signal sources captured by the device. Achieving a comfortable and functional wearable interface relies heavily on electrode count and design.
This figure illustrates an electrode configuration supporting spatial differentiation. The major challenge for a wrist worn neural interface is electrode detachment from the skin. A three electrode configuration can provide accurate classification and reliable functionality even when one of the electrodes loses skin contact.
The band for a wrist-worn neural interface should have dimensions and geometry similar to a typical watch strap or wristband wearable. It should be designed to accommodate various hand and wrist movements without causing discomfort. The band's thickness should not exceed 8mm, and its width should not exceed 20mm to maintain a sleek and comfortable profile.
Choosing a wrist form factor offers greater comfort and convenience, as the wrist is a natural and familiar location for wearing devices. It is socially acceptable and aesthetically appealing, resembling a traditional watch band. This design allows compatibility with existing watches or wristbands, making it a natural extension for wearable accessories.
Additionally, it facilitates improved sensor placement close to the skin surface without interfering with other sensors, enhancing the accuracy and reliability of biopotential signal readings from subtle finger movements.
The band's geometry is typically designed with the form-follows-function approach, considering common usage scenarios. The material used for the band should possess key attributes like flexibility, durability, and biocompatibility.
Integrating flexible electronics into the band presents practical challenges. Creating a bendable, malleable, durable, and biocompatible bracelet is an active area of research. Bending and elongation are specific challenges to address, as they impose different stress conditions on the band.
A dynamic design with both rigid and flexible electronics is susceptible to various physical stresses, with elongation potentially putting high stress on flex-rigid connectivity. The design is typically limited to a certain radius of flexion.
Band geometry considerations also come into play when integrating technology into existing smartwatch bands. Some components may become redundant, leading to new PCB layouts and functionality requirements. Band dimensions, power connections, communication connections, and algorithms may require adjustments to accommodate these changes. Achieving a ubiquitous band-geometry-dimensioning necessitates close interdependency between hardware, software, and functionality design.
To summarize, we have looked at these six parameters (4 system and 2 product) from the perspective of building a device that satisfies the following goals: comfort, accuracy and functional input for all day wear, that is durable, and can handle electrode detachment.
In the next blog we will dive into sensor fusion. We will present different types of sensors, and explore which ones are optimal for neural input wristbands.
*All figures shown in this blog are taken from our white paper, available for download here.