Twin‐Wire Sensor Networks

Abstract

A fundamental limit to high‐density sensing is that adding sensors increases the number of wires, pads, and interconnections. This problem is even worse for low‐values resistive or impedance sensors which, for high accuracy, require 4‐wire measurements. Here, twin‐wire sensor networks are described which enable 4‐wire measurements with significantly fewer wires, pads, and interconnections. The effects of the resistor noise can be minimized by minimum‐resistance sensing paths. A single chopper switch and straightforward digital operations can reject the equivalent input offset and low‐frequency noise voltages of the instrumentation amplifier, thus requiring no hardware changes. The inclusion in the network of a reference resistor can compensate the errors of both the biasing current and the instrumentation amplifier voltage gain. For validation, an extremely compact, robust, and easy‐to‐connect flexible‐PCB twin‐wire 29‐temperature‐sensors network requiring only 32 pads is demonstrated. This device is placed on an anthropomorphic head phantom for detecting the temperature and location of a touching object or on the hand of a volunteer for monitoring skin temperature during mental and physical stimulations. A twin‐wire 29‐photoresistors network is also presented. The strategies reported here can be applied to any high‐density array of resistive or impedance sensors (temperature, strain, blood flow, light, etc.) and may find wide application in robotics and wearable or epidermal devices.

Fundamentals of Skin Bioimpedances

“…The significant series parasitic impedances due to the insulating nature of the stratum corneum would result in large errors for 2-wire characterizations of the tissues beyond the stratum corneum (Note S2, Supporting Information). In principle, this issue can be solved by 4-wire (or tetrapolar) bioimpedance measurements [36][37][38] which, only ideally, allow to measure an impedance of interest independently on the parasitic series impedances (Notes S3 and S4, Supporting Information). However, even with ideal 4-wire measurements, a key problem in multilayer structures is to understand which parameters significantly or negligibly contribute to the measured bioimpedance.…”