- Francesco Pini
- Kevin McCarthy
The development of BioSensors for recording electrical activity and stimulating different sites of the human body has seen an incredible acceleration in the last decade. In the last few years, the recording and stimulating electronics, fundamental to interfacing single biosensors to a control device (computer, external actuator, wheelchair), were situated outside the human body and occupied a relatively large amount of space, rendering impossible any type of portable application.
The miniaturization of BioSensors, in conjunction with the maturity of microelectronic technology (especially CMOS technology), has made possible the realization of micro-prototypes of BioSystems, all implantable inside the human body. This condition has opened the door to a large amount of applications and projects that could not have been thinkable even a few years ago.
An example of a BioSystem consists of the realization of a totally integrated fully implantable System to interface a Biosensor (Cuff Electrodes) with an actuator device. The integrated system will be part of a BioElectrical Closed Loop Functional Electrical Stimulation (FES) System that has many applications in the rehabilitation field.
The Cuff Electrodes consist of a cylindrical tube of insulating material (silicone rubber) which is fitted with metal electrodes and placed around a nerve or a nerve bundle. By making those cuffs tripolar (that means three electrodes inside the cuff) it’s possible to record a high nerve signal (ENG or ElectroNeuroGram), while decreasing as much as possible the interference pick-up (EMG signal and network interference at 50Hz).
Fig: Cuff Electrodes placed around a nerve bundle
The interfacing system, connected directly to the Cuff Electrodes, and situated as near as possible to the recording site, consists of a very low noise analogue front end, necessary to amplify the weak ENG signal and avoid the addition of further white noise, a stage of selective filtering and amplification, a stage of analogue to digital conversion and a further stage of digital signal processing and data transmitting.
The realization of the analogue electronics is very challenging in term of power consumption, noise performances, CMRR, PSRR and size of the system. Nonetheless the final purpose of the project is to realize a lifelong product that should be implanted in the human body for decades. For this reason it is unfeasible to think to supply the system with a normally battery supply and it’s mandatory to think about finding other ways to supply the implanted electronics.
Status of the project
The first step in developing a recording system for cuff electrodes is the analysis of the signals that the sensor (cuff electrodes) is picking up. The main components involved in the input signal are:
- The nerve signal (ENG or Electroneurogram) that has an amplitude of a few microvolts (3-10µV) and main components concentrated in the kHz bandwidth (1-10kHz);
- The muscle interference (EMG or Electromyogram) that has an amplitude in the millivolts range (1-10mV) and frequencies that vary from 1Hz to 3kHz (main power concentrated at 250Hz);
- White noise due to interstitial fluid and electrode-tissue interface.
By making the cuff electrodes tripolar (three electrodes) and connecting them in an opportune way (Quasi Tripole Scheme), it is possible to reject the EMG interference so that only the ENG signal will be amplified.
Fig.2: Cuff Electrodes Electrical Model and Quasi Tripole Scheme
Since the amplitude of the signal of interest (ENG) is very small and is just above the noise level, as a first step it is mandatory to design a low noise stage of amplification to boost the level of the signal without adding more noise. For this purpose the design and comparison of different topologies of differential low noise amplifier has been afforded. The object of this first stage of amplification is to amplify the differential input signal by a factor 100 (40dB) and add the least amount of noise possible without improving too much the power consumption. Giving a maximum power dissipation of the stage of 1mW, with a supply voltage of 3.3V, the maximum available current results in 300µA. With this current budget, sizing opportunely the transistor devices, it is possible to obtain an input-referred RMS noise voltage of 300nV in the bandwidth of the signal (1-10kHz). An example of the topology of the low noise amplifier is shown in the following figure. The input pair (pMOS transistors) has been sized to work in the weak inversion region while the current mirror (nMOS transistors) has been designed to work in the strong inversion region. In this way it is possible to minimize the input-referred voltage noise without increasing too much the current consumption.
Fig.3: All CMOS low noise preamplifier
The designed preamplifier shows power consumption and input-referred RMS noise voltage as reported in the specifications on a reasonable area.
Plan for the future
The preamplifier topology is now at the layout phase and it will be realized in the next few months. Together with the amplifier, other stages in the analogue front end have to be developed (filtering and amplification, ADC) and integrated on the prototype.
The following stage will be the test of the designed prototype in the laboratory for electrical purposes and in-vivo for obtaining experimental results.