Sensores Mosfet

Proceedings of the 3rd International
IEEE EMBS Conference on Neural Engineering
Kohala Coast, Hawaii, USA, May 2-5, 2007

ThD1.15

Spiking Chemical Sensor (SCS): A new platform
for neuro-chemical sensing
Pantelis Georgiou∗ , Iasonas F. Triantis† , Timothy G. Constandinou† and Chris Toumazou†
† Institute

of Biomedical Engineering, ∗ Department of Electrical and Electronic EngineeringImperial College London SW7 2AZ, UK
Email:{pg200, i.triantis, t.constandinou, c.toumazou}@imperial.ac.uk

Abstract— A spiking chemical sensor (SCS) is presented for
detection of neurogenic ion concentration associated with active
nerve fibres in nerve bundles. Based on the ”integrate-and-fire”
circuit, the SCS uses a chemically-modified ISFET front end
encoding the sense data in the spikedomain. Used in an array,
it provides a spatio-temporal map of chemical activity around
the nerve bundle which may be relayed off chip using low
power asynchronous communication hardware. The circuit is
shown to be tunable to yield a linear relation with either pH
or actual hydrogen ion concentration. Furthermore, its compact
pixel footprint in addition to efficient use of the sensing surface,makes it ideal for use in neuro-chemical imaging.

are intrinsically sensitive to hydrogen ions, the selectivity may
be changed by depositing various ionophores on the surface
[5].

I. I NTRODUCTION
Devices for monitoring neural activity can be used for
a range of applications including studies of neurophysiology and neuropathology, diagnostics, drug monitoring and
rehabilitation [1].Although nerve conduction is based on
ionic current flow, most neural monitoring studies focus on
bioelectric recording (recording of electroneurogram or ENG)
with very few studies using ionic sensing in-vitro or in-vivo.
This is almost entirely due to the conventional apparatus for
ion sensing in chemical laboratories being cumbersome, slow
and innapropriate for small concentrations [2].
ISFET(Ion Sensitive Field Effect Transistor) based chemical sensors, first introduced by Bergveld [2], have gained
considerable interest due to their ion sensitivity, fast temporal
response, compact size and prospect of monolithic integration
[3]. Although much of the initial literature predicted ISFETs
to be mainly used as future tools for electrophysiological
measurements, the sensors were notfurther developed for
biomedical applications in particular, but rather for ion sensing
in general [2].
In order to develop an ISFET-based interface for measuring neurogenic ion concentration variations [4], certain
customization is needed. The main requirements of a neurochemical interface include robustness, compactness, spatiotemporal selectivity and ionic specificity. To achieve this,
customdesigned ISFETs based on CMOS technology are integrated with compact local processing to realise an “intelligent
chemical sensor”. This can be combined with a multitude of
such sensors in an array, providing spatio-temporal sensing
and the added advantage of sensor failure immunity (by
redundancy). The next step is to characterise the sensors by pH
measurements which can then be adjusted tobe selective to
specific ions that relate to neural conduction. Although ISFETs

1-4244-0792-3/07/$20.00©2007 IEEE.

Fig. 1. Potassium ion sensing from a nerve bundle. The ISFET array is used
to generate a spatio-temporal map of chemcial activity.

Presented here is an integrated chemical image sensor
with local signal conditioning and pre-processing circuitry.
Fabricated in acommercially available CMOS technology, the
SCS includes an ISFET-based device modified for potassium
ion sensitivity in addition to bias, drive and signal conversion
circuitry. Moreover, a matrix of sensors has been tesselated on
a single chip, forming the basis of a chemical image sensor
and additionally improving robustness and accuracy through
redundancy. Furthermore, by embedding additional local...