Spy Pills

AN INGESTIBLE ELECTRONIC PILL FOR REAL TIME

ANALYTICAL MEASUREMENTS OF THE GASTRO-

INTESTINAL TRACT

Erik A. Johannessen1, Tong-Boon Tang2, Lei Wang1, Lili Cui1, Mansour

Ah mad ian2, Nizamettin Aydin2, Alexandros Astaras2, Alan F. Murray2, Brian

W. Flynn2, Tugrul Aslan2, Steve P. Beaumont3, David R. S. Cumming1 and

Jonathan M. Cooper1.

1Department of Electrical Engineering, The University of Glasgow, UK.

2Department of Electrical Engineering, The University of Edinburgh, UK.

3Institute for System Level Integration, The ALBA Centre, Livingston, UK.

Abstract

A state-of-the-art electronic "pill" has been developed for in situ studies of the gastro-

intestinal (GI) tract using integrated circuit and system level integration technologies.

The measurement parameters include real time analysis of temperature, pH, conductivity

and dissolved oxygen.

Keywords: ingestible pill, sensor array, microchip control, wireless communication

1. Introduction

Ingestible analytical microsystems have extended the application of lab-on-a-chip

devices to environments with restricted access [1]. There is now a great potential in

developing non invasive microelectronic pills capable of monitoring physiological

processes in situ for the detection of disease and abnormalities in medical research.

The device comprises 4 microelectronic sensors. A microfabricated silicon diode

measures the body core temperature whilst an ISFET sensor [2] measures local pH of

the stomach acid and the intestine. A pair of direct contact gold electrodes measures the

conductivity of, and indirectly, the contents of the stomach. The fourth sensor consists

of a three-electrode electrochemical cell [3], which is used to detect the level of

dissolved oxygen and to identify the activity of aerobic bacteria in the small and large

intestine. All the sensors are controlled by an application specific integrated circuit

(ASIC), which sample the data at a resolution of 8 bit prior to communication off chip

as a single interleaved data stream.

2. Experimental

The electronic pill comprise a biocompatible capsule, which consists of a chemically

resistant polyether-terketone (PEEK) coating, the four microfabricated sensors, the

ASIC control chip and a discrete component radio transmitter (Fig. 1). The unit is

powered by two SR44 Ag2O batteries (3.1 V), which provides an operating time of 35

hours at the rated power consumption of 15 mW.

The sensors were fabricated on two separate 5x 5 mm2 silicon chips located at the

front end of the capsule. The temperature sensor is embedded in the substrate, whereas

the conductivity sensor is directly exposed to the surroundings. The pH and oxygen

sensors were enclosed in two separate 8 nL electrolyte chambers containing a 0.1M

KOH solution retained in a 0.2 % calcium alginate gel. The electrolyte maintains a

stable potential of the integrated Ag/AgCl reference electrodes used by the two sensors.

The oxygen and pH sensor are covered by a 12 µm thick film of teflon and nafion

respectively, and protected by a 15 µm thick dialysis membrane of polycarbonate.

The signals were conditioned by the ASIC and then transmitted to a local receiver

(base station) at 40.01 MHz prior to data acquisition on a PC. The applied simplex

communication link, based on a direct sequence spread spectrum communication

system, can handle data from several pills at the same time.

ASIC

Sensors

Ratio

transmitter

Capsule

(front section)

Batteries

Viton rubber

O-ring

Chip carrier

(PCB)

Fig. 1: Electronic pill comprising sensors, control chip (ASIC), radio transmitter

and batteries. The unit is encapsulated in bulk-machined PEEK. The rear

cap is removed for clarity.

3. Results and discussion

The signal resolution was 0.4oC, 0.6 pH units, 0.02 mS cm-1 and 0.1 mg O2 mL-1 over

the specified dynamic ranges (Fig. 2). Real time in situ tests (Fig. 3) have demonstrated

a temperature dependency of the pH sensor, thus emphasising the additional importance

of the temperature sensor to correlate for signal changes from the neighbouring sensors.

The generic nature makes the pill adaptable for use in corrosive environments related

to environmental