Design of Health TechnologiesDesign of Health Technologieslecture 12lecture 12

John CannyJohn Canny10/17/0510/17/05

Advanced Sensing SystemsAdvanced Sensing Systems

Biosensors: Glucose monitoring Other systems

Magneto-elastic sensors Magneto-elastic sensors (Grimes)(Grimes)The magneto-elastic material resonates at a

characteristic frequency when excited by a magnetic field.

Magneto-elastic sensorsMagneto-elastic sensors

The magneto-elastic ribbon is made of a commercial sheet called Metglas.

The polymer is a custom co-polymer made by the Grimes group. It is believed to work because glucose bonds to sites on polymer chains that separate them from other chains. This allows the polymer to absorb water.

Magneto-elastic sensorsMagneto-elastic sensorsIts frequency response (in air) shows a sharp peak

which is determined by the density of the polymer layer

Magneto-elastic sensorsMagneto-elastic sensorsResonant frequency in a liquid is lower, and the

peak is not as sharp.

Magneto-elastic sensorsMagneto-elastic sensorsFrequency response in water varies with the

glucose concentration, in an almost perfectly linear curve.

Sensor measurementSensor measurementThe electronics are simple. A sharp spike is applied

to a driving coil, and a response is measured in a sense coil.

Sensor measurementSensor measurementThe magnetic spike is short, about 3 gauss for 16

micro-seconds (earth’s magnetic field is about 0.5 gauss, and a refrigerator magnet about 10 gauss).

The pickup coil measures sensor activity for a further 8 milli-seconds. The response is transformed with an FFT to determine the frequency peak.

This should be easy to do with a small, battery-powered device. Because the sensor’s response is quite slow (tens of minutes to respond), it is enough to take readings every few minutes.

Biosensor statusBiosensor statusThere are many promising systems on the horizon,

but the only commercially-deployed biosensors are glucose monitors (~$4B). 3 main types:

Single Use: Disposable sensing material, often “static” measurement. Cheap and portable, but low sensitivity and accuracy.

Intermittent Use: Often use hydrodynamics – generally much better performance from sensing a moving fluid. Its still a challenge to move these out of the lab and onto a chip.

Biosensor statusBiosensor statusContinuous (In Vivo) Sensors: Very economical,

but very hard to calibrate and may suffer from unknown amount of drift.

Biosensor designBiosensor designWe give a brief introduction to micro-fluidic sensor

design.

While these were originally fabricated in silicon using MEMS techniques, the trend is toward glass and plastic as the substrate.

Both glass and many plastics allow optical measurements, but silicon is opaque to visible light.

Glass and plastic are also more resistant to contamination from the chemicals used in the measurement.

Biosensor designBiosensor designSurface immobilization: The first step is sensing

is creating a selective surface to react to the sensed agent

Biosensor designBiosensor designBead immobilization: A variation that uses beads

to increase relative surface area.

Biosensor designBiosensor designDetection: Several methods, including resonant

frequency of MEMS cantilevers. But amperometry (current measurement) is the most widely used approach. Typical mechanisms for current flow include redox cycles between the target group and variants.

Biosensor designBiosensor designOptical Detection: A 2D

array of agent/antigen reactions produces fluorescent traces:

Biosensor designBiosensor designMagnetic Detection: The antibodies are

immobilized on a surface and magnetic beads bind to sites where the analyte is attached.

Enzyme-Linked Immunosorbent Assay

ReuseMost immunosensors use bound antibodies and Most immunosensors use bound antibodies and immobilization. Removing the bound species can be immobilization. Removing the bound species can be difficult without destroying the sensors. difficult without destroying the sensors.

Methods and results vary, but a recent detector for Methods and results vary, but a recent detector for Chagas disease used glycine-HCl to wash the Chagas disease used glycine-HCl to wash the sensor, and reported efficacy for more than 30 sensor, and reported efficacy for more than 30 cycles. cycles.

Biosensor designBiosensor designSystems-on-a-chip: are promising but coming

slowly. Biosensing still seems a long way from commercial viability. But there are some promising prototypes:

Discussion QuestionsDiscussion Questions

1.1. It may be a while before we have highly It may be a while before we have highly integrated sensors for many pathogens (and integrated sensors for many pathogens (and economics dictates that they will come for first-economics dictates that they will come for first-world diseases first). Can you think of world diseases first). Can you think of telemedicine/information tools to help facilitate telemedicine/information tools to help facilitate traditionaltraditional (but simple) lab methods? (but simple) lab methods?

2.2. Sensors for medical diagnosis may always be a Sensors for medical diagnosis may always be a difficult economic proposition. Can you think of difficult economic proposition. Can you think of other models that might work? E.g. home other models that might work? E.g. home testing? testing?