Use of membranes in sensor technology

Use of membranes in sensor technology In this article, Editorial

Board member Dr Peter Ball discusses how microporous

can be used in a variety of applications

in the construction

of sensors.

membranes

The

This includes their use

relationship

required

as a surface to which molecules involved in the detection function of the sensor are coupled, as

pore

a reading surface in the sensor for measurement

membrane

undesired components microporous properties

of a signal and as a barrier to passage of

of the fluid being analysed.

membranes

The physical and chemical properties

make them well-suited

and gives examples of how membranes

to this role. This

article

discusses

of

these

may be utilised in sensors.

between

size is constant

method

type.

for

The

it is worth

a number

of methods

of media.

to cover any optical,

optoelectronic

device used to detect

a component

contact

with

system.

rhis definition

very

broad

systems

the sensor range

utilised

uivo, devices

as part

of in

area of the membrane,

of a fluid in

when

materials

etc.) and industrial

applications

in

enzymes

involving

andiot-

use of

physically

hostile

‘l‘his diverse

BET

broad-ranging

approach

dclibcrately role that

play in sensor

to

has

demonstrate

microporous

technology

to stimulate

potential

new applications.

the

very

membranes

and hopefully,

process,

been

the

reader

can in the

to as the bubble

pointing

ro think

of

membrane,

determines

The membranes films, typically ombination

of either a single polymer

of polymers. depends

Ggures

100 - 200 pm thick, which

about

1

or a of the

on the type of polymer 2

and

epresentativc

The structure

show

surface

types

of membrane

t his case respectively

a Pall Nylon

used.

views

of

material,

in

66 and a Pall

(P’ITE) membrane, to 1:&ytetrafluoroethylene represent typical hydrophilic and hydrophobic membranes. consists

In of

contains which

both

cases,

thickness.

has to be imparted

make it robust

enough

incorporating

into

tihrous

polymeric

the

membrane.

including

to

in use, e.g. by

media

a nonwoven

are both

important

characteristics

FOI- example,

the

of the membrane

characteristics

Mechanical

sub\trare

performance

thickness

which

to the membrane

ro handle

Pore size and thickness the

membrane

structure

a small but finite range of pore sizes and i\ of a specific

strength

the

a microporous

fat-

Membrane Technology No. 101

pore

define

particulate

microorganisms.

of

‘l‘hr

to the

size and its removal

contaminants, pore

size and

to wetting size,

in

hydrophobicity

of

the

of

the

the

ability of such

fluids

be

retained

by the

microporous

in

most

fluid

wetted

in

from that

fluid.

pt-essure than

Smaller

larger

This

for

point

measured

bubble

between

of

results different

methods.

‘Pore size is important for sensor application” Pore size is particularly applications

in which

is for

is to

to force

of a membrane pores

require

pores 70 force fluid

a

liquids

important

a particle

berween

for sensor

of a known

and must be excluded

and retention

is measured method

required

the pores

that

(microns)

and

common

the gas pressure

known

mcmbtane. pm

membranes The

observation

may give different

The relationship the size of particle

expressed

particularly

the first visible bubble,

is present

the membrane.

usually

the bubble

methods,

the

pore

out of the pores and allow air passage.

used in sensors are thin, porous

an be composed

the

Pore size indicates

higher

the

to resist penetration

indirectly.

Structure and properties of membranes

the

point.

out that there are

for measuring

These

and

by this

In this context,

is resistant

fluids, with

measure

nembrane

i.e. one that

conjunction

will

to

In the case of a hydrophobic

aqueous

through

may he important molecules

surface area (also referred to as

area).

membrane

environments. Selected

it’s internal

with

which

specific

surface

the pore size of the membrane,

membrane,

monitoring

(bacteria,

the

the total internal

to use it as a detector.

the smaller higher

including

used for environmental

in chemically

a

measurements

binding

membrane

encompasses

applications,

making

also influence

or electronic of a detection

therefore

for e.g. biological devices

thickness

of this article is intended

fluid

obtained

those which are based on manual ‘Sensors’ in the context

pressure

a given

value

is usually referred

However, point

the

to force the fluid out of the pores and

pore size, particle

ic based on a tests performed

as the

carrier

size

from the sensor.

tluid

for

the

size, using

particle

challenge.

These tests may be based on retention

of viable

particles

viable particles In a gas mechanisms retention. inertia

(microorganisms)

or

non-

(e.g. latex particles). stream,

other

are involved These

more

mechanisms

of small particles

For very small particles,

complicated

in membrane depend

to changes Hrownian

particle on

the

in direction. motion

of the

p.uti&

.lllO\1;~ a mcmbranc

highct- capture from

efficiency

rhc relationship

si7.c rating

IO operate

than would between

.~nd the particle

the liquid

t0

used

membranes,

manufacture

including

polyvinylidene

which

can

microporous

various

fluoride

nylon

(I’VL?F) P’I‘FE.

resins,

polyether-

sulphone

(I’%),

polymers

has

chemical

properties,

which

can

membrane

particularly

suztable

for applications

requiring

and

pore

size.

I’hetc is a wide range of polymers, be

at much

be expected

certain

resistance

prcvcnrion

Each

intrinsic

to

polymer

these and

make

chemical

of passage of liquids

he membrane

of

physical

the

attack,

erc. In addition,

can be surface-modified

o give it new properties,

beyond

those

of the

I:,asr polymer. .i\vo examples re activated Find specific

An

which

exhibit

of specific

example

suited

biologically ‘or

(via

an

An

ot

is Pall which

is downstream

of very small

molecules,

such

amino

a5

of

which

the

*I

low

latter

exhibits

in

\idc ot the membrane.

very

binding.

level

of

nonspecific

Orher

low nonspecific

Membranes as an ‘active layer’

compounds)

‘Active’ in this context

selective

layer forms

a functional

system. Typically,

part

this means

is used

as a high

internal

porous

structure

to which

molecules

the detection

process

because

binding

biologically

peptides,

proteins,

Binding

Certain

are

to

well

to

typically

important nucleic

use

depend

either

molecules

high reflectance Sciences

of aqueous

subsequently

@

molecules, through with

to

of cations,

arc also available of anions

rhat

through

rhe

application

Membranes as a ‘protection layer’

from

the

with of

e.g.

proteins,

a detection

to on

membranes

normally

used

hydrophobic

membranes.

Typically,

the for pass the

more

selective

exposed

(CIP)

operations,

often

use

role

are

a class

allow

of use.

fully later

fairly

of

rhis

are

the

The

selective

an

sensors

effective

flow

(An

means

agents

(e.g.

and/or

alkalis

are The these

chemicals hypochlorites (typically

as and

hydroxide-

a\

and

combine,

materials.

\UCh

‘I’heTc

made

from

polyptopylenc.

materials

allow

performance-related the choice

protection

rate/differential

the

parameters

of :I membrane

layer pressure

These variables

arc

defined

area

are considered

which

differential rcmperaturc downstream.

01 air

How

below.

is a measure

gas tlows

thl-ough

the

by determining

rhe

of air to pass &rough

membrane pressure,

‘1nd

air

and it’s water intrusion

‘This is measured

rime ir rakes a volume

material

it5

The air flow rare of a memhranc membrane.

very

such

main

pressure.

handling

typically

etc. Such

of the ease with

of protecting CIP

a

ale

the

01 differential

ro be sealed by a variety of methods.

influencing as

cleaning-in-place

particulates.

aggressive

peroxides)

mechanisms

makes

The

\upporr

media, nylon

driving

to minimize

wirh

membranes

and cast of

of rhe tnembrane

rhe manut&rurel

materials

nonwoven

flow Irates, ~LII

i)f scaling

gas tlow through

a given and

membranes

in

strength.

product.

difficulties,

support

of

under

in this section.

to moisture, and

of

permeability

feature

oxidising based).

in

rypes

the passage of liquids

retention

membranes

in the presence The

that

offer better

incorporation

for

polyester,

materials

this

conditions,

on the

for the membrane

to maximize

membrane

as a protection

membranes

conditions

described a

e.g. to react phase

etc.

during

into the finished

loading,

chemicals

membranes

handling

environmental

particulates

normal

requirements

this can bc at die expense

used

aggressive

governing

become

it may be important the membrane

to

allowing

side

adverse

gases while resisting

Similarly,

wetted

are typically

under

microporous

opacity,

is co a large extent dependent

performance

terms of gas flow rate and mechanical

Hydrophobic

materials

membranes,

potentially

distal to the detector.

unhindered

and/or

membranes

membrane particular

important

when

of

instances,

as reflectance Some

detection In certain

a material

surface-modified

fluids,

signal

the passage

membranes

fl ow rates”

membrane.

e.g. high

in use.

Supor

II PVDF

range

such as

type

hydrophilic.

transmitting

of

of leakage

ligand

have particularly

light

cationic

pressure

of the sensor.

Pall Fluorodyne

For example

scaling

if these are factors

highly

will allow

confers

used

the functioning

intrinsically

of ions.

“Thinner membranes offer better

in

which

layer when the sensor is a gas sensor and is to be

and/or

them

membranes

will

such

make

passage

for

a layer,

membrane

in selecting

ate I’!33 membranes

role

provide

Membranes

bound

which

ammonium

The

properties,

Pall Gelman

(e.g. quaternary

or non-covalent.

is its surface

e.g.

wide

acids or nucleotides.

considering

opacity,

to a very

‘Knncr

rhis

capable

on the consequences

of any non-covalently Also worth

suited

they are intrinsically

may be covalent

decision

for

be

exhibiting

to attach ro polymeric

only allow rhe movement

area

required

binding

possible

is to

while anionic

are immobilised.

materials

process

surface

should

are also available.

membranes

of the detection

binding

known

surfaces

Another

that the membrane

matrix

membrane

and other

means that the membrane

111.111 OIIC with a vcr)

types of membrane

range of molecules

sensors

101 cx.unpic

more

In this case, a membrane

evaluated.

very

containing

‘;C1ISOI

component.

terminus)

example

is Pall LoProdyne,

low protein

approach

to the coupling

oligonucleotides. approach

low levels

such as proteins.

membrane,

important

peptides

and ‘passivated’

former

ABC

particularly

will covalently

very

molecules,

of the

Immunodyne

membranes

which

types of molecule

membranes, binding

of surface-modified

membranes,

at usually

atmosphcrtc

‘1-0 dccuratclv

a

a

defined at

room

prc5\urc i<,mparc

two

Membrane Technology No. 101

which

places

membrane

fluorocarbons

surfaces,

on

imparting

energy characteristics

exposed

low

to virtually

surface

any membrane

polymer. Table 1, above, summarises properties

for

some

the main physical

typrcal

hydrophobic

membranes.

Incorporating membranes into sensors An

important

method(s) the

sensor.

described

membranes,

the same factors must be considered

on a like-for-like The

basis for both membranes.

differential

pressure

membrane

is defined

measured

between

upstream gauge

flow a

difference

gauge

is

constrained.

approxrmatron,

across

as the pressure a pressure

located

can

the flow through

be likened

which

the

capillary

As

to flow

capillaries

between

is expressed

rough

a membrane

in true

relationship

diameter

a

downstream

of the

flow

and

as:

is highly

influenced

porous A

high

measured

membrane

Al,

is creating

indicates

resistance

that

the

to gas flow. This

material

resistance fluids

to

membrane

rate per unit area of membrane

intrusion

small

a number

membranes,

of

pores

particdates

or fluid trapped Water

clogging

of the force required

the

through

required in units

pressure

sensor caused

are particularly is exposed

of water

flow.

membrane’s

water

intrusion

characteristic

membrane

properties.

and the nature

A

is a function factors

of the hydrophobic

water the

are pore size

to cause

tluid

force acting

flow because

wetting

being wetted. lower than

the material. intrusion,

fluid

a porous The

through

polymers

critical

with

afrer manufacturing.

the fluid

surface

energy

of membranes as

materials

groups

onto

It has not related

their

or to

of functional These

may need to consider of

different fluids,

for attachment aspects

feasible

important

of

with different

membranes.

information

it has of the

to their selection,

practical

which

are

ail

an interested depending

application.

is typicall\-

membrane

broad

been

other

important nature

the

this article,

compatibility

methods

range a diverse

available

on Such

from

the

supplier.

Only liquids with surface as demonstrated can

ultimately

can wet by water be forced

regardless

pressure

the surface

of its

is applied tension

needed

be

to

of a

to force that

Examples

polymer,

or

Acknowledgements The author

would like to acknowledge

of his colleagues

MS Elisabeth

from

the

groups

Scientific

chemistry

are PTFE, Repel

the input

Pall. particularly

lander and

Laboratory

in Port Washington,

Dr Jane

and Dr Sima Jafari Services

New York and at

UK,

using

or they can be

energy

within

Janas,

Portsmouth,

manufactured

low surface

a driving

under

ro describe

such describe

in

to give an overview

aspects

the

versatile used

Within

applications.

the

with low surface energ),

post-treated

possible

this review

fluid

or surface energy, of the

be

applications

only been

researcher

or

the membrane. can

membrane

that are available

.I

can

of ‘sensors’.

the

fluid, the lower the pressure Membranes

of

that can be employed.

represent which

of

heading

tension,

membrane,

lower

materials

use

which are

Concluding remarks

is based on

and

the

various

The

into

of methods.

and

utilised.

the surface

surface energy, if sufficient the fluid.

intrusion

that of the material

However, any

through

liquid

flow. A balance

of a material

between

energy

of various

with small pores require

The hydrophobicity

welding,

and support

membrane

be considered.

the relationship

material

water

membrane

the air flow rate and water should

of

the pores of the membrane.

Membranes

to air

of the

under

sonic

routinely

within

a small-pore

a higher

a large-pore

resistance

surface tension,

or high-

As already mentioned,

contributing

higher

energy,

relevant

water-

by depth

pressure

two principal

to

than

wetting

(e.g. lb/in’ (or psi) mbar or bar. Water the

pressure

to

a membrane.

are expressed

intrusion/breakthrough

pressure

water

respectively

pressure

and

b enveen

to push water into the pores

cause bulk flow of water

within

of

pressure

and

are measures,

and

.l’hese characteristics

when

area

the membrane,

pressure

pressure

of a membrane

pressure

unit

on or in the membrane.

intrusion

breakthrough

per

have

is the

membranes

polymers

possible

surface-tension

‘Therefore. to

the

hydrophobic,

of high

tends

media

size. When

is in addition

passage

is increased.

can be caused by too small a pore size for the flow being utilised, too

by pore

of

techniques

range a microporous

types

adhesives are some common

of media,

(ffl), i.e.: flow through

the

can be sealed bv a variety

Membranes

Therefore,

of

increase the sealing options

(I =d’

membrane

consideration

in

(/‘z] and a pressure

of the membrane

located

(AP)

path

All

sealing,

Heat

Table 1. Comparison of Different Membrane Properties

practical

to be used or sealing membranes

a low

treatment,

For further information Scientific

and Laboratory

Ltd., Walton

Road,

contact: Dr Peter Ball, Services,

Portsmouth,

Pall Europe Hants,

1RH, UK. E-mail [email protected]

GU32