The design of permeation cells for use with glove – and skin samples under conditions of continuous and intermittent exposure
D.W. Bromwich
Griffith University, Brisbane, Australia
Background
Over seventy designs of permeation cells for gloves and skin have been published, but each has its limitations, so there was scope to design a better cell. This study reviewed these designs and developed detailed ideal design criteria for an open-loop permeation cell and an automated intermittent permeation cell.
Methods
Following the design and testing of prototype open-loop permeation cells for continuous and intermittent exposure, a comprehensive list of design criteria were developed for both designs. These criteria were based on the experience with an ASTM F739 cell, review of all known existing permeation cell designs and known problems and the experience gained with the prototype cells. A list of the ideal design criteria is given below.
1. Test any portion of CPC, intact or excised, including seams. Skin can be seen as a delicate membrane without seams.
2. Use with solid, liquid or gas test chemical
3. Use with solid, liquid or gas collection medium
4. Small dead volume to ensure fast response
5. Capable of being used from low flows (50 mL min-1) to high flows (10 Lpm) of carrier gas without significant pressure on sample.
6. Flow pattern of collection medium to remove boundary layer from the sample
7. Easy to decontaminate
8. Quick to assemble and use
9. Resistant against solvents, acids, alkalis
10. Little training in use to get reproducible results
11. Rugged – undamaged by a 1 m drop test onto a hard floor.
12. Replaceable components are “off the shelf”
13. Cheap and easy to fabricate
14. In the public domain (no copyright or royalty)
15. Can be used in field
16. Capable of being used for intermittent exposure testing
17. Adequate sample size to prevent edge effects
A computer controlled testing rig and software had been developed for testing up to eight prototype cells simultaneously and both the rig hardware and software were upgraded to permit more extensive testing of the new cell designs.
An investigation into temperature of the membrane sample was necessary as the air flow to quickly dry the samples between exposures caused significant evaporative cooling that temporarily affected the permeation rate. The effect of pre-exposing the sample to solvent vapour on initial non-zero permeation rates is evident in data published with the ASTM F1383 standard for intermittent exposure and this effect has been investigated.
To determine the collecting flow patterns in the cells, water was used as the collecting fluid and a bolus of red dye was injected into this flow. The collecting flow patterns and dye clearance times were observed with a clear plastic disk in place of the test membrane and the cell clamping frame modified to permit photography.
The permeation measurements in these trails were with reference neoprene supplied by ASTM Committee F23 and the collecting fluid was air, as the detector was a photo-ionisation detector (HNU 101) and the flow sensors were designed for air. However, the test rig could be adapted for other fluids like water or saline, with different detectors and flow sensors. The room temperature was 20±1˚C.
Results
Continuous exposure cell
A number of designs of open-loop permeation cells for continuous exposure were considered. A design that did not distend the test sample as the flow rate increased was selected. Not all design criteria were fulfilled, particularly the use of the cell with solids.
Figure 1. New permeation cell in frame with certified torque wrench
Construction is of brass, as this is easy to machine, but the cell could be made of stainless steel. Some parts such as the body, are simple thick walled tubes, and could be made of glass to permit the sample to be observed. The cell can be assembled and clamped in its heavy frame in seconds. The collecting fluid – gas or liquid is forced at pressure (Figure 2) though the quick connect port “A” where it is forced through a 0.1 mm annular slit onto the test membrane at the point it is clamped. This arrangement ensures the stresses on the test membrane are minimised and the jet of air moves uniformly towards the centre of the test membrane.
Figure 2. Flow of collecting fluid in new cell
The flow is very turbulent and is designed to remove much of the boundary layer from the test membrane. The cell design also ensures that the cell has a small dead volume and no dead zones where the collecting flow stagnated. This was demonstrated with a bolus of red dye with water as a collecting fluid.
Validation with ASTM F739 Cell
The new cell was validated against data provided in ASTM F739 (acetone, reference neoprene) and was within the acceptance limits.
Intermittent Cell
An automated intermittent exposure variant of the cell was designed with only one moving part, to meet the additional design criteria:
1. External control over filling of cell.
2. The entire test sample should be wetted.
3. Wet periods should be precisely and accurately controlled
4. The drying flow must evaporate test chemical efficiently.
Figure 3. New intermittent exposure cell.
Some experimentation was needed to make the nozzle wet the test membrane, and ensure the drying flow removed any wetness from the test membrane. The operation of the cell is shown below.
Figure 4. Operation of new intermittent exposure cell
Data using reference neoprene and acetone is shown below with data published for the standard ASTM F1383 intermittent cell.
The glitches in the permeation curve with the new intermittent permeation cell correlated with evaporative cooling of the sample, though this cooling made little difference to the temperature of the effluent collecting air.
Figure 5 Comparison between experimental and ASTM F1383 data
The non-zero initial permeation rate with the ASTM data is most probably due to exposure of the test membrane to acetone vapour before the membrane was exposed to liquid acetone. This was simulated with the new intermittent exposure cell by not turning on the drying flow or the intermittent exposure mechanism for the first 90 minutes. The initial permeation curve and the subsequent “normal” trial are shown on a log scale as the initial vapour permeation rate was low.
Figure 6. Exposure to acetone vapour, then intermittent exposure to liquid acetone.
Conclusions
A new open loop permeation cell that can be used for a wide range of liquid chemicals with both liquid and gaseous collection media has been designed and tested. The cell can operate under computer control with either continuous or intermittent exposure to a test chemical and is particularly suited to delicate membranes. The design offers a very small collecting volume and a high flow velocity at the collecting surface of the test sample, to maximise disruption of the boundary layer. The cell is equivalent to the ASTM F739 cell and appears to have a smaller collecting volume and less potential for dead zones than all other cell designs reviewed, including Franz-type cells.
An automated intermittent exposure variant of the cell has also been developed, that gives data similar to the ASTM F1383 Intermittent cell, but strictly controls wet and dry parts of the exposure cycle. However the enhanced drying of any chemical temporally cools the test sample, producing a glitch in the permeation rate. There is no simple way to avoid the glitch and change quickly from wet to dry conditions, but the drying flow does ensure unintended vapour exposure does not occur.
The cells should be capable of testing skin in vitro.
Content last modified: 26 May 2005
