TEN POINT MANUFACTURING GUIDE TO THE USE OF
BIOGEL HIGH WATER CONTACT LENS MATERIALS
1. Blank storage
Store in the sealed foil pouch until needed. After opening, any surplus blanks should be left in the pouch and be resealed by folding and using a clip or sticky tape, and placing in a plastic box with a tight fitting lid (e.g. Tupperware). Note that high water (HW) materials are more hygroscopic than many desiccants such as silica gel. Any part-cut blanks must be polished before storage in plastic boxes or in an oven at 30-35°C
2. Environment
Due to the moisture sensitivity of these materials the laboratory environment may need to have the humidity controlled to 55% ± 5% RH. This is in addition to any air-conditioning for temperature control.
3. Handling
Dry handling is of major importance. Firstly due to their great affinity for water, they can absorb it without notice in the dry state, only to appear after hydration as 'orange skin' or random spots. To labs inexperienced in their use most problems are from unseen moisture on fingers during general handling or from the breath during examination with a loupe magnifier. Only moisture contact between lathing and polishing produces any defect. It is suggested that rubber finger cots (finger gloves) are used and holding the lens 'above' eye level when examining it.
The second handling problem is stressing the lens in the dry state, which shows itself as cracks or broken lenses only after hydration or sometimes autoclaving. The most frequent cause of this is when measuring dry lens thickness using a mechanical thickness gauge. Preferably only the half cut button or the hydrated lens should be measured for thickness, NOT a finished dry lens.
4. Machining
Poly HEMA dust will stick irreversibly to dry HW materials, it cannot be removed without damaging the lens. Do not mix production of the two materials, or ensure separate handling and good dust extraction. Also have separate polishing materials. The HW material cuts much easier than HEMA, more like PMMA. Adjust lathing spindle speeds and cutting rates to produce continuous 'streaming' of swarf. HW material is generally easier to cut, less brittle and gives longer diamond tool life.
5. Polishing
Polish in any of the usual ways - plastic foam, pitch, or cloth. Since less hard than HEMA, care should be taken to use top quality polish, without 'lumps' which can cause scratches. Because of the higher swell factors the surface finish has to be better than HEMA. Use mineral oil based polish rather than silicone. Because the HW material is softer than pHEMA it is important to polish for the minimum time possible, and not to let the polish 'dry'.
6. Cleaning
Clean dry lenses with petroleum ether 80-100 C boiling range as first choice (Care-Flammable!). Do not use anything containing aromatics (i.e. Benzene C6H6). Some silicone oils have been used, however they can cause subsequent wetting/dry patch problems. In the past the Fluorohalogens (e.g. Freon/Arklone) were preferred, however these are now becoming unavailable (ozone thinning!). It will probably be worth trying the replacements as they become more generally available.
However the lenses are cleaned in the dry state, cotton bud or ultrasonic, the lenses should always be cleaned after hydration, preferably using a non-ionic surfactant, between thumb and forefinger or in the palm of the hand. This manual cleaning will also reveal any latent defects such as cracks in the lens (see 3), and is the most effective way of producing top quality lenses free of manufacturing residues.
7. Hydrating
The HW materials hydrate very rapidly. They can be considered to be fully hydrated in 60 minutes. The first hydration in approx. 20 ml of saline per lens, will extract unreacted residue from the lens as well as swelling to finished size. At this time the lens should be cleaned with a non-ionic surfactant (see 6) and rinsed in fresh saline and may then be measured to confirm final lens parameters, before packing in a vial (or 'blister') containing a further quantity of fresh saline and autoclaving.
Good manufacturing practice and medical device regulations should be followed with regards any microbiological controls and testing that need to be carried out, from the point of hydration through to autoclaving. National and International standards give methods, guidance and tolerances for contact lenses and processes. The quality of the saline used and any environmental controls required should be considered in consultation with a qualified microbiologist knowledgeable of medical device manufacture.
8. Measurements
Any measurements of the hydrated lens should comply with the test methods given in International standards, and the measuring equipment should be properly calibrated. It is most important that the lens is both conditioned before measurement and measured, at a temperature of 20° ± 0.5° C. The HW materials will change their swell substantially with a change in temperature of only a few degrees. To equilibrate the lens it should be maintained at the measurement temperature for at least 30 minutes before measurement.
9. Packing
The HW materials may be packed in any primary container that is compatible with the method of sterilization and will maintain sterility throughout its shelf life. The HW materials themselves will not undergo any physical or chemical change while hydrated. However the slow loss of moisture from the package will slowly change the tonicity of the saline packing solution, until this alters the parameter of the lens sufficient to fall outside its specification. Shelf-life studies or historical experience with a particular package will enable an expiry date to be determined. There have been occasions when lenses have gone bright green, yellow or orange after autoclaving, this has usually been traced to impurities in rubber stoppers used to cap lens vials.
10. Patient handling and care products
The Biogel HW materials are compatible with any contact lens care product that is suitable for use with an FDA Class II material (>50% water content - non-ionic). It may also be disinfected using heat, provided the lens has been thoroughly cleaned using a surfactant contact lens cleaner.
Compared to pHEMA lenses the HW materials may appear more fragile due to the rapidity of 'tear propagation', any small edge damage usually leads to lens breakage. For this reason both fitters and wearers should be cautioned to handle the lens carefully and to keep finger nails short and smooth. The HW materials have an affinity for any colorants e.g. nicotine from the fingers of heavy smokers, cosmetics etc. Some care products, notably those containing sorbic acid will also turn lenses yellow.
Because of the temperature effect on swell, HW lenses will shrink when placed on the eye. Therefore the lens should be fitted significantly flatter than a pHEMA lens. The higher the water content the flatter the lens must be fitted. A prospective wearer should be examined after wearing the lens for 30 minutes before deciding the lens parameter to be dispensed. HW lenses should not be made less than 0.09 mm centre thickness or corneal desiccation may result. Since HW lenses are much thicker than current pHEMA lenses, a spherical lens will mask up to 2.00 D of corneal astigmatism.
Owing to their high oxygen permeability, compared to pHEMA lenses, the HW lenses have been prescribed for extended wear (EW) applications. It is important to note that EW is associated with a much higher incidence of sight threatening complications, in particular corneal ulcers, than for cosmetic daily wear. The prescribing of HW materials for EW should not be undertaken without a full knowledge of the risks involved and the precautions to be taken by both the practitioner and the wearer to minimise any threat to ocular health.
© I H Polymeric Ltd. 1996
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