Keeping an eye on the quality of Ocular Delivery Products using Texture Analysis

Treatment of some diseases is best achieved by direct administration near the affected area, particularly with diseases involving ophthalmic, optic, dermal, oral cavity, and anorectal tissues.

Ocular drug delivery is an interesting and challenging opportunity for pharmaceutical development. 

The eye is protected by a series of complex defence mechanisms, which usually result in poor bioavailability of drugs administered via the ocular route. These include tear production and the blinking reflex in addition to short residence time and reduced drug permeability. The drug bioavailability can be enhanced either by providing prolonged/sustained delivery to the eye or by facilitating transcorneal penetration. 

The application of soft contact lenses, ocular inserts, viscosity-enhancing/phase transition systems and mucoadhesive polymers have been demonstrated as promising approaches to extend the residence time of various drugs in the eye. Nowadays, hydrogels based on  a range polymers are commercially produced and widely applied as soft contact lenses. The unique features of these hydrogels are their biocompatibility and excellent physicochemical and mechanical properties. 

The mechanical properties of a contact lens material are rarely the first thing on a practitioner’s mind when selecting the ideal contact lens with which to fit their patients. However, a contact lens is subject to external forces both on the eye by the lids, during handling and also during the manufacturing process. The success of the contact lens material and the impact of these external forces are governed by the mechanical properties of the material. 

With the increasing use of silicone hydrogels, attention is starting to be refocused on mechanical properties and the ocular complications that can arise as a result of stiffer, less flexible materials. Indeed, with the very wide variety of gas permeable, conventional hydrogel and silicone hydrogel materials available and with the differing mechanical behaviour of each of these classes of material, it seems reasonable that the mechanical properties of contact lens materials should warrant further attention.

From the perspective of many contact lens wearers, comfort and good vision are the two main qualities to be achieved with any contact lens. Comfort can most easily be achieved from a flexible hydrogel contact lens that drapes easily over the cornea and has minimal interaction with the eyelids during blinking. The comfort achieved by high levels of flexibility is offset by the reduction in durability. Conventional hydrogel materials have poor handling characteristics in comparison to their gas permeable counterparts, with low tensile and tear strength.

According to French (2005), a high degree of flexibility can also be a disadvantage when trying to achieve optimum vision. However, an increase in stiffness or rigidity, such as that associated with gas permeable lenses, will achieve this, but at the expense of initial comfort.

The rigidity of a lens material affects the flexure of the lens on a toric cornea with the resulting reduction in optimum vision, as well as the movement of the lens on the eye and tear exchange. While considering gas permeable lens materials, one may also like to bear in mind that the hardness of a lens may be a factor in its scratch resistance and durability.

The ease of manufacture of a contact lens, along with the reliability of the quoted parameters and dimensional stability, will also be affected by the mechanical characteristics of the contact lens material. 

Measuring Contact Lens Mechanical Strength

Typical tensile test of
dumbbell-shaped sample

The tensile properties of a material are perhaps the most frequently considered and measured in the polymer industry. To measure the tensile strength of a material, a dumbbell-shaped sample of known cross-sectional area is clamped between two opposing jaws (such as those shown); the Texture Analyser then measures the amount of force used to stretch the sample.

Elongation is measured as the % change in length of the sample relative to the sample’s original length. The area under the stress-strain curve (as illustrated) is called the toughness and is a measure of how much energy a sample can absorb before it break.

The shape of the stress-strain curve gives an indication as to whether a material is strong and tough or strong and brittle. Obviously, resistance to breakage (as in the case of rigid contact lenses) is important for the durability of a contact lens both in manufacturing and patient handling, but if this toughness is obtained by increasing the flexibility of the material and thus increasing the likelihood of on-eye flexure and potential manufacturing problems, the balance of ideal mechanical properties may not be achieved.

The modulus (or Young’s Modulus) describes how well it resists deformation and it is taken as the initial slope in the elastic region of the curve (i.e. at low strains). The Young’s Modulus will influence both patient comfort and visual performance. While a material with a lower modulus will offer improved comfort in comparison with a stiffer material of high modulus, the increased flexibility can lead to residual astigmatism as a result of lens flexure on toric corneas.

It also usually follows that as Young’s Modulus increases, oxygen permeability decreases, and vice versa. One may therefore expect a greater flexibility in lens materials of higher oxygen permeability.

Left: A stress-strain curve illustrating a brittle material; the toughness is given by the area under the curve.
Right: A stress-strain curve illustrating a material that is both strong and tough.

Soft contact lenses are completely different in their mechanical behaviour to rigid lenses. In their dehydrated state they are hard and brittle. Immediately they become hydrated, hydrogels take on a much softer, rubber-like consistency and have very low tensile and tear strength. This low mechanical strength is the reason for the lack of durability of soft lenses, although with frequent replacement of soft lenses that now dominate the market this is less of a problem.

Hydrogels have a very low Young’s modulus, which offers several advantages. The way the lens drapes over the cornea means that the fitting of the lens is not as dependent on the lens parameters. There is much greater comfort initially for the patient. There is a much lower incidence of mechanically-induced ocular complications. The low modulus, however, also means that the lens material has poor handling characteristics and also is not able to mask corneal astigmatism.

Compression of a contact lens
to assess mechanical strength

The modulus of a material can also be measured under compression. This type of test also allows the recovery of the material to be measured with time after the load has been removed. There may be difficulties associated with the measurement of clinically relevant mechanical properties, and it is therefore most accurate to obtain measurements with the material in its final lens form. This can present practical problems if the sample to be tested is required to have a specified size or shape, such as a dumbbell for tensile testing.

 Testing of mechanical properties should be done with test conditions that come as close as possible to the conditions in which the material is to be used and therefore in the testing of contact lenses one of the most useful tests that can be carried out is the time-dependent deformation and the subsequent recovery under successive periods of load (using a test situation similar to that shown. The viscoelastic nature of contact lens materials may result in permanent deformation under repeated load, which will obviously have a bearing on contact lens parameter stability during wear. 

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We can design and manufacture probes or fixtures for the TA.XTplus texture analyser that are bespoke to your sample and its specific measurement.

Once your measurement is performed, our expertise in its graphical interpretation is unparalleled. Not only can we develop the most suitable and accurate method for the testing of your sample, but we can also prepare analysis procedures that obtain the desired parameters from your curve and drop them into a spreadsheet or report designed around your requirements.

For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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