The importance of poroelasticity in soft contact lenses

The importance of poroelasticity in soft contact lenses

Evidence of the pore space of a silicone hydrogel contact lens as transport path for pore diffusion and molecular diffusion – Part 1.

Scleral lenses, hybrid lenses or Ortho-K: To ensure safe use of these types of contact lenses, the main focus is on the oxygen permeability of the plastic material of the lens. The metabolism of the cornea and the limbus should suffer as little irritation as possible. The aim of this article is to explain how this transport mechanism occurs and why it is so important. By Alina Kinder

In an age of retro looks and vintage design, even conventional systems are being updated offering newer approaches to solutions. Scleral lenses have made a comeback in contact lens optics. Using these lenses where only the edge is seated on the sclera, and which thus can be adapted to more complex corneal geometries [1], the discomfort to the physiology of the already stressed cornea through contact lens wear is kept as low as possible thanks to new materials and higher oxygen permeability. [2] In addition, lens systems such as hybrid lenses and also piggyback lenses catering for more complex corneal geometries have made a comeback in contact lens optics. Here, too, the benefit of lenses made of newer materials is to ensure a better oxygen supply to the cornea. [3] Current studies show that treating keratoconiosis, keratoplasty and some forms of corneal degeneration/dystrophy using contact lenses made of modern materials not only stabilizes the vision but the lenses are also physiologically safer for wearing all day long. Other contact lenses, such as the orthokeratology contact lenses (Ortho-K), have also gained in significance since the introduction of modern materials and have been able to reposition themselves in contact lens optics since the advent of higher oxygen permeable materials. Furthermore, the importance of corneal swelling is not insignificant, especially with Ortho-K lenses.

OXYGEN PERMEABILITY AS A KEY FACTOR

To ensure safe use of the above-mentioned systems, the main focus is on the oxygen permeability of the plastic material of the contact lenses. The lens material must perfectly satisfy the optical requirements in addition to the requirements

regarding the comfort of the contact lens wearer, without irritating the metabolism of the cornea and the limbus; or with only minimal irritation. For this reason, oxygen permeability is already taken into account during the production of the plastic material. An inadequate supply of oxygen to the cornea can lead to • too high acidity of the cornea resulting in lactate build up in the stroma. • osmotic water influx into the cornea resulting in edema formation. • structural changes of the cornea, such as loosening of the cell bond in the epithelium with an associated increase in the rate of loss of epithelial cells.

Thus one of the main aims when fitting contact lenses is to preserve the anatomical and functional integrity together with the rigidity of the cornea, in order to minimize the physiological changes mentioned above. [4,5] Contact lens optics use the permeability or transmissibility of the contact lens material as a suitable parameter for describing or predicting whether the plastic material has a high or low impact on the physiology of the cornea.

SOFT MATTER

In science, soft matter is used to describe a state where a distinction cannot easily be made between liquid and solid. Soft matter can also be described as amorphous. Polymer melts and solutions as well as elastomers (rubber), for example, are classified as soft matter, and the same is also true for soft contact lens materials. [6] In the literature, the transmissibility of solutes in soft matter is described by permeability. In chemistry, with regard to soft matter, permeability also refers to the aggregate state of the material. Permeability alone cannot adequately define the solute transmissibility of a soft matter material. [7] Such amorphous polymers are produced by polymerization processes based on polyaddition or polycondensation. The particular polymerization process used in turn determines the properties of the plastic.

DK/T AND DK VALUES

Permeability describes the transmissibility of dissolved particles in the material. The oxygen permeability of a contact lens material is referred to by manufacturers as the Dk value (permeability), where D is the diffusion constant of the material and k the solubility constant of the material. For a better analogy, in contact lens optics the property of oxygen transmission in soft contact lenses is given by the Dk/t value. For this purpose, the geometric thickness of the center of the contact lens to be compared with a power of -3.00 dpt is set as the reference distance. Fig. 1: Schematic visualization of oxygen permeability. Lotrafilcon B, left with -3.00 dpt and right with -10.00 dpt [8]

Nowadays, modern equipment can track the potential oxygen transmissibility over the entire surface of a soft contact lens in relation to the dioptric effect of the lens. This theoretical approach, aimed at visualizing the oxygen permeability of the contact lens, is realized purely theoretically via the ratio of the permeability to the corresponding thickness of the lens with a dioptric effect. For this, the transmissibility, i.e. the Dk/t value of the individual points of the lens, is allocated a color code designed to indicate the permeability (Fig. 1). The color codes correspond to the internationally valid minimum Dk/t values. As early as 1984, Holden and Mertz identified a critical limit value of 84 Dk/t for extended wear, in order to keep corneal swelling overnight as low as possible. In 2009, Harvitt and Bonnano's study recommended higher limits, such as a minimum value of 35 Dk/t for daytime wear and a minimum of 125 Dk/t for extended wear (EW), with a minimum of 140 Dk/t for EW. [9,10] With hard contact lenses, which are clearly in a solid state, the analogy with the Dk/t value clearly cannot be maintained. In such cases, comparison of the oxygen permeability of the respective materials is given purely by the Dk value. Here contact lens optics attributes the reason for good oxygen transmission through hard contact lenses to the high amounts of silicone and fluorine as oxygen carriers. Accordingly, a comparison of soft and hard contact lenses with regard to material permeability is only possible based on the Dk value. A study by Hideji Ichijima and H. Dwight Cavanagh compared the permeability of a hard contact lens with a value of 90 Dk/t *10-9 ml(O2) * cm/ml * hPa*s with a soft silicone hydrogel contact lens with a value of 125 Dk/t *10-9 ml(O2) * cm/ml * hPa*s and found that the hard contact lens delivered more oxygen in total to the cornea than the highly oxygen-permeable silicone hydrogel contact lens. [11] This result was not due to the respective permeability of the contact lens materials, but rather to the different diameter sizes of the lenses, and thus to the different areas of the cornea covered.

Fig. 2: Model of the interpenetrating network (IPN) of a silicone hydrogel contact lens. The aqueous phase runs through the channels (blue arrow). The channel system is embedded in a silicone hydrogel material (white arrow).

The size of the lens diameter of a hard contact lens may be 20% to 30% smaller than the diameter of the cornea. The exposed area of the cornea thus enables oxygen to reach the cornea via the physiological way, through the tear film by diffusion, driven by the partial pressure difference of the oxygen.

HYDROGEL VS. SILICONE HYDROGEL

The product descriptions of contact lens optics emphasize the benefits of the individual elements such as silicone and fluorine as oxygen carriers. The selling point of the good oxygen permeability of modern contact lens materials lies in the high silicone content and the lattice structure of the plastic polymer. On the one hand, silicone acts as an oxygen carrier and, on the other hand, the regular lattice structure of the silicone hydrogel material acts as a water reservoir and thus as an oxygen carrier, as in conventional soft lenses. In conventional contact lenses made of hydrogel material, the clear polymer lattice structure serves as a good water reservoir. Thus the water stored in the so-called cavities (lattice interspaces) can serve as an oxygen carrier. [12] However, based on experience with hydrogel contact lenses, it is well known that the oxygen permeability performance of such hydrogel lenses does not satisfy the demands of the cornea; thus this stored water alone is not sufficient as an oxygen carrier. Due to the internal structure of the silicone hydrogel material, the cross-linking, the interpenetrating network (IPN) of the two constituents silicone and hydrogel, the ion exchange (according to the manufacturer's information) takes place in the water channels (Fig. 2). At the macroscopic level, the crystalline lattice structure of the silicone hydrogel polymer is said to be responsible for the diffusion of the oxygen. The cross-linking of the two components is said to be responsible for the improved oxygen permeability of the material. This, at least, is consistent with the current view of oxygen transmission through polymeric contact lens materials. Current studies of contact lens materials do not suggest any other means for transporting solutes. It is interesting to note that the product description does not make any distinction between the different aggregate states "solid" and "soft". The Dk/t value is rarely mentioned for hard contact lenses, because it is difficult to imagine that oxygen can diffuse through the solid contact lens material within a reasonable wearing time. In this regard, the industry does not seem to make use of the transmissibility method, whereby the Dk value is only divided by the center thickness of the contact lens, for hard contact lenses.

SOLUTE TRANSPORT PATH

The transport path of solutes through the material can also be illustrated using another model. The first possible model is that of capillaries – cavities running through the contact lens. Oxygen dissolves in the tear film, enters the material through the surface openings (pores) and flows through the lens pore system to the rear surface of the lens, thus reaching the epithelium of the cornea. This is another way of describing the physiological path and is similar to the process of pore diffusion. Pore diffusion or diffusion of gas into liquid lends itself to, and can realistically describe, the process of oxygen diffusion through the contact lens material. Oxygen flow through the silicone is not the simplest of diffusion processes. The hypothesis of pore diffusion cannot be applied to all materials, only to materials that exhibit porous characteristics. In an interpenetrating polymer network (IPN) consisting of a combination of two polymers in the form of a network, the relevant polymerization process creates a suitable cavity that can be used as a pore space for pore diffusion. [13] IPN technology is a combination of silicone with a hydrogel material and results in a plastic mixture which is cloudy due to dispersion and thus opaque. The solution to the transparency problem lies in the network structure that connects the two phases separately. There is a broad range

CURRENT STUDIES CONCERNING CONTACT LENS MATERIALS DO NOT SUGGEST ANY OTHER POSSIBLE WAYS OF SOLUTE TRANSPORT.

of applications of IPN technology, including biomedical applications, coatings and adhesives. Some of these IPNcrosslinked plastics exhibit a two-phase structure in the range of several 100 angstroms. This corresponds to a magnitude of several 10 nm, meaning that the void space available is also in the range of soft matter pore spaces. In contact lens optics, pores – and thus openings of 1 - 10 nm (10 - 100 Å) – are known. [15,16] According to the International Union of Pure and Applied Chemistry (IUPAC), the pores of polymer hydrogel materials, which are openings on the outer surfaces of soft matter, are in the range of mesopores.

Pore diameter Nomenclature (IUPAC) Examples

< 2 nm Micropores Activated carbon 2 - 50 nm Mesopores > 50 nm Macropores Polymer hydrogels Plants

Tab. 1: Pore size according to IUPAC. [17]

Film dressings used in the treatment of severe skin burns are an example of the use of IPN technology for biomedical materials. Made of polymeric material, the transparent dressings transport fluid away from the damaged tissue while facilitating the ingression of oxygen to preserve the tissue cells which are still alive and enabling new tissue to form. Diffusion in the dressings is of great importance in this process, allowing a healing environment (i.e. maintaining the moisture balance) to develop at the damaged tissue site. In addition, transparent dressings allow the treating physicians to optimally follow the healing process. [13, 14] The process of pore diffusion is the process that contributes to better healing due to these transparent film dressings. It is only thanks to this film covering that the moist wound environment can be created appropriately and help the tissue to form new cells. If the processes of oxygen transport and fluid removal were to take longer, then the healing process would be considerably slowed down and thus the benefit of such dressings would be significantly reduced. Diffusion through the pores serves as a transport path for the dissolved particles. To avoid scarring after keratoplasty or other penetrating eye surgery, a bandage lens is subsequently placed on the cornea to protect it. [18] Here, too, the healing process is protected and encouraged by pore diffusion.

This is part 1 of this article. Read part 2 in the upcoming issue of GlobalCONTACT and in

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Alina Kinder (EurOptom, Master of Science (Klin. Optometry)) is a lecturer at the Professional School for Optometry in Cologne (HFAK). She also works as an examiner on the examiner board of ECOO diploma and optometrist (ZVA/ HWK).