2/16/16 When designing and fabricating micro molded devices and implants for the human eye, the physical characteristics and material consistency of the components of the eye are critical to understand. Understanding the body’s reaction to polymeric implants is complex. Not only is the natural response affected by the chemical properties of the polymer but also by the physical properties of the implant. Development of an ophthalmologic drug delivery device requires design criteria compatible with the delicate nature of the eye, including proper materials, size, shape and porosity.
Some materials, although tested for bio-compatibility, may still cause inflammation and immune-responses leading to long term effects on the eye. It is advantageous for safety, regulatory robustness and speed, to market reasons to select not only a predicate material (PMMA, silicone), but also the predicate grade used in an intraocular application. Families of materials vary greatly from grade to grade in terms of both physical and chemical properties. For example, leachables and extractables over time can vary greatly with different grades of silicone and PMMA and these factors are critical to long term implant and device safety and efficacy. Additional material selection consideration include materials that are slippery, flexible, and non-hydroscopic for compliance adherence.
Ophthalmologic implants or devices must be very small and pliable to fit into the sections of the eye. For example, a glaucoma drain must fit into the sclera which is from 0.3-1.0mm thick. A part’s thickness and size is dependent on the ocular area and location of the implant, but also chosen on the melt flow of the materials.
Implants within the wall of the sclera are radial in nature to rest within the semi-circular outer wall of the eye. Glaucoma is a result of the increased fluid pressure in the eye due to the reduction or blockage of fluid from the anterior to posterior chambers. Devices such as these are possible using micro-injection molding and micro-automation.
A pupil expander device has micro features and surface finishes which are necessary to fit comfortably in the eye and provide tensile strength with fine alignment and mechanical strength to hold them in place.
A delivery device that sits on the cornea – it’s a thin membrane-like, silicone structure with radial design, and a 50 micron wall thickness to fit inside the upper eyelid. The cornea has 5-6 payers varying from 2-20 microns in thickness, made up of highly sensitive pain receptors. Cornea pain receptor density is up to 600 times that of skin, which is why even a slight injury to the eye is extremely painful.
In micro molding ocular implants and devices, parting lines where the mold’s halves come together, and surface finish of the molds that create the molded parts, must meet stringent comfort standards required for them to be worn or implanted. The implications for compliance are clear. Surface finish, blending parting lines, spherical radii, and matching cores and cavities to ultra-precision tolerances (A2 or A1 finishes) are the keys to creating implants that can stand the test of time in an intraocular environment.
In the context of an ocular implant, smooth materials can have very different tissue and nerve responses compared with micro-structured materials. Tissue encapsulation of a foreign body (such as the implant) is higher with rougher surfaces because there is more surface area for the implant to attach to tissue. Nerve response to surface finish needs to be considered in implant design. Wear or degradation of a rough surface is more prevalent as well because the smaller porous particles in the surface can be toxic to tissue, can spread throughout the eye, and also trigger an allergic reaction.
The anatomy and physiology of the ye is one of the most complex and unique systems in the human body. Micro molding is a scalable process with particular design criteria met, including proper size, three-dimensional shape, wall thickness, material selection and surface finish.
Micro injection molding is a viable and scalable process for fabricating ultra-precise, micro-sized, ultra-thin, yet robust implants and devices located in a highly complex environment such as the eye.
Scalability (as discussed in our 2/69/16 blog) is an important consideration at the initial product and process design phase in order to achieve the economies of scale – tens to hundreds of thousands of parts, to millions annually – that micro molding offers.
Careful consideration of surface finish, feature size and material selection is paramount to the successful integration of marrying micro molding technology with the internal chambers and inter-connective functions of the eye.