New class of material for intraocular lens manufacture: polyisobutylene-based polymer

Biocompatibility refers to the ability of a material to perform with an appropriate host response in a specific situation. —David F. Williams Materials currently used in the manufacture of intraocular lenses (IOLs) are usually categorized in 2 general classes: acrylic and silicone. Rigid acrylic lenses are manufactured from poly(methyl methacrylate). Foldable acrylic lenses are usually described as hydrophobic or hydrophilic, according to surface and bulk properties such as contact angle and water content. There is a large variation of copolymers that can be used for the manufacture of these lenses though; therefore, all hydrophobic or hydrophilic acrylic lenses are not the same. In a similar manner, different generations of silicone materials have been used in the manufacture of silicone lenses, with modern lenses usually made from silicone materials with a higher refractive index.1 Other materials have been used or considered in the past for IOL manufacture. As an example, IOLs made of glass were available in the 1970s but were swiftly abandoned because of their potential for shattering during Nd:YAG laser posterior capsulotomy procedures.2 Another example is represented by a transparent polyether urethane, which was considered as a material for foldable IOLs, but ultimately found unsuitable for this application due to signs of degradation when implanted intraocularly and subcutaneously.3 Studies evaluating a different class of material used in the manufacture of IOLs have recently become available in the literature.4,5 The new IOL material is composed of crosslinked polyisobutylene, used in the manufacture of the monofocal IOL referred to as xPIB IOL (Eyedeal Company). The material is described as a thermoplastic elastomer crosslinked by poly(styrene-block-isobutylene-block-styrene) (SIBS), formed through chemical or radiation-induced bonding of polyolefin chains, creating a 3D network structure that, according to the manufacturer, significantly enhances mechanical properties, thermal stability, and chemical resistance of the IOL. The material would not contain cleavable groups (hydrolysable or oxidizable groups) anywhere in the polymer including its side and end groups, which would ensure long-term resistance to degradation and embrittlement. The refractive index of the new material is 1.52; it is hydrophobic, as indicated by a contact angle of 97.2 degrees, and water content of 0.05%. It is also highly flexible (glass transition temperature of −62.3 °C, and elongation at break of 250% to 300%) and exhibits relatively reduced chromatic aberration (its Abbe number is 50).4,5 Besides application in IOL manufacture, polyisobutylene-based polymers have been used in other fields of medicine, such as manufacture of stents, and glaucoma drainage devices.6–10 Work on SIBS for long-term medical implants began in the early 1990s, and since then, different in vitro testing and in vivo testing have confirmed the biocompatibility and biostability of SIBS in these applications.6–10 The optical surface of the single-piece, monofocal IOL manufactured from this material is aspheric and biconvex, designed to provide no spherical aberration. A benzotriazole derivative ultraviolet blocker is crosslinked into the lens polymer. The lens is available with a dioptric power from +6.0 to +34.0 diopters (D) (in 0.50 D steps), with overall diameter of 13.0 mm and optical diameter of 6.50 mm. The haptic design consists of a double C-loop platform with no angulation.4,5 An in vitro study using an accelerated aging methodology to induce glistening formation showed an average glistening (microvacuoles [MV]) density of 7.46 ± 3.78 MV/mm2 for the new lens compared with 142.42 ± 72.47 MV/mm2 for the control (model SN60WF, Alcon Laboratories, Inc.). The same study showed a smooth optical surface under scanning electron microscopy, as well as power and modulation transfer function values that were conformed to ISO standards.4 In this issue, Qassoom and Kleinmann (page 909) reported a preclinical study designed to evaluate uveal and capsular biocompatibility of the new IOL in a rabbit model in comparison with a commercially available single-piece hydrophobic acrylic lens (model SA60AT, Alcon Laboratories, Inc.). Bilateral phacoemulsification was performed in 10 New Zealand white rabbits, with 1 eye implanted with the test IOL, and the contralateral eye implanted with the control. Slitlamp examinations were performed at postoperative weeks 1 and 6, followed by euthanasia of the animals, and enucleation of the eyes with subsequent gross, postmortem examination with the Miyake-Apple view, and complete histopathological examination. All IOL implantations were uneventful. No significant differences were noted in the incidence and severity of ocular findings between the 2 groups, including long-term findings usually observed in rabbit studies, such as posterior capsule opacification and synechiae formation. Soemmering ring formation observed during gross, postmortem examination was found to be statistically higher in the control group. Histopathological examination of the tissue samples revealed an overall normal histological appearance in both groups of lenses. Mild findings only observed in the test IOL group at the end of the study were as follows: giant cell deposits on the anterior optic surface in 2 eyes, and proliferation of cortical cells in 1 eye (observed during the week 6 slitlamp examination), as well as presence of pigment at the haptic or in the vitreous, in 3 eyes (observed during gross, postmortem examination). In the September 2025 issue of the journal, Zhao et al. reported the results of the first clinical study on this new IOL material.5 This was a prospective, parallel-controlled, randomized clinical study evaluating the safety and stability of the new lens, in comparison with a commercially available single-piece hydrophobic acrylic lens (model SN60WF). The test IOL was implanted using Medicel Accuject 2.0 injection systems and delivered through 2.0 mm corneal incisions. Nineteen eyes of 19 patients per IOL group were included; the follow-up schedule was 1 day, 1 week, 1 month, 3 months, 6 months, and 1 year after implantation. No significant differences in intraocular pressure or corneal endothelial cell count were observed between test and control IOL groups. Visual acuity improved significantly in both groups at all postoperative follow-ups. One eye with the test IOL showed persisting slight aqueous flare for 2 months postoperatively, with spontaneous resolution thereafter. There were no significant differences between the groups regarding postoperative aqueous depth or tilt at the different timepoints, as well as tilt changes over time; IOL decentration was lower in the test group at week 1, but no significant differences were observed at subsequent follow-up visits (all 3 parameters measured with Casia 2, Tomey Corp.). Interestingly, the anterior capsulorhexis opening area decreased during the trial, with significant more shrinkage in the test than in the control IOL group at all timepoints. This decrease was apparently uncorrelated with changes in spherical equivalent throughout the study.5 The definition of biocompatibility is not a fixed one, but rather highly context-dependent, as a material considered biocompatible for one application might not be suitable for another. An IOL is overall expected to induce minimal inflammatory reactions after implantation, including foreign-body reaction; show appropriate interaction with surrounding ocular tissues, including uveal tissue, and residual lens epithelial cells within the capsular bag (for in-the-bag fixated lenses); and remain optically clear, as well as resistant to any form of degradation. The aforementioned attributes are also expected to endure for many years because IOLs can be implanted early on during a patient's lifetime.1 The new IOL material described here shows promising results. Studies describing larger series and longer clinical follow up will ultimately establish its place as another viable option in IOL manufacture.