Rabbit eye irritation caused by wearing toxic contact lenses and their cytotoxicities: in vivo/in vitro correlation study using standard reference materials

In vitro assessment of the biocompatibility of chemically treated silicone materials with human lens epithelial cells

Cytotoxicity testing is a regulatory requirement for safety testing of new ocular implants. In vitro toxicity tests determine whether toxic chemicals are present on a material surface or leach out of the material matrix. A method of evaluating the cytotoxicity of ocular implants was developed using fluorescent viability dyes. To assess the assay’s sensitivity in detecting toxic substances on biomaterials, zinc diethydithiocarbamate (ZDEC) and benzalkonium chloride (BAK) were deposited on silicone surfaces at different concentrations. Human lens epithelial cells (HLEC) were added to the surface of these treated silicone surfaces and were assessed for viability. The viability of both the adherent and non-adherent cells was determined using confocal microscopy with, annexin V, ethidium homodimer, and calcein. Cell metabolism was also evaluated using resazurin and the release of inflammatory cytokines was quantified using a multiplex Mesoscale Discovery platform. Confocal microscopy was shown to be a sensitive assay for evaluating material toxicity, as significant toxicity (p < 0.05) from ZDEC and BAK-treated surfaces compared to the untreated silicone control was detected. Patterns of cytokine release from cells varied depending on the toxin evaluated and the toxin concentration and did not directly correlate with the reduction in cell metabolic activity measured by alamarBlue.

Hyaluronic acid and graphene oxide loaded silicon contact lens for corneal epithelial healing

Hyaluronic acid (HA) eye drop solution is widely used to treat and manage various corneal diseases like keratoconus (after corneal cross-linking) and dry eye syndrome. However, ocular dosage forms like eye drop solution affect the routine life style of patients due to frequent dosing schedule. In this study, HA and reduced graphene oxide (rGO) was directly loaded in the silicon contact lenses (HA-GO-DL) and compared with the conventional soaking method (HA-GO-SM). The contact lenses at lower level of rGO showed permissible swelling and transmittance properties. The water retention property of HA-GO-DL contact lenses was confirmed by water evaporation studies. The flux data of HA-GO-SM contact lenses showed high burst release with 24 h release duration. While, HA-GO-DL lenses confirmed low burst with sustained release up to 96 h. In ocular irritation study, the HA-GO-DL-2 lenses was found to be safe. The HA-GO-DL-2 batch showed high HA-tear fluid concentration (rabbit model) and improvement in the rabbit tear fluid volume (Schirmer strip studies) in comparison to the soaking method (HA-GO-SM-2) and eye drop solution. The study successfully demonstrate the potential of HA-GO loaded contact lenses to improve tear fluid volume to manage various ocular diseases like dry eye syndrome.

In vitro methods of assessing ocular biocompatibility using THP-1-derived macrophages

Macrophages play an important role in the elimination of infections, the removal of debris and in tissue repair after infection and trauma. In vitro models that assess ocular biomaterials for toxicity typically focus on the effects of these materials on epithelial or fibroblast cells. This investigation evaluated known ocular toxins deposited on model materials for their effects on the viability and activation of macrophages. THP-1-derived macrophages were cultured onto silicone films (used as a base biomaterial) deposited with chemical toxins (benzalkonium chloride (BAK), zinc diethyldithiocarbamate (ZDEC) and lipopolysaccharide (LPS)). Utilizing three fluorescent dyes calcein, ethidium homodimer-1 (EthD-1) and annexin V, the viability of macrophages attached to the biomaterial was determined using confocal microscopy. Propidium iodide (PI) staining and alamarBlue® (resazurin) reduction were used to assess cell death and metabolic activity. CD14, CD16, CD33, CD45, and CD54 expression of adherent macrophages, were also evaluated to detect LPS activation of macrophages using flow cytometry. The sensitivity of this test battery was demonstrated as significant toxicity from treated surfaces with ZDEC (0.001-0.01%), and BAK (0.001%-0.1%) was detected. Also, macrophage activation could be detected by measuring CD54 expression after exposure to adsorbed LPS. These in vitro methods will be helpful in determining the toxicity potential of new ocular biomaterials.

Reference biomaterials for biological evaluation

The variability associated with most of the biological evaluation studies is relatively large and hence experimental controls are frequently employed to ensure that the results are reliable. These experimental controls, either positive or negative controls, could be considered as reference materials in the context of biological evaluation. Currently, reference biomaterial needs across the world are not addressed satisfactorily. Affordability and lack of availability of certified reference materials necessitates the development and the use of laboratory developed RMs. A well defined characterization strategy is essential for the successful development of in house reference materials. This article provides an overview of the current international status in the area of reference biomaterials and enumerates the steps involved in the development of an in house reference material. The production and characterization of commercially pure titanium pins for use as negative controls during muscle implantation studies is presented as a case study.

Studies on segmented polyetherurethane for biomedical application: effects of composition and hard-segment content on biocompatibility

Segmented polyetherurethane (SPEU) materials based on polytetramethylene oxide (PTMO, Mw 1000 and 2000) with various hard-segment contents were synthesized and their biocompatibilities studied via different tests. The static contact angle data reveal that the higher hard-segment-content SPEU material possesses a lower contact angle, implying that the surface of the higher hard-segment-content SPEU is more hydrophilic than its low hard-segment-content SPEU counterpart. The catalyst- and additive-free PTMO-based SPEU materials in this study possess neither a hemolytic nor a cytotoxic response that could be considered non toxic for biomedical applications. By using L-929 cell lines, a cell-seeding test indicated that the higher hard-segment-content SPEU material possesses quicker cell attachment and proliferation behaviors. In vitro platelet adhesion tests indicated that the lower hard-segment-content SPEU possesses less platelet adhesion than the high hard-segment-content SPEU material. Both ex vivo canine artery-artery (A-A) and arterio-venous (A-V) shunting tests revealed that the extent of platelet adhesion reaction is less for lower hard-segment content SPEU. In addition, the blood compatibility of SPEU material synthesized from PTMO 1000 excels over PTMO 2000 SPEU material by near the same levels as the hard-segment-content SPEU.

Cytotoxicity of medical materials sterilized with vapour-phase hydrogen peroxide

A new sterilization system using vapour-phase hydrogen peroxide (VPHP) was recently developed. The cytotoxicity of various medical materials sterilized by the VPHP sterilization system was investigated. After VPHP sterilization, polystyrene, polyurethane (PU8), blend material of silicone and polyurethane (Sil/PU6), poly(methyl methacrylate) (PMMA), fluorosilicone acrylate and poly(2-hydroxyethyl methacrylate) (HEMA) showed strong cytotoxicity, whereas polyethylene and polypropylene did not. Although the cytotoxic potential of most materials is reduced by extension of the aeration time, HEMA and PMMA still retained strong cytotoxicity after 12 h aeration. Addition of catalase to the cell culture eliminated the cytotoxicity of sterilized polystyrene and PU8. Hydrogen peroxide (H2O2) residues remaining in the sterilized materials were determined. Large amounts of H2O2 (5.1-186 micrograms) were detected in HEMA, PU8, Sil/PU6 and PMMA. In contrast, silicone and polyethylene contained low levels of H2O2. The amounts of residual H2O2 in the materials decreased with increasing aeration time, but the elimination rate of H2O2 differed among the test materials. The cytotoxic potential of the VPHP-sterilized materials correlated with the amounts of residual H2O2 present. These results indicated that the cytotoxicity of VPHP-sterilized materials was caused by the residual H2O2. To generalize the developed VPHP sterilization system for various medical devices, it is important to validate the aeration of materials for removal of cytotoxic residuals.

Studies on the standardization of cytotoxicity tests and new standard reference materials useful for evaluating the safety of biomaterials

Standard reference materials (SRM) made of polyurethane films containing various amounts of cytotoxic compounds such as zinc diethyldithiocarbamate (ZDEC) and zinc dibutyldithiocarbamate (ZDBC) are proposed. The cytotoxicity indices of five dithiocarbamates and the SRM films obtained by colony assay were compared with those by published standards, i.e., neutral red assay (NF S 90-702), agar diffusion assay (USP XXII), and millipore filter diffusion assay (DIN-V 13 930). Among them, colony assay using 3 cell lines of Balb 3T3, L929 and V79 was found to be most sensitive. The extraction method and direct contact method were proposed as two test methods of colony assay. In the former, culture media after incubation with test materials were used as test solution, and in the latter, cells were directly cultured on test materials. The extraction method gave a linear relationship between the cytotoxic potentials of the SRM and its concentration of ZDEC or ZDBC. The cytotoxic potentials of the SRM correlated well with the thickness of inflammatory layer in rabbit muscle implantation tests of the SRM. The direct contact method was found to be useful to detect weak cytotoxicity because of its high sensitivity. Based on these data, we propose the colony assay and the two kinds of new SRMs as Japanese standard test method for evaluating the cytotoxicity of biomaterials.