Contact Angle as Characterization Method for Vitreous Humor and Vitreous Humor Substitute Gels

Purpose: In clinical ophthalmology, intraocular injections and long-acting implants are currently utilized to facilitate controlled drug release into the eye. However, in vivo pharmacokinetic testing not possible due to the inaccessibility of intraocular tissue. This has created a need for bio-relevant in vitro dissolution methods that reflect in vivo pharmacokinetic studies. To achieve that, tissue properties need to be assessed for relevancy to drug release and, if necessary, incorporated into the in vitro dissolution method design and media selection. Currently, synthetic gel substitutes are utilized in ocular drug release studies and release medium viscosity is the primary property that is adjusted to biorelevant ranges to mimic human vitreous humor. Here, surface characterization of vitreous humor is studied as a potential marker to develop biorelevant in vitro release media. To achieve this, contact angle measurements of human vitreous humor, as well as synthetic vitreous humor substitutes, were obtained using an optical goniometer (DataPhysics Instruments USACorp.). It is hypothesized that obtaining contact angle values of these substances allows for more accurate predictability of drug release. Water contact angle is a simple and efficient method to obtain characterization of the hydrophobicity or hydrophilicity of surfaces and this technique has yet to be utilized in vitreous humor studies. Methods: An analysis of eight different samples of human vitreous humor (donated by Dr. Abe Clark, North Texas Eye Research Institute) and a vitreous humor substitute gel formulation made of agar and hyaluronic acid was completed. The synthetic gel formulation has been reported in the literature as a vitreous humor substitute. Both vitreous humor samples and gel samples were deposited on glass microscope slides and allowed to air dry at room temperature for 24 hours. Water contact angle was then measured at multiple points on each slide and the average values of each run were compiled. Human vitreous humor demonstrated two different phases: a viscous, gel-like portion and a thin, watery portion. These phases were separated in each sample and were tested independently. Contact angles were measured at different hyaluronic acid content of the synthetic gel formulation. Results: Water contact angle measurements were reproducible in both the vitreous humor and synthetic gel samples. The gel-like, more viscous phase of vitreous humor had an average contact angle of 77.46±25°. In contrast, the less viscous, watery phase produced an average contact angle of 38.31±25°. The synthetic gel formulation demonstrated a contact angle of 66.79±10°. Additionally, when altering the concentration of hyaluronic acid in the gel formulation, contact angles were reduced by 8±2° with every .05% reduction in concentration of hyaluronic acid. Conclusions: Water contact angle shows promising capabilities as a reliable method of surface characterization of vitreous humor and vitreous humor substitute gels. Our data showed that vitreous humor-mimicking gels can still have contact angles that are different than vitreous humor, but they can be corrected by simple formulation adjustments. Future studies will utilize the method developed here to study the effect of contact angle on controlled drug release studies of intraocular formulations.