Advances in ocular implants: From geometry design to controlled drug release mechanisms

The unique physiological barriers within the eye greatly impede the effective delivery of traditional drug therapies to specific intraocular target sites, leading to low bioavailability and a short duration of action. Intraocular implants placed at targeted locations via minimally invasive surgery can directly bypass these barriers. Moreover, these implants employ their morphological features to facilitate long-term, stable, and controlled drug release at the disease site. This review systematically covers the preparation processes and drug release characteristics of sheet-shaped, irregular-shaped, and cylindrical implants. Lamellar implants with a large surface area offer rapid drug release, thereby enhancing therapeutic efficiency; irregular implants provide more precise and personalized therapeutic solutions by adapting to the anatomical structure of the eye; and cylindrical implants demonstrate stable and slow release with a uniform diffusion pathway, which can maintain the drug concentration for several months. The in-depth discussion of the release mechanism regarding the implant-based drug delivery is also included. Nevertheless, contemporary research on ocular implants continues to encounter challenges, including limited drug-loading capacity and inadequate accuracy in predicting in vivo release kinetics. In the future, a combination of computational simulation, smart-responsive materials, and nanotechnology with the current strategies for implant preparation will be necessary to optimize implant design and accelerate their translation to clinical applications. STATEMENT OF SIGNIFICANCE: The field of ocular drug delivery faces persistent challenges in overcoming physiological barriers. Ocular implants, when placed at targeted sites via minimally invasive surgery, can bypass these barriers directly. Their tailored geometries and controlled release mechanisms further enable long-acting therapeutic strategies. The current review systematically elaborates on drug release kinetics and tissue integration capacity of ocular implants with different geometric characteristics, e.g., sheet-shaped, cylindrical, and irregular implants. By stating current advances and addressing existing research gaps, this review aims to establish a theoretical framework for the rational design of ocular implants, ultimately facilitating the improvement of clinical outcomes in the management of ocular diseases.