Abstract: The field of the invention is site-specific drug delivery capsules (SSDC's). The disclosure relates to a closure member (14) for a substance reservoir (12) of an SSDC (10). The SSDC (10) includes a retainer that provides a non-linear force resisting opening of the closure member (14). The non-linear force ensures that the closure member (14) unseals the reservoir (12) only when an opening force exceeds a maximal value of the resisting force, thereby preventing premature or accidental emptying of the reservoir (12). The preferred means of providing the resistive force is a rolling, elastomeric o-ring (64) that additionally seals the closure member (14) into an aperture (13).