Release on cue: stimuli-responsive carriers
A stimuli-responsive delivery system is designed to hold its drug tight while travelling through the body, then let go only where a trigger tells it to. The trigger is some local signal that differs between healthy and target sites — pH, temperature, an enzyme, light, or a redox difference. The carrier is built from a material that physically changes when it meets that signal: a polymer that swells, dissolves, or falls apart, springing the drug loose right where you want it.
The most widely used trigger is pH. The body offers sharp pH steps: the stomach is strongly acidic (~1.5–3.5), the small intestine mildly alkaline (~6–7.5), tumour and inflamed tissue slightly acidic, and endosomes inside cells acidic again. A pH-responsive delivery material can be tuned to stay shut at one pH and open at another. This is the principle behind colon-targeted delivery: a coating that survives the stomach and small intestine but dissolves at the higher pH or under the bacterial enzymes of the colon, delivering drug locally for conditions like ulcerative colitis.
Place once, dose for weeks: implants and depots
The other advanced idea is duration. An implant is a small solid placed under the skin (or in a tissue) that releases drug over weeks, months, or years — contraceptive rods are the familiar example. A depot injection does the same from an injected reservoir: drug suspended in oil, or trapped in biodegradable microspheres that slowly erode, forming a local store that meters drug out long after the needle is gone. Both turn a daily-pill regimen into a single visit, which is a powerful boost to adherence.
The prize is zero-order release: a constant amount of drug per unit time, holding blood levels in a flat, steady band instead of the peaks and troughs of repeated dosing. A controlled-release reservoir implant approaches this because diffusion through its membrane is roughly constant while the inside stays saturated. This is the most refined form of modified release — but it comes with a hard constraint you must respect.
Sizing a depot for a target duration
Goal: deliver 0.5 mg/day for 90 days from a microsphere depot.
Total drug needed (assume ~constant release):
D = rate x time = 0.5 mg/day x 90 days = 45 mg
Account for non-ideal release:
- encapsulation efficiency ~ 80% -> must load 45 / 0.80 = 56.25 mg into spheres
- initial burst ~ 10% of load released on day 1
burst = 0.10 x 56.25 = ~5.6 mg dumped early -> check it stays below toxic level
Approx dose remaining for steady phase:
56.25 mg loaded - 5.6 mg burst = ~50.6 mg metered out over the 90 days
effective steady rate = 50.6 / 90 = ~0.56 mg/day (a touch high; trim the load)
Lesson: a depot is sized backwards from duration x rate, then padded
for efficiency loss and burst, and the burst is checked for safety.