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Gels can help concentrate a medicine at a chosen site or control how quickly it is released. In drug delivery, hydrogels—water-swollen polymer networks—can act as local depots, slow cargo movement, or respond to biological conditions and external cues. “Targeted” does not necessarily mean that a gel travels through the body and selectively homes to diseased tissue: placement, retention, and controlled release are distinct strategies, and responsive designs are not by themselves proof of clinical benefit.
What “targeting” means when a gel delivers a drug
In this context, targeting can describe different ways of controlling exposure. A gel may be placed at or near the intended site, retain a drug locally, or release cargo in response to a cue. These approaches can be combined, but none should be confused with a guarantee that the material will find a disease site on its own.
- Local placement: The gel is delivered at or near the intended site, for example as an injected depot or an implant. The delivery route and any required device matter as much as the material.
- Retention and release control: The gel network can hold cargo and slow its movement, while diffusion, degradation, or both determine how it leaves the matrix.
- Responsive release: A formulation is designed to change its behavior when it encounters a biological condition or an externally applied stimulus.
These distinctions are useful when assessing claims about a “targeted” gel: ask whether the claim concerns where the gel is placed, how long it retains the drug, what prompts release, or selective delivery to a particular tissue.
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How a hydrogel controls drug release
The polymer network shapes movement
A hydrogel is a hydrated polymer network that can contain a therapeutic cargo. Its composition, crosslinking, network architecture, and mesh—the spaces within that network—affect how readily the cargo moves. Interactions between the drug and polymer also matter. Together, these features influence retention, diffusion, swelling, mechanical integrity, degradation, and the resulting release profile.
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The 2018 review Designing hydrogels for controlled drug delivery describes hydrogel design across network, mesh, and molecular scales. The practical point is that release is not a property of the polymer name alone: it emerges from the material’s structure, the drug it carries, and the intended delivery profile.
Degradation can contribute to release
Some systems are designed so that the gel degrades as the drug is released. The rate and conditions of degradation can therefore affect how long the depot persists and when cargo becomes available. A formulation needs to balance retention with release: keeping a drug in the matrix is not useful if the intended dose cannot leave it at the needed rate.
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The loaded drug can change the response
A stimulus-responsive polymer tested on its own may not behave the same way after a drug is loaded into it. Drug-polymer interactions can alter the material’s sensitivity to a stimulus and change its actuation or release pattern. A 2020 review, Stimuli-sensitive cross-linked hydrogels as drug delivery systems: Impact of the drug on the responsiveness, highlights this formulation issue. Performance should be characterized in the loaded system rather than inferred from the unloaded polymer.
What can trigger release?
Responsive hydrogels are engineered to change their behavior in response to a cue. A 2025 systematic review of stimuli-responsive hydrogels in targeted cancer therapy discusses biological and external triggers as design strategies. Their inclusion in a formulation does not establish that the cue will be selective, accessible, or reliable in a patient.
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Biological conditions
- pH: A gel may be designed to respond to acidity. The presence of an acidic environment does not, on its own, show that release will occur only at a tumor or only where treatment is needed.
- Enzymes: Enzymatic conditions can be used as a proposed signal for changing a material or releasing cargo. Whether the relevant enzyme is sufficiently available at the target site is a practical design and delivery question.
- Redox conditions: Differences in redox conditions can be used to design a response. The response still depends on the formulation and on whether the cue is present at a useful level where the gel reaches.
External stimuli
- Heat, light, and ultrasound are among the external cues discussed in reviews of responsive delivery systems. An external trigger may offer a way to control release, but its usefulness also depends on whether it can reach the gel, whether a device is needed, and whether the response fits the treatment setting.
For any trigger, the relevant questions are how strongly and consistently the gel responds, how the drug-loaded formulation behaves, and whether the intended site can be reached and exposed to the cue. A trigger mechanism is a design feature, not evidence of improved outcomes in people.
How to assess a gel delivery system
A single label such as “targeted” does not tell a reader whether a delivery system is suitable for a treatment. Compare the system across the factors that determine its performance and feasibility:
- Placement and route: Where is the gel put, how does it reach that location, and does delivery require an injection, implant, or other device?
- Release mechanism: Is release driven primarily by diffusion, degradation, a biological cue, an external stimulus, or a combination?
- Release profile: What are the onset and duration of release? Is the pattern sustained or does it include an initial burst? Does that profile match the therapeutic need?
- Cargo fit: What drug or other therapeutic cargo is loaded, and how do loading, stability, and drug-polymer interactions affect the response?
- Material behavior: How do composition, crosslinking, swelling, mechanical integrity, and degradation affect placement and persistence?
- Evidence and feasibility: What stage of evidence supports the system? Are there clinically meaningful comparisons? What manufacturing, sterilization, device-access, or adoption constraints apply?
These criteria reflect design and translation considerations discussed across the hydrogel reviews. Individual studies may not report every item, so an unreported characteristic should not be assumed to be favorable.
What the literature says about translation
Engineering a gel to respond in a laboratory setting is only one part of developing a therapy. A 2026 review in Chemical Society Reviews, first published on May 29, 2026, identifies challenges that include material complexity, tissue penetration, device accessibility, economic constraints, and clinical adoption. These factors can determine whether a technically interesting system can be manufactured, delivered, and used in a real treatment setting.
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The same review reports that light was the most popular external stimulus in its analysis: 44% of the papers, reported as 361 trials. That figure describes the review’s analysis of stimuli-responsive nanomedicines and microscale therapeutics. It does not establish that light-triggered gels are more effective, that the figure applies to all gel delivery research, or that such systems are standard care.
A 2026 review in Communications Materials provides further context on using stimuli-responsive biomaterials to deliver biomedicines. Across this literature, a responsive mechanism or promising preclinical result should be distinguished from evidence of benefit in people. Claims about a specific candidate therapy require product-specific evidence on its indication, route, development stage, and human results.
What gels can—and cannot—promise
Gels offer a flexible way to localize a drug, tune its release, or design a response to a biological or external cue. Their performance depends on the complete formulation: the polymer network, degradation behavior, drug-polymer interactions, intended route, and release profile. A gel that releases cargo under a chosen condition is not automatically selective for diseased tissue, practical to use, or clinically effective. Those are separate questions that must be answered with evidence for the particular system.
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