What if a cosmetic active ingredient did not have to be released all at once?

A conventional cream places an active ingredient on the skin, and the ingredient begins interacting with the surrounding formulation and skin immediately. But some cosmetic ingredients are sensitive, poorly compatible with the base formula, or better suited to being released gradually rather than in a single burst.

This is where controlled release technology becomes interesting.

Instead of simply adding an active ingredient to a cream or serum, the active can be incorporated into a delivery system designed to influence when, how quickly and under what conditions it becomes available.

The technology is already quite broad. Microcapsules, polymeric particles, liposomes, microspheres, silicone elastomers and other carrier systems have all been investigated for personal care applications.

The important point, however, is that controlled release is not one single technology.

It is a formulation strategy.

What Does Controlled Release Mean in Cosmetics?

Controlled release means designing a delivery system so that an active ingredient is released in a controlled or sustained manner rather than being immediately available after application.

The Delivery System Handbook for Personal Care and Cosmetic Products distinguishes controlled or sustained release from simpler forms of encapsulation.

An encapsulated ingredient may simply be protected until the product is applied and then released in one event. A controlled-release system, by contrast, is intended to provide delivery over a period of time or at a desired rate. Sequential-release systems go a step further by releasing different ingredients at different stages.

That distinction is important.

Encapsulation and controlled release are not necessarily the same thing.

Encapsulation can be used simply to protect an active.

Controlled release adds another objective: controlling its availability.

Why Would a Cosmetic Formulator Want Controlled Release?

There are several potential reasons.

Some active ingredients are chemically unstable and may benefit from being separated from the surrounding formulation.

Others may be incompatible with other ingredients in the formula.

In some cases, gradual release can be desirable because the formulator wants the active to remain available at the skin surface for a longer period.

The delivery-system literature identifies several potential functions of encapsulation and controlled-release technologies, including:

• protecting sensitive actives;
• improving formulation stability;
• preventing incompatibility between ingredients;
• controlling release;
• extending the duration of delivery;
• and improving the overall performance of the delivery system.

This creates an interesting shift in formulation thinking.

Instead of asking only:

"How much active should we add?"

the formulator can also ask:

"How should that active be made available after application?"

How Can an Active Ingredient Be Released?

The release mechanism depends on the delivery system.

For microcapsules, the Delivery System Handbook describes four basic mechanisms:

Mechanical rupture – the capsule wall breaks as a result of physical forces.

Dissolution – the capsule wall dissolves under appropriate conditions.

Melting – the wall or matrix changes state and releases the encapsulated material.

Diffusion – the active gradually moves through the capsule wall or matrix.

These mechanisms can produce very different release profiles.

A capsule designed to rupture when a consumer rubs a cream into the skin behaves very differently from a polymeric system in which an active slowly diffuses through a matrix.

This is why selecting the carrier is such an important part of formulation development.

Microcapsules: A Simple Idea With Many Possibilities

Microencapsulation is one of the more established approaches to controlled delivery.

The basic concept is straightforward: the active ingredient becomes the core, while another material forms a surrounding shell or matrix.

The carrier can then protect the active before use and determine how it becomes available after application.

The handbook describes microcapsules containing both water-soluble and oil-soluble actives and discusses different shell and matrix materials, including systems based on polymers and coacervation.

The applications are surprisingly broad.

Examples described in the literature include encapsulated fragrances, vitamins, pigments, exfoliating materials and other personal care ingredients.

The technology can therefore serve more than one purpose.

A capsule can simultaneously provide protection, controlled release and formulation compatibility.

Diffusion-Controlled Release: When the Carrier Acts Like a Reservoir

One particularly interesting approach is diffusion-controlled delivery.

Instead of breaking open, the carrier allows the active to move gradually through the material surrounding it.

The handbook describes Chronospheres®, a polymeric delivery system designed to incorporate both lipophilic and hydrophilic actives and release them to the skin through diffusion.

The system is described as providing diffusion-controlled delivery influenced by skin lipids and moisture.

This creates a useful concept for topical formulation:

the delivery system can act as a reservoir for the active.

Rather than placing the entire quantity of an ingredient immediately at the skin surface, part of it can remain incorporated in the carrier and become available later.

But this approach creates another formulation challenge.

The surrounding vehicle matters.

The Formula Around the Delivery System Matters

It is easy to think of an encapsulated active as being completely isolated from the rest of the formulation.

It isn't.

The surrounding cosmetic base can influence the stability of the delivery system and the movement of the active.

The Chronospheres® example illustrates this particularly well.

According to the source, water-soluble actives incorporated into the system should not simply be suspended in a water-based vehicle, and oil-soluble actives should not simply be placed in an oil-based vehicle, because the active can diffuse out of the carrier during storage.

The concentration gradient between the carrier and surrounding formulation therefore becomes an important consideration.

This is a valuable formulation lesson:

A delivery system cannot be designed independently of its final vehicle.

The carrier, active ingredient and cosmetic base have to be considered as one system.

Polysaccharide Microspheres: Controlled Release With Biopolymers

Another interesting approach uses polysaccharide-based microspheres.

The handbook describes chitosan-based microspheres designed for topical delivery.

Chitosan is a cationic polysaccharide that can form polymeric matrices and has bioadhesive properties. The described technology uses these properties to create microspheres capable of encapsulating active ingredients and providing sustained or controlled release.

The source also describes potential advantages such as:

• prolonged contact with the stratum corneum;
• improved stability of sensitive actives;
• separation of incompatible ingredients;
• reduced irritation in some formulations;
• and a reservoir effect.

One example discussed is retinol delivery using chitosan-based microspheres.

This illustrates an important point: the carrier can influence not only release rate, but also where and how long the active remains associated with the skin.

Liposomes: Another Route to Controlled Delivery

Liposomes are another well-known delivery technology used in cosmetic science.

They are vesicular structures formed primarily from phospholipids and can incorporate different types of active materials depending on their composition and structure.

The handbook's section on liposomes in personal care products discusses the loading of both hydrophilic and hydrophobic actives, as well as issues such as lipid composition and stability.

Liposomes are particularly interesting because their structure can influence how an active interacts with the surrounding formulation and skin.

However, they also introduce their own formulation challenges.

Lipid oxidation, hydrolysis, physical stability and active loading efficiency all need to be considered.

Again, the most sophisticated delivery system is not automatically the best one.

Can Silicone Systems Control Active Release?

Yes.

The delivery handbook also describes silicone-based technologies designed for entrapment and controlled release.

Silicone elastomers can be used to entrap or absorb certain ingredients, with release occurring under the shear forces generated when a product is rubbed onto the skin. Silicone vesicles have also been investigated as encapsulation systems.

This is a good example of a triggered release mechanism.

The delivery system is not simply releasing the active at a constant rate from the moment it is manufactured.

The mechanical action associated with application can influence release.

This opens up an interesting range of possibilities for topical products.

Controlled Release vs. Sequential Release

Controlled release is not the most advanced form of delivery described in the literature.

There is also sequential release.

In a sequential system, different active ingredients can be isolated and released at different stages.

The Delivery System Handbook describes sequential release as a method that can be used to separate incompatible actives and meter them out at defined times.

Multiple-phase emulsions are one example of how different actives can be incorporated into different parts of a formulation system.

This is particularly interesting when two ingredients would be difficult to formulate together in a conventional system.

Instead of forcing incompatible ingredients into the same environment, the delivery system can help keep them separated until the appropriate stage of application.