Design of Functionalized Bi-Layered Swellable PCL/Eudragit-Based Matrix Tablets for Controlled Drug Release: Crank’s Diffusion, Kinetic Modeling and Mechanistic Insights

Medicine often arrives in the body as a sudden flood, a large dose released all at once that must then be managed by the liver and kidneys. To avoid this spike and the subsequent drop, scientists have long sought ways to build drug delivery systems that act like a slow, steady tap, releasing a therapeutic agent over hours or days. The key to this control often lies in the material surrounding the drug. By choosing specific polymers—long-chain molecules that can swell when wet or dissolve slowly—researchers can engineer a matrix that dictates exactly when and how a medicine escapes. A particularly clever strategy involves attaching the drug to these polymers through a chemical link that is sensitive to the body's environment. If the link breaks only when it encounters a specific level of acidity, the drug remains locked away until it reaches the right location, such as the stomach, before being freed to do its work.

In a recent study, researchers set out to refine this approach by creating a new type of pill designed to release a model drug called p-anisidine in a controlled manner. They built these pills as bi-layered tablets, essentially two distinct layers pressed together, using a blend of two common materials: Eudragit RL100, a synthetic polymer known for letting water pass through easily, and poly(ε-caprolactone), or PCL, a biodegradable plastic that is semi-crystalline and hydrophobic. The active ingredient was not just mixed in; it was chemically bonded to the system. In one set of experiments, the drug was attached to a small molecule carrier, while in another, it was grafted onto a larger polymer chain. Both carriers were linked to the drug via a Schiff base, a chemical bond that acts like a molecular safety latch, known to break apart quickly in acidic environments but stay intact in neutral or basic ones. The team wanted to see how the mix of these two polymers and the acidity of the surrounding fluid would influence the speed and amount of drug released.

The researchers prepared these tablets by compressing the powder mixtures into small, cylindrical discs. They then placed these discs into three different liquid environments to mimic the conditions found in the human body: a highly acidic fluid similar to stomach juice with a pH of 1.2, a mildly acidic fluid at pH 4.0, and a basic fluid at pH 8.0, representing the intestines. As the tablets sat in these liquids, they began to absorb water and swell. The scientists measured how much drug came out over time, tracking the process carefully to understand the mechanics behind the release. They found that the acidity of the liquid played the most dramatic role. In the highly acidic environment, the chemical latch holding the drug broke apart rapidly, leading to a much higher release of the drug compared to the other conditions. This confirmed that the system was indeed sensitive to pH, releasing the most drug where the acid was strongest.

However, the composition of the tablet itself proved to be just as important as the acidity. When the researchers added the semi-crystalline PCL to the Eudragit RL matrix, the tablets released significantly more drug than those made with Eudragit alone. This was not because the drug moved faster through the material in a simple sense, but because the PCL changed the internal structure of the tablet. The addition of this second polymer made the matrix more open to water penetration, allowing the liquid to swell the tablet more effectively. This increased hydration helped break the chemical bonds holding the drug and provided more pathways for the released molecules to travel out of the tablet. In fact, the tablets containing the PCL blend released up to 78 percent of their drug load in the acidic environment, whereas the tablets made with only the single polymer released less than 60 percent. This improvement was consistent across all pH levels, showing that the blend created a more efficient delivery system regardless of the surrounding acidity.

To understand exactly how the drug moved, the researchers analyzed the data using mathematical models that describe how substances diffuse through a gel-like material. They found that the release followed a predictable pattern known as Fickian diffusion, where the drug moves through the swollen polymer network at a rate determined by the concentration difference between the inside and outside of the tablet. This meant that once the tablet was wet and the chemical bonds had broken, the drug simply diffused out in a steady, controlled stream. The study also revealed a subtle but important distinction between the two types of drug carriers they tested. When the drug was attached to a small molecule, the release was driven almost entirely by the speed at which the chemical bond broke. But when the drug was attached to a large polymer chain, the process became a three-step dance of the bond breaking, the tablet swelling with water, and the drug finally diffusing out. In this second scenario, the physical structure of the tablet played a larger role in controlling the final outcome.

The results of this work highlight that designing a successful drug delivery system requires balancing chemical sensitivity with physical structure. By combining a pH-sensitive chemical link with a polymer blend that swells effectively, the researchers created a platform that could be tuned to release drugs at different rates. The study demonstrated that while the chemical environment dictates when the drug is freed, the physical makeup of the tablet determines how much of it can actually escape. This dual control offers a promising path for creating future medications that can be tailored to release their contents precisely where and when they are needed, ensuring that the right amount of medicine reaches the body without the waste or side effects associated with sudden, uncontrolled doses.