Transdermal patches are medical devices that deliver medicine through the skin and into the bloodstream. They can provide steady dosing for hours or days, which helps avoid the peaks and drops that can happen with pills or injections. This technology is used for medicines such as nicotine, motion sickness drugs, pain medicines, and some hormones.
The design matters because the skin is a strong barrier that only certain drug molecules can pass through safely and predictably.
A typical patch has several layers, including a backing layer, a drug reservoir or drug-in-adhesive layer, a rate-controlling membrane in some designs, adhesive, and a removable liner. After the liner is removed and the patch is applied, drug molecules diffuse from a region of higher concentration in the patch toward a lower concentration in the skin and blood. The stratum corneum, the outermost skin layer, is usually the main barrier to diffusion.
Engineers control dose by changing drug concentration, patch area, membrane properties, adhesive chemistry, and wear time.
Understanding Medical Technology: Transdermal Patches
Skin is not a simple flat wall. Its outer cells are packed with oily materials that slow the movement of many substances. Beneath this layer are living cells, small blood vessels, and body fluids.
A medicine must divide between the patch material, the oily outer skin, and the wetter tissue below. This is why a useful drug needs a balance of properties.
If it prefers water too strongly, it may not enter the outer layer. If it prefers oils too strongly, it may become trapped there instead of reaching blood vessels.
Delivery does not begin at its full rate the instant a patch touches skin. There is often a delay while medicine enters the skin layers and builds up a pathway through them. This delay is called lag time.
After that, a well designed patch can approach a nearly steady rate. Temperature changes can disturb this pattern. Fever, heating pads, hot baths, and vigorous exercise may increase blood flow or change the skin barrier.
More medicine may then enter the body faster than intended. This is why patch instructions often warn users to avoid direct heat.
Engineers choose between several patch designs. In a reservoir patch, medicine is held in a separate compartment and a membrane helps limit its release. In a matrix patch, medicine is mixed through a polymer layer.
In a drug in adhesive patch, the sticky layer itself contains the medicine. Each design must stay attached without causing major irritation. It must protect the drug from air, moisture, and light.
Some drugs need chemical helpers called penetration enhancers. These can temporarily make the outer skin easier to cross, but they must be used carefully because they can irritate skin or change delivery in unpredictable ways.
Students may encounter patches in nicotine treatment, travel sickness treatment, hormone treatment, and long term pain care. Correct use is part of the technology. Skin should usually be clean, dry, and free of lotions.
The application site is often rotated so one area does not become sore. A used patch can still contain medicine, so it should be folded with the sticky sides together and disposed of safely.
Children and pets can be harmed by used patches. A patch should not be cut unless the medicine instructions specifically permit it, because cutting can change the release pattern.
When studying patches, connect the science to measurable variables. A larger contact area can deliver more medicine when other conditions stay similar. Longer wear time usually gives a larger total dose, but only within the approved schedule.
Thickness matters because molecules take longer to cross a thicker barrier. Concentration matters because a greater difference in concentration tends to produce stronger movement.
Real skin adds variation from age, hydration, location on the body, and skin damage. These factors explain why medical directions are precise and why a patch is not simply a sticker carrying medicine.
Key Facts
- Diffusion moves drug molecules from high concentration to low concentration.
- Fick's first law: J = -D dC/dx, where J is flux, D is diffusion coefficient, and dC/dx is the concentration gradient.
- Approximate delivery rate: rate = J A, where A is the patch contact area.
- Total delivered dose can be estimated by dose = rate × time when the rate is nearly constant.
- The stratum corneum is the main skin barrier for most transdermal drugs.
- Good patch candidates are usually potent, stable, and able to dissolve in both lipids and water to some degree.
Vocabulary
- Transdermal patch
- A medicated adhesive device that delivers a drug through the skin into the body over time.
- Diffusion
- The net movement of particles from an area of higher concentration to an area of lower concentration.
- Flux
- The amount of drug crossing a unit area of skin per unit time.
- Stratum corneum
- The thin outer layer of the epidermis that forms the skin's strongest barrier to drug entry.
- Rate-controlling membrane
- A patch layer designed to limit how quickly drug molecules leave the patch.
Common Mistakes to Avoid
- Assuming any drug can be put in a patch is wrong because many drugs are too large, too irritating, or unable to cross the skin well.
- Thinking a larger patch always means a stronger medicine is wrong because dose depends on drug concentration, patch design, skin permeability, and contact area.
- Cutting a patch without instructions is wrong because it can damage the rate-controlling system and cause too much or too little drug release.
- Applying a patch to damaged or irritated skin is wrong because broken skin can increase absorption unpredictably and raise the risk of side effects.
Practice Questions
- 1 A patch delivers nicotine at a steady rate of 0.80 mg per hour for 16 hours. What total mass of nicotine is delivered?
- 2 A transdermal patch has a drug flux of 0.025 mg/(cm^2 h) and a contact area of 40 cm^2. What is the drug delivery rate in mg/h, and how much is delivered in 24 h?
- 3 Explain why a transdermal patch is designed with multiple layers instead of simply placing liquid medicine directly on the skin.