science

Transdermal

Transdermal is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Transdermal rather than just read about it. In short: Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution. Examples include transdermal patches used for medicine delivery.

Key takeaways

Transdermal belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Transdermal to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Transdermal from memory before moving on to harder problems.

Reference excerpt

Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution. Examples include transdermal patches used for medicine delivery. The drug is administered in the form of a patch or ointment that delivers the drug into the circulation for systemic effect.

Techniques

Obstacles Although the skin is a large and logical target for drug delivery, its basic functions limit its utility for this purpose. The skin functions mainly to protect the body from external penetration (by e.g. harmful substances and microorganisms) and to contain all body fluids. There are two important layers to the human skin: (1) the epidermis and (2) the dermis. For transdermal delivery, drugs must pass through the two sublayers of the epidermis to reach the microcirculation of the dermis. The stratum corneum is the top layer of the skin and varies in thickness from approximately ten to several hundred micrometres, depending on the region of the body. It is composed of layers of dead, flattened keratinocytes surrounded by a lipid matrix, which together act as a brick-and-mortar system that is difficult to penetrate. The stratum corneum provides the most significant barrier to diffusion. In fact, the stratum corneum is the barrier to approximately 90% of transdermal drug applications. However, nearly all molecules penetrate it to some minimal degree. Below the stratum corneum lies the viable epidermis. This layer is about ten times as thick as the stratum corneum; however, diffusion is much faster here due to the greater degree of hydration in the living cells of the viable epidermis. Below the epidermis lies the dermis, which is approximately one millimeter thick, 100 times the thickness of the stratum corneum. The dermis contains small vessels that distribute drugs into the systemic circulation and regulate temperature, a system known as the skin's microcirculation.

Transdermal pathways There are two main pathways by which drugs can cross the skin and reach the systemic circulation. The more direct route is known as the transcellular pathway.

Transcellular pathway By this route, drugs cross the skin by directly passing through both the phospholipid-rich membranes and the cytoplasm of the dead keratinocytes that constitute the stratum corneum. Although this is the path of shortest distance, the drugs encounter significant resistance to permeation. This resistance is caused because the drugs must cross the lipophilic membrane of each cell, then the hydrophilic cellular contents containing keratin, and then the phospholipid bilayer of the cell one more time. This series of steps is repeated numerous times to traverse the full thickness of the stratum corneum.

Intercellular pathway The other more common pathway through the skin is via the intercellular route. Drugs crossing the skin by this route must pass through the small spaces between the cells of the skin, making the route more tortuous. Although the thickness of the stratum corneum is only about 20 μm, the actual diffusional path of most molecules crossing the skin is on the order of 400 μm. The 20-fold increase in the actual path of permeating molecules greatly reduces the rate of drug penetration. Recent research has established that the intercellular route can be dramatically enhanced by attending to the physical chemistry of the system solubilizing the active pharmaceutical ingredient, rendering a dramatically more efficient delivery of payload and enabling the delivery of most compounds via this route.

Microneedles A third pathway to breach the Stratum Corneum layer is via tiny microchannels created by a medical micro-needling device of which there are many brands and variants. Investigations at the University of Marburg, Germany, using a standard Franz diffusion cell showed that this approach is efficient in enhancing skin penetration ability for lipophilic as well as hydrophilic compounds. The micro-needling approach is also seen as 'the vaccine of the future'. The microneedles can be hollow, solid, coated, dissolving, or hydrogel-forming. Some have regulatory approval. Microneedle devices/patches can be used to deliver nanoparticle medicines.

Devices and formulations Devices and formulations for transdermally administered substances include transdermal patches and transdermal gel.

Illustrations

Worked examples

Example 1 — a first encounter with Transdermal

Start with the simplest possible case. Write down what Transdermal claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Transdermal before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Transdermal ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Transdermal

In research: Transdermal appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Transdermal in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Transdermal is common in secondary-school and first-year university syllabi. It links to neighbouring topics Medical treatments, Routes of administration, so understanding it makes those chapters shorter.
In everyday life: Look for Transdermal outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Transdermal in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Transdermal means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Transdermal out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Transdermal in simple terms?

Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution. Examples include transdermal patches used for medicine delivery.

Why does Transdermal matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Transdermal?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Transdermal.