A Review on Transdermal Drug Delivery System

The transdermal drug delivery system (TDDS) is an advanced and controlled drug delivery approach designed to deliver therapeutically effective drug concentrations across the skin into the systemic circulation. Over the past few decades, TDDS has emerged as a promising alternative to conventional drug delivery routes such as oral and parenteral administration. Although the oral route is the most commonly used method for drug administration, it is often associated with several limitations including gastrointestinal degradation of drugs, poor bioavailability, hepatic first-pass metabolism, frequent dosing, and patient non-compliance. Parenteral routes, on the other hand, are invasive, painful, and may lead to reduced patient acceptance, particularly in long-term therapy.^1-3 Transdermal drug delivery overcomes many of these drawbacks by providing a non-invasive, painless, and convenient method of drug administration. In TDDS, drugs are administered in the form of patches that are applied to the surface of the skin, from where the drug is released in a controlled manner and absorbed into the systemic circulation.³ This approach allows maintenance of relatively constant plasma drug concentrations over an extended period of time, thereby minimizing peak-and-trough fluctuations associated with conventional dosage forms. As a result, TDDS improves therapeutic efficacy while reducing adverse effects.^3-6 The skin, being the largest organ of the human body, serves as both a protective barrier and a potential route for drug delivery. However, the presence of the stratum corneum in the epidermis poses a major challenge to transdermal drug permeation. Only drugs with suitable physicochemical properties such as low molecular weight, appropriate lipophilicity, and adequate potency are considered ideal candidates for transdermal delivery. Therefore, a thorough understanding of skin anatomy, drug absorption pathways, and factors influencing skin permeability is essential for the successful design of transdermal formulations.^1-4 Various formulation strategies have been employed to enhance transdermal drug permeation, including the use of polymers, penetration enhancers, and novel delivery systems. Transdermal patches are broadly classified into matrix-type, reservoir-type, and micro-reservoir systems based on their design and drug release mechanism. Each type offers distinct advantages in controlling drug release and improving patient compliance. In recent years, technological advancements such as microneedle-based patches, iontophoresis, and smart transdermal systems have further expanded the scope of TDDS, enabling the delivery of macromolecules and biologics through the skin.^2-5 The present review aims to provide a comprehensive and systematic overview of transdermal drug delivery systems with emphasis on skin anatomy, mechanisms of drug absorption, factors affecting transdermal permeation, types and components of transdermal patches, pharmaceutical applications, and recent advances in transdermal technology. This review is intended to serve as a useful academic reference for undergraduate and postgraduate pharmacy students as well as

1) Anatomy of Skin Related To Transdermal Absorption

The skin is the largest organ of the human body, covering approximately 1.5–2.0 m² and accounting for nearly 7% of total body weight. Structurally, it consists of three major layers: epidermis, dermis, and hypodermis. Each layer plays a significant role in regulating drug permeation.⁵

1.1 Epidermis

The epidermis is the outermost layer composed mainly of keratinized stratified squamous epithelium. The stratum corneum, the outermost sub-layer, is the principal barrier to drug penetration. It consists of dead keratinocytes embedded in a lipid matrix, restricting the passage of most hydrophilic and large molecular weight drugs.^1-5

1.2 Dermis

The dermis is a vascular connective tissue layer containing blood vessels, lymphatics, nerves, hair follicles, and sweat glands. It provides nutrients and acts as a sink for drugs that have crossed the epidermis, thereby maintaining a concentration gradient necessary for continuous permeation.^1-5

1.3 Hypodermis

The hypodermis consists mainly of adipose tissue and serves as mechanical protection and thermal insulation. While it does not significantly influence drug permeation, it supports the dermis and epidermis structurally.^1-8

Fig 1: - Transdermal drug absorption pathways

2. Mechanism Of Drug Absorption Through Skin

Drug molecules penetrate the skin through three major pathways:

2.1 Transcellular Pathway

In this route, drugs pass directly through the corneocytes. This pathway is mainly followed by small, moderately lipophilic molecules.⁹

2.2 Intercellular Pathway

This is the predominant route of penetration, where drugs diffuse through the lipid matrix between corneocytes. Most transdermal drugs utilize this pathway due to its relatively larger surface area.^2-9

2.3 Appendageal Pathway

Drugs permeate through hair follicles and sweat glands. Although this route contributes minimally due to limited surface area, it is significant for ions and large polar molecules.^2-10

Fig 2: - Transdermal drug delivery pathways

3. Factors Affecting Transdermal Drug Absorption

Transdermal drug absorption is influenced by several interrelated factors. These factors can be broadly classified into physicochemical properties of the drug, biological factors related to the skin, and formulation-related factors. Proper optimization of these parameters is essential for the successful development of an effective transdermal patch.

3.1 Physicochemical Factors

The physicochemical characteristics of a drug play a crucial role in determining its suitability for transdermal delivery.

Molecular Weight:

Drugs with low molecular weight (generally below 500 Da) permeate the skin more easily compared to large molecules.

Partition Coefficient:

An ideal transdermal drug should possess balanced lipophilicity and hydrophilicity. Excessively lipophilic drugs may remain in the stratum corneum, whereas highly hydrophilic drugs may fail to penetrate the lipid-rich barrier.

Melting Point:

Drugs with lower melting points exhibit higher solubility and improved permeation through the skin.

Drug Concentration:

An increased drug concentration enhances the diffusion rate by maintaining a higher concentration gradient across the skin.^1-7

3.2 Biological Factors

Biological characteristics of the skin significantly influence transdermal absorption.

Skin Condition:

Damaged or hydrated skin shows higher permeability compared to dry and intact skin.

Age:

Skin permeability is higher in infants and elderly individuals due to thinner stratum corneum.

Skin Site:

Different anatomical sites exhibit variable permeability. Areas with thin stratum corneum, such as behind the ear, show enhanced absorption.

Blood Flow:

Increased blood circulation enhances systemic uptake of drugs after permeation.^2-5

3.3 Formulation-Related Factors

Formulation design greatly affects drug release and permeation.

Type of Polymer:

Selection of polymer determines drug release rate and mechanical strength of the patch.

Natural polymer: -

1. Chitosan

2. sodium alginate

3. gelatin

4. starch

5. cellulose, etc

Natural polymer are biocompatible and biodegradable

:- semi-synthetic polymers are common for  controlled drug release.

:- synthetic polymers are strong, stable, long-term release.

Penetration Enhancers:

Penetration enhancers are agents that temporarily and reversibly increase the permeability of the skin to allow greater transport of drugs across the skin without causing permanent damage.

Mechanism of Action

Penetration enhancers work by:

1. Disrupting lipid structure of the stratum corneum
2. Increasing skin hydration
3. Altering protein structure
4. Increasing drug solubility in skin.

Ideal Characteristics of Penetration Enhancers

1. Non-toxic

It should not show any toxic effect on the skin or systemic circulation, even after repeated application.

2. Non-irritant and Non-allergenic

It should not cause skin irritation, redness, itching, burning sensation, or allergic reactions.

3. Reversible Action

The effect of the penetration enhancer on skin permeability should be temporary, and the skin should return to its normal barrier function after removal.

4. No Permanent Skin Damage

It should not cause permanent damage to the stratum corneum, epidermis, or dermis.

5. Pharmacologically Inert

It should not possess any pharmacological activity of its own and should not interfere with the therapeutic action of the drug.

6. Compatible with Drug and Polymer

It should be chemically and physically compatible with the drug, polymer, and other excipients used in the formulation.

7. Effective at Low Concentration

It should enhance skin penetration even at low concentrations to minimize the risk of irritation and toxicity.

8. No Effect on Drug Stability

It should not cause degradation, precipitation, or instability of the drug.

9. Good Solvent Properties

It should increase drug solubility in the skin and formulation, thereby improving drug diffusion.

10. Acceptable Odour and Appearance

It should be cosmetically acceptable, with no unpleasant smell or discoloration.

11. Easy to Formulate

It should be easy to incorporate into transdermal patches, gels, or ointments without affecting formulation properties.^1-9

Patch Thickness:

Thicker patches may retard drug release, whereas thinner patches allow faster diffusion. Patch thickness is an important physical parameter of a transdermal patch that directly affects drug release, mechanical strength, flexibility, and patient comfort.

Importance of Patch Thickness

1. Uniform Drug Distribution

Uniform thickness ensures equal distribution of drug throughout the patch.

2. Controlled Drug Release

Increased thickness results in slower drug release, whereas decreased thickness leads to faster drug release.

3. Mechanical Strength

Proper thickness provides sufficient strength and prevents breaking or tearing of the patch during handling.

4. Flexibility and Patient Comfort

Very thick patches may be uncomfortable, while very thin patches may lack durability.

5. Batch-to-Batch Reproducibility

Uniform thickness ensures consistency between different batches of patches.

6. Dose Accuracy

Patch thickness directly affects the amount of drug per unit area. Uniform thickness ensures accurate and consistent dosing, which is essential for therapeutic effectiveness and patient safety.

Factors Affecting Patch Thickness

• Polymer concentration

• Amount of drug and plasticizer
• Volume of solvent used
• Area of casting surface
• Drying temperature and time

Measurement of Patch Thickness

Patch thickness is measured at 3–5 different points using:

1. Vernier caliper
2. Digital micrometer screw gauge

The average thickness is calculated and reported.

Ideal Thickness Range

Generally, 0.1 – 1.0 mm, depending on formulation and polymer type.

4.  Types Of Transdermal Drug Delivery Systems

Based on design and drug release mechanism, transdermal patches are classified into various types.

4.1 Matrix-Type Transdermal Patches

Matrix-type transdermal patches are one of the most commonly used designs in transdermal drug delivery systems, where the drug is uniformly dispersed within a polymer matrix that controls drug release.

Basic Structure

A matrix-type transdermal patch generally consists of:

1. Backing layer – protects the patch from the external environment
2. Drug-polymer matrix – contains drug uniformly dispersed in polymer
3. Adhesive layer – helps the patch stick to the skin (may be part of matrix)
4. Release liner – removed before application.

Mechanism of Drug Release

1. Drug release occurs mainly by diffusion
2. Follows Fick’s law of diffusion
3. Release rate depends on:

• Polymer type and concentration
• Patch thickness
• Drug solubility
• Presence of penetration enhancers

Polymers Used in Matrix Patches

• HPMC
• Ethyl cellulose
• PVA
• PVP
• Eudragit RL/RS
• Chitosan

Advantages

Simple and economical to manufacture

Better flexibility and patient comfort

Lower risk of dose dumping compared to reservoir systems

Suitable for both hydrophilic and lipophilic drugs

Disadvantages

Difficult to achieve zero-order release

Drug release rate decreases over time

Limited drug loading capacity

Evaluation Parameters

Thickness uniformity

Weight variation

Drug content uniformity

Folding endurance

In-vitro drug release

Skin irritation study

Role of Patch Thickness in Matrix Patches

Thicker matrix → slower drug release

Thinner matrix → faster drug release

Uniform thickness ensures dose accuracy and controlled release

Examples

Nitroglycerin matrix patch

Diclofenac sodium matrix patch

Nicotine transdermal patch

4.2 Reservoir-Type Transdermal Patches

A reservoir-type transdermal patch is a system in which the drug is enclosed in a separate reservoir (liquid, gel, or suspension) and its release is controlled by a rate-controlling membrane before reaching the skin.

 1. Structure and Components: -

• 1. Backing Layer
• Outermost protective layer
• Impermeable to drug, moisture, and oxygen
• Provides mechanical support
• Materials used: Polyethylene, Polyester, Aluminum-laminated films

1.2.  Drug Reservoir

Contains drug in solution, suspension, or gel form May include:

• Solvent
• Penetration enhancers
• Stabilizers

Maintains constant drug activity throughout use

1.3.  Rate-Controlling Membrane

• Controls drug diffusion rate

Determines release kinetics (often zero-order) Made of:

• Ethylene-vinyl acetate (EVA)
• Polyurethane
• Silicone membranes

 Drug release depends mainly on membrane thickness and permeability, not on drug concentration.

1.4.  Adhesive Layer

• Keeps patch attached to the skin
• Must be biocompatible and non-irritant
• Sometimes placed below or above the membrane

1.5. Release Liner

• Protects adhesive layer during storage
• Removed just before application.

5. Mechanism of Drug Release

• Drug diffuses from the reservoir
• Passes through the rate-controlling membrane
• Enters the stratum corneum
• Reaches systemic circulation

• Release follows Fick’s law of diffusion
• Often shows zero-order kinetics (constant release)

3. Factors Affecting Drug Release

• Membrane thickness
• Membrane permeability
• Drug concentration in reservoir
• Presence of penetration enhancers
• Skin condition

4. Advantages

• Precise and controlled drug release
• Can achieve zero-order release
• Suitable for potent drugs with short half-life
• Better control compared to matrix patches

5. Disadvantages

• Complex and costly manufacturing process
• Risk of dose dumping if membrane is damaged
• Less flexible and bulkier
• Not suitable for high-dose drugs

6. Evaluation Parameters

• Membrane integrity test
• Drug content uniformity
• In-vitro drug release study
• Adhesive strength
• Skin irritation test

Example

• Scopolamine transdermal patch
• Nitroglycerin reservoir patch
• Clonidine patch
  3. Micro-Reservoir Transdermal Systems

A micro-reservoir transdermal system is a drug delivery system in which the drug is enclosed in microscopic reservoirs (micro-droplets or micro-spheres) that are uniformly dispersed within a polymeric matrix, and drug release is controlled by both diffusion and partitioning.

1. Concept and Principle

1.1 Drug is dissolved or dispersed in an aqueous phase.

1.2 This drug solution is dispersed as micro-reservoirs within a hydrophobic polymer matrix using high-shear mixing.

1.3. Each micro-reservoir acts as a miniature drug reservoir, but unlike classical reservoir systems, there is no single large reservoir.

2. Structure and Components

2.2. Backing Layer:-

• Protects patch from external environment
• Prevents drug loss and moisture entry
• Materials: Polyethylene, Polyester

2.3. Micro-Reservoir Drug System (Core Layer)

Drug present in micro-reservoirs

Drug may be in:-

1. Solution
2. Suspension

Micro-reservoirs are uniformly dispersed in polymer matrix Polymers used:-

• Silicone elastomers
• Polyisobutylene
•  Ethylene-vinyl acetate (EVA)

3. Rate-Controlling Polymer Matrix

• Surrounds each micro-reservoir
• Controls diffusion of drug from micro-reservoirs to skin

4. Adhesive Layer

• Maintains contact with skin
• Often incorporated into the matrix itself

5. Release Liner

Removed before application

5.1. Mechanism of Drug Release

• Drug diffuses from micro-reservoirs
• Passes through the surrounding polymer matrix
• Penetrates the stratum corneum
• Enters systemic circulation

???? Release is controlled by:

• Diffusion
• Partition coefficient
• Polymer permeability

5.2. Release Kinetics

• Near zero-order release
• More uniform than matrix systems
• Safer than reservoir systems (no dose dumping)

6. Advantages

• Combines benefits of matrix and reservoir systems
• Reduced risk of dose dumping
• Better control over drug release
• Improved drug stability
• Suitable for potent drugs

7. Disadvantages

• Complex manufacturing process
• Difficult to achieve uniform micro-reservoir size
• Higher production cost

8. Evaluation Parameters

• Patch thickness
• Micro-reservoir size and distribution
• Drug content uniformity
• In-vitro drug release
• Skin irritation test

9. Examples

• Nitroglycerin micro-reservoir patch
• Scopolamine micro-reservoir system

Fig 3: - Different types of transdermal patches.

5. Pharmaceutical Applications of Transdermal Drug Delivery System

Transdermal drug delivery systems (TDDS) have gained significant importance in pharmaceutical therapy due to their ability to provide controlled, sustained, and non-invasive drug administration. By delivering drugs through the skin directly into systemic circulation, TDDS overcome several limitations associated with oral and parenteral routes, such as gastrointestinal degradation, first-pass hepatic metabolism, and poor patient compliance. The applications of transdermal drug delivery extend across various therapeutic areas, particularly in chronic disease management where long-term and controlled drug release is required.

5.1 Pain Management

One of the most successful applications of TDDS is in the management of chronic and acute pain. Transdermal patches containing opioid and non-opioid analgesics provide sustained analgesic action over an extended period.

Drugs such as fentanyl, buprenorphine, and diclofenac are commonly administered via transdermal patches. These patches maintain constant plasma drug levels, thereby reducing fluctuations that may cause side effects or inadequate pain control. Additionally, transdermal analgesic patches reduce gastrointestinal irritation and improve patient compliance, especially in patients requiring long-term pain management such as those with cancer or arthritis.^1-3

5.2 Hormonal Replacement Therapy

Transdermal drug delivery is widely used in hormone replacement therapy due to its ability to provide steady hormone levels without first-pass metabolism. Hormones administered orally often undergo extensive hepatic metabolism, which may lead to reduced bioavailability and adverse effects. Transdermal patches containing estradiol, testosterone, and progesterone are commonly used in the management of menopausal symptoms, hypogonadism, and other endocrine disorders. The transdermal route ensures controlled hormone release, improved therapeutic efficacy, and reduced risk of hepatic side effects compared to oral administration.³

5.3 Cardiovascular Disorders

TDDS plays an important role in the treatment of cardiovascular diseases, particularly in conditions requiring continuous drug delivery. Nitroglycerin transdermal patches are widely used in the prevention and management of angina pectoris. Transdermal delivery of nitroglycerin provides sustained vasodilation, reduces the frequency of anginal attacks, and avoids extensive first-pass metabolism. The ability to terminate therapy easily by removing the patch is an additional advantage in cardiovascular emergencies.^1-5

5.4. Neurological and Central Nervous System Disorders

Transdermal drug delivery has shown promising results in the management of neurological disorders where continuous drug delivery is essential. Rotigotine transdermal patches are used in the treatment of Parkinson’s disease to provide continuous dopaminergic stimulation. By maintaining constant plasma drug concentrations, transdermal systems help reduce motor fluctuations and improve patient compliance. TDDS also minimizes gastrointestinal side effects often associated with oral CNS medications.³

5.5. Smoking Cessation Therapy

Nicotine transdermal patches are one of the most widely used TDDS products. These patches deliver controlled amounts of nicotine over prolonged periods, helping to reduce withdrawal symptoms and cravings associated with smoking cessation. Transdermal nicotine therapy provides a safer alternative to smoking by eliminating exposure to harmful combustion products and improving patient adherence to cessation programs.¹

5.6. Dermatological and Local Applications

TDDS is also used for local drug delivery in dermatological conditions such as inflammation, fungal infections, and psoriasis. In these applications, the drug primarily acts at the site of application, minimizing systemic exposure and side effects. Drugs such as lidocaine and ketoprofen are commonly formulated as transdermal or topical patches for localized therapeutic action.^2-5

5.7. Anti-Motion Sickness Therapy

Transdermal patches containing scopolamine are widely used for the prevention of motion sickness. These patches provide prolonged drug release and are particularly useful during long-distance travel. The transdermal route offers improved patient convenience and reduces the need for repeated dosing compared to oral formulations.³

5.8. Emerging Applications and Future Therapeutic Use

Recent advances in transdermal technology have expanded its pharmaceutical applications beyond small molecules. Novel systems such as microneedle-based patches, iontophoretic systems, and smart transdermal patches have enabled the delivery of peptides, proteins, vaccines, and other macromolecules. These advanced transdermal systems hold great potential in personalized medicine, chronic disease management, and vaccination programs, highlighting the growing importance of TDDS in modern pharmaceutical therapy.^4-5

6. Marketed Transdermal Patches

Several transdermal products are commercially available, demonstrating the clinical success of TDDS.