Topical Drug Delivery System

 

Kote Rupali Balasaheb, Jinat Maniyar*, Jadhav Ravindra S, Mule Asmita, Khetmalis Aditya,

Monika Magar, Nikita Borse

Department of Pharmaceutics, Pravara Rural Education Society's Institute of Pharmacy, Loni, MS, India.

*Corresponding Author E-mail: rupalikote236@gmail.com

 

ABSTRACT:

Topical drug delivery systems (TDDS) have evolved significantly over the past several decades, offering targeted and localized therapy through the skin while minimizing systemic exposure. The skin, as the largest organ and a complex barrier, provides both accessibility and selectivity for drug administration. Conventional topical formulations, including ointments, creams, gels, and lotions, have proven effective for certain dermatological conditions; however, they often face limitations related to drug penetration, bioavailability, stability, and patient compliance. Advances in pharmaceutical technology have led to innovative delivery strategies such as nano-based carriers, liposomes, ethosomes, niosomes, transferosomes, solid lipid nanoparticles, and microneedle systems, which enhance therapeutic efficacy and overcome limitations of traditional formulations. TDDS also leverages physical and chemical enhancement methods, including sonophoresis, electroporation, iontophoresis, and chemical penetration enhancers, to improve drug permeation and retention. Future perspectives emphasize the integration of transdermal patches, hybrid chemical-physical approaches, and optimized formulation design to ensure stability, targeted delivery, and minimal side effects. The transition from conventional to carrier-mediated and advanced topical systems represents a promising avenue for effective treatment of localized and systemic conditions, including hydrophilic and macromolecular drugs, thereby improving patient adherence and overall therapeutic outcomes.

 

KEYWORDS: Topical, Trandsdermal, Dermal, Skin, Permeation, Penetration.

 

 

 

INTRODUCTION:

Over the past several decades, drugs have been administered to the human body through a variety of routes, including oral, sublingual, rectal, parenteral, inhalation, and topical applications.1 Topical drug delivery, in particular, involves applying a medicinal formulation directly to the skin to treat localized conditions such as acne or dermatological manifestations of systemic illnesses like psoriasis. The primary aim of this approach is to confine the pharmacological effects of the drug to the skin or underlying tissues, minimizing systemic exposure.2

 

Formulating an effective topical drug is a complex task.3 A topical preparation must be capable of delivering the active ingredient to a targeted site while accommodating diverse chemical entities that may possess different, and sometimes incompatible, physicochemical properties.4 Additionally, the formulation must maintain chemical stability within an appropriate container, ensuring the integrity of the drug until it is applied.5

 

Upon application, the formulation must interact efficiently with the skin to facilitate adequate absorption, which directly affects the drug release rate.6 The skin, which constitutes approximately sixteen percent of an adult's body weight, is the largest organ and plays a crucial role in maintaining homeostasis.7 It serves as a physical, chemical, and biological barrier against external environmental challenges. Conventional topical and transdermal formulations, such as creams and ointments, are effective for certain dermatological conditions but often face limitations regarding drug targeting and bioavailability.8

 

Advancements in pharmaceutical technology have led to the development of innovative transdermal and topical delivery systems, including nano-based formulations.9 These approaches address common formulation challenges, such as drug solubility, and enhance therapeutic efficacy in dermatological treatments. Moreover, they help overcome drawbacks associated with oral administration, including reduced patient compliance, first-pass metabolism by the liver, and longer dosing intervals.10

 

Modern and emerging strategies to improve topical delivery of both small and large molecules include chemical penetration enhancers, biopolymers like sodium hyaluronate, liposomes, foams, topical sprays, occlusion with dressings or patches, topical peels, thermal methods, iontophoresis, and ultrasound. These advanced systems provide improved drug efficacy and tolerability, enhance patient adherence, and offer solutions to unmet needs in the dermatology market.11 This review focuses on the evolution, current status, and future perspectives of topical drug delivery systems.12

 

Anatomy and Physiology of Skin:

 

Figure 1: Anatomy and Physiology of Skin

The biggest organ in the human body, with a surface area of 1.5 to 2 square meter, the skin acts as the most complex barrier shielding the biological system from the outside world. The largest exterior defense mechanism is the skin. Although the skin serves as the body's outermost covering, it also serves additional purposes.13 Depending on the surroundings, skin temperature might range from 30 to 40 degrees Celsius. 14It is composed of three layers: the outer epidermis (biological barrier), which is 50–150΅m thick, the 250΅m thick dermis (heat barrier), and the innermost subcutaneous fatty tissue (mechanical cushion). The superficial layers of the stratum corneum, which make up the epidermal region, have a unique cellular structure that prevents molecules larger than 500 Da from passing through and provides physical protection against microbial invasion.15

 

Layer of skin:

A) Epidermis:

1.     Stratum corneum

2.     Stratum lucidum

3.     Stratum granulosum

4.     Stratum spinosum

5.     Stratum basale

 

B) Dermis:

 

C) Hypodermis:

 

A) Epidermis16

The epidermis, a constantly renewing squamous epithelium made up mainly of squamous cells and live cells or cells of the squamous layer (existing epidermis), corneal layer dead cells, also known as the stratum corneum, covers the entire body's exterior.

Types of epidermis layers

 

1. Stratum Corneum:17

Also known as the stratum corneum, this is the outermost layer of skin. It is a flow restriction barrier that restricts the amount of material that can enter and exit. The composition of the stratum corneum, which is composed of 7580% protein, 515% lipid, and 510% ondansetron on a dry weight basis, has a significant impact on its barrier qualities. The stratum corneum will expand several times its dry thickness of roughly 10 mm when fully moistened.

 

2. Stratum lucidum:18

The compressed cells of the epithelium make up the stratum lucidum. Many cells have a damaged nucleus, and some have none at all. The layer has the appearance of a homogeneous translucent zone due to the glossy nature of these cells. The layer is hence called stratum lucidum (lucid = clear).

 

 

3. Stratum Granulosum:

This is a thin layer of two to five flattened rhomboid cells.  The cytoplasm contains kerneletohyaline granules. The protein keratohyaline is the precursor of keratin.

 

4. Stratum Spinosum:

This layer is frequently called the prickly cell layer because the cells inside it have protoplasmic projections thatresemble spines. These projections connect the cells to one another.

 

5. Stratum Base:19

Stratum germinativum, another name for this thick layer, is made up of polygonal cells close to the surface and columnar or cuboidal epithelial cells in the deeper layers. In this case, the ongoing process that produces new cells is called mitotic division. The newly formed cells continue to travel toward the stratum corneum. The stem cells are called keratinocytes. The dermis is reached by certain projections from this layer. These protuberances function as a supporting and structural component. The color of the skin is determined by this layer, which contains the pigment melanin.

 

B. Dermis:

The dermis is a hydrophilic deposit that is between 0.1 and 0.5cm thick. Sweat glands, blood arteries, lymphatic and nerve ends, pilosebaceous units, and a network of collagen and elastin fibers contained in the mucopolysaccharide matrix comprise the dermis. The collagen fibers in the connective tissue provide support, while the elastic tissue provides flexibility.20

 

C. Hypodermis:

The hypodermis is the innermost layer of the skin. It is the layer that separates the skin from the internal parts of the body, such as the muscles and bones. Sweat glands, sebaceous glands, and hair follicles are all encased in the epidermis, despite their dermal origin.21

 

Skin Penetration:

Effective penetration of drug molecules is crucial for transdermal drug delivery systems (TDDS). Drugs can traverse the skin via three main routes: the intracellular (transcellular) pathway, the intercellular pathway, and the transfollicular (transappendageal) pathway. Lipophilic molecules typically favor the intercellular route, navigating between cells, while the intracellular route allows drugs to pass directly through corneocytes. Both hydrophilic and lipophilic molecules can utilize this route depending on their polarity, though it is more complex due to alternating lipophilic and hydrophilic barriers.22 The transfollicular pathway, involving sweat glands and hair follicles, contributes minimally due to its small surface area (~0.1%), but it may aid the initial diffusion of larger molecules.23

Several strategies have been developed to enhance skin permeation. These include physicochemical approaches such as drug-vehicle interactions (prodrug design, ion pairs, eutectic mixtures) and vesicular systems like liposomes, niosomes, ethosomes, and transferosomes.24  Chemical penetration enhancers (CPEs) such as water, alcohol, terpenes, azone, sulfoxides, surfactants, phospholipids, and urea can modify the stratum corneum to improve drug transport. Physical methods include microneedle-assisted ablation, and electrically powered techniques such as iontophoresis, electroporation, magnetophoresis, ultrasound, and photomechanical waves, all of which temporarily disrupt the skin barrier to facilitate drug delivery.25

 

 

Figure 2:  Drug penetration through skin

 

Factors Affecting Drug Permeability through the Skin:

A) Physiological factor:

1.     Temperature of skin

2.     Lipid content

3.     Density of sweat glands

4.     Thickness of skin

5.     pH of skin

6.     Hydration of skin

7.     Blood flow

8.     Inflammation of skin26

 

B) Physiochemical factors:

1.     Molecular weight of drug

2.     Degree of ionization

3.     Partition coefficient

4.     Vehicle effect27

 

Function of Skin:

1. Protective Barrier & Homeostasis : The skin shields vital organs from environmental hazards, minimizes water loss through perspiration, and helps maintain body temperature by regulating blood vessel dilation and contraction.28

 

2. Immune Function: Langerhans cells in the skin detect and process antigens, allowing the skin to trigger immune and inflammatory responses against external threats.29

 

3. Sensory & Endocrine Role: Skin receptors sense heat, touch, and pain, while the skin also acts as an endocrine organ by producing vitamin D and responding to hormones like insulin and androgens.30

 

4. Sebum & Sweat Production: Sebaceous glands secrete sebum, providing antibacterial and water-repellent properties, while apocrine and eccrine glands produce sweat for thermoregulation and pheromonal signaling.

 

5. Metabolic Functions: The skin contributes to respiration, biotransformation of xenobiotics, and metabolism of lipids, carbohydrates, vitamin D, collagen, melanin, and keratin.31

 

TOPICAL DRUG DELIVERY:

Topical drug delivery systems are designed to administer therapeutic agents directly to the skin to treat localized conditions. Available in liquid, semisolid, and solid forms, these formulations enhance absorption when the drug has suitable lipid/water properties. Semisolid bases carry active ingredients along with stabilizers, emulsifiers, or antioxidants. Drugs can produce local effects on the skin or systemic effects. Topical delivery avoids oral administration issues like first-pass metabolism and plasma fluctuations.32

 

Topical Preparation Classification

A) System of topical drug classification (tcs):

Q1 (Qualitative sameness): Products have the same ingredients, regardless of their proportions.

Q2 (Quantitative sameness): Products have the same amount of each ingredient.

Q3 (Microstructure (semi-solid) sameness):Products have the same physical structure, like viscosity, rheology, and particle size.33

 

B) Conventional Topical Dosage Forms:

1.     Ointments: Greasy, semi-solid preparations for local or systemic effect.

2.     Creams: Oil-in-water (O/W) or water-in-oil (W/O) emulsions for topical application.

3.     Gels: Semisolid systems with polymer networks for local delivery.

4.     Pastes: Thick, stiff preparations with high solid content for protective action.

5.     Lotions: Liquid emulsions or suspensions applied to large skin areas.

6.     Plasters: Adhesive medicated patches providing prolonged local effect.34

7.     Liniments: Liquid or semi-liquid preparations rubbed on the skin for therapeutic effect.

8.     Foams: Aerated semi-solid systems for easy spreading and absorption.

9.     Sprays/Solutions: Liquid solutions for application on the skin or mucous membranes35

C) Novel Topical Drug Delivery System:

1.     Liposomes: Phospholipid vesicles that encapsulate drugs for targeted delivery.

2.     Ethosomes: Soft, malleable vesicles containing high ethanol content for enhanced skin penetration.

3.     Niosomes: Non-ionic surfactant vesicles similar to liposomes for controlled release.

4.     Transfersomes: Ultra-deformable vesicles for deep skin penetration.

5.     Solid Lipid Nanoparticles (SLNs): Lipid-based nanoparticles for sustained drug release.

6.     Nanostructured Lipid Carriers (NLCs): Second-generation lipid nanoparticles with improved drug loading.

7.     Microspheres and Nanoparticles: Biodegradable polymeric carriers for controlled topical delivery.

8.     Hydrogels: Water-swollen polymer networks for localized and sustained release.

9.     Microemulsions and Nanoemulsions: Thermodynamically stable systems for improved solubility and penetration.36

10. Transdermal Patches: Adhesive patches providing controlled drug release over time.

11. Iontophoresis and Electroporation Systems: Electrically assisted methods for enhanced drug permeation.

12. Microneedle-based Systems: Minimally invasive arrays for targeted dermal delivery.37

 

Challenges for Designing Topical Dosage Form:

The difficulty in creating a topical product lies in the numerous conditions that a formulation must satisfy, including all of the following needs:

 

1. Skin penetration:

The main obstacle to delivering bioactive agents into the skin via skin penetration is Fick's first law of diffusion, which describes the solute transfer rate as a function of the concentration of the different ingredients, the area that has to be treated, and the skin's permeability. The relationship between percutaneous absorption and molecular weight, which influences the coefficient of diffusion. Furthermore, some of the excipients included in the formulation, which have moisturizing, drying, or occluding actions, can also alter permeability, which in turn changes the drug release at the treatment site.38

 

2. Skin pH:

It is very difficult for drugs to permeate the stratum corneum if their molecular size is greater than 500 Daltons. Both high and low pH formulations can be harmful to the skin. For topical distribution, a moderate pH value is therefore appropriate. Additionally, the level of ionization at a specific pH is significant.

 

3. Stability:

During the development stage, it provides database studies to assist in the selection of formulation, excipient, and container closure systems; to ascertain shelf life and storage conditions; and to verify that no changes to the formulation or manufacturing process will adversely affect the product's stability.39

 

4. Acceptability:

Nowadays, patients are searching for topical drugs that are safe, effective, easy to use, and acceptable from a cosmetic perspective.  Acne routines increase topical system effectiveness and compliance since they are convenient and cause minimal skin disruption.40

 

Physical Enhancement Methods for Topical Drug Delivery:

1. Ultrasound/Sonophoresis/Phonophoresis:

Sonophoresis employs ultrasound waves to enhance transdermal drug absorption. It is a noninvasive, safe, and versatile method suitable for many drugs. Ultrasonic waves, when applied using coupling agents like hydrogels, produce cavitation, heat, and mechanical effects that temporarily disrupt the stratum corneum (SC), forming microchannels for drug penetration.41

 

2. Laser Radiation and Photomechanical Waves: Controlled laser exposure removes portions of the stratum corneum without damaging deeper layers, improving drug diffusion. The effect depends on laser parameters such as wavelength, pulse duration, and energy. This technique is often used for both dermatological therapy and enhanced topical delivery of hydrophilic and lipophilic drugs.42

 

3. Electroporation: Electroporation involves applying short, high-voltage pulses to create transient pores in the skin barrier. These reversible pores enable the delivery of large molecules like peptides, proteins, and vaccines. The technique allows controlled and efficient penetration with minimal skin damage.43

 

4. Iontophoresis: Iontophoresis utilizes a mild electric current to drive charged drug molecules through the skin via electrorepulsion. It is a safe and patient-friendly approach for delivering large, charged, or poorly permeable molecules such as peptides and oligonucleotides.44

 

5. Radio-frequency: Radio frequency uses high-frequency alternating current (~100 kHz) to create heat-induced microchannels in the epidermal membrane, similar to laser radiation. The device's drug delivery rate is determined by the number and depth of its microchannels, which are influenced by the microelectrodes' characteristics.45-46

 

Future Prospectives of Topical Delivery:

Transdermal drug delivery (TDD) is a non-invasive, patient-friendly approach suitable for all age groups, bypassing oral bioavailability issues and first-pass metabolism. It offers sustained drug release, improved therapeutic efficacy, and reduced systemic interactions. Challenges remain in enhancing penetration, understanding molecular and cellular barriers, and ensuring formulation safety. Future strategies include transdermal patches, gels, nano-formulations, and chemical penetration enhancers (CPEs). Physical methods and hybrid chemical-physical techniques are promising, particularly for hydrophilic drugs and macromolecules. Optimized formulations preserve drug stability, enable skin partitioning, allow targeted delivery, and minimize side effects. Focused formulation design is key to fully exploiting novel TDD technologies.

 

CONCLUSION:

This review highlights the historical evolution, current status, and emerging trends in topical drug delivery systems (TDDS). Over recent decades, TDDS has garnered considerable attention due to the skin’s unique attributes, offering both accessibility and a selective barrier for local and systemic drug administration. The development of topical formulations has progressed from simple solutions and ointments to advanced multiphase systems incorporating nanotechnology and adjunctive enhancement strategies. Conventional topical formulations exhibit limitations in terms of efficacy, stability, and patient compliance. In contrast, novel carrier-based systems demonstrate superior safety, therapeutic efficiency, practicality, and extended shelf life. Transitioning conventional formulations to carrier-mediated delivery platforms necessitates extensive research, representing a promising avenue for effective transdermal delivery of hydrophilic and other challenging drug molecules.

 

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Received on 24.10.2025      Revised on 26.11.2025

Accepted on 28.12.2025      Published on 12.02.2026

Available online from February 14, 2026

Res.J. Pharmacology and Pharmacodynamics.2026;18(1):81-86.

DOI: 10.52711/2321-5836.2026.00010

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