Herbal Extracts and Plant Biomarkers: A Promising Approach in Formulation Development of Dermal and Transdermal Dosage Forms

From several decades Herbal extracts have been used to treat various skin disease conditions. Dermal and Transdermal drug delivery allows an improved patients safety compliance and minimizes the ecological footprint of pharmaceutical manufacturing processes. A key strategy in this evolution is the incorporation of plant-based excipients in formulation development of dermal and transdermal dosage forms. “Herbs” are obtained from plants as a raw material or their extracts. They are used in the form of active ingredient and phytoconstituents derivatives, fragrance, Flavors, colorants, anti-inflammatory, antifungal, and antibacterial agents. The Phytoconstituents derived from different extraction processes needs a strategical approach to deliver the drug particles through skin barrier in the form of micro, macro, and Nanosized particles which are termed as Nanoparticles. Novel drug delivery approach leads to the formulation development of more Bioavailable, excipient compatible, very stable, and sustainable products. Incorporating herbal nanocarriers made up of biodegradable lipids, polysaccharides, and polymers in the form of microemulsions, creams, gels, lotions, and transdermal patch transfer of drugs allows a highly absorbable, permeable, and increased onset of action on the site of action on skin. To pass the drug ions and particles in soluble or suspended form through the tissues by the passage of a direct electric current through an electrolyte solution is known as “Electrophoresis” which in turn used as a pretreatment process in transdermal patch delivery system.

Structure of the Skin and Its Role in Skin Defence Mechanism [2,4]:

Human skin is the outer covering of the body which is the largest organ in the body [Figure:1]. It is also the first line of defence. Skin contains many specialized cells and structures. It is divided into three main layers known as epidermis, dermis, and hypodermis. The subcutaneous tissue is the outer layer of the fat and connective tissue that houses larger blood vessels and nerves. This layer is the important in the regulation of temperature of the skin itself and the body.  The size of this layer varies throughout the body and from person to person. Hair follicles, sweat glands and sebaceous glands are the main skin appendages. Many medicinal plant species worldwide to treat different diseases. Skin Plays an important role to protect body from pathogens and bacteria and excessive loss of water and keep the skin hydrated. It helps to regulate body temperature, sensation, insulation, storage, and synthesis of Vitamin D by action of ultraviolet radiations. Skin keeps body water resistant and helps to absorb oxygen and helps to repair the damaged tissues and cells. Some of the common Skin Problems and treatments [Table No:1] are:

Acne; a skin condition that occurs when hair follicle become plugged with oil and dead skin cell. It causes whiteheads, blackheads, or pimples.

Eczema; It is a skin condition where patches of skin become inflamed, itchy, cracked, and rough. Some types can also cause blisters. It is an inflammation-based diseases are atopic dermatitis which is a chronic disease. The most common side effects of are skin dryness, itching, redness, scaly patches, and thickened plaques with excoriation.

Hives; Urticaria is also known as hives is a swollen, pale, red bumps or plaques on the skin that appear suddenly because of the body’s reaction to certain allergens.

Rosacea; Rosacea is a common chronic skin disorder observed when face, neck, chest, or ears with flushing, persistent erythema, telangiectasias, eruption of inflamed papules, pustules, and hypertrophy with fibrosis of the sebaceous glands of the nose.

Psoriasis; Psoriasis is a common chronic immune mediated skin condition which includes redness, swelling, scaling, flasking, pruritic, skin tightness, pain and bleeding which can all have a significantly negative impact on patients physical and mental well-being.

Sunburn, Skin Eruptions and hyperpigmentation; Due to prolonged exposure of skin under sunlight leads to skin burns and suntan resulting the hyperpigmented skin which will further lead to Skin Cancer.

Insect bite and Bee Stings; a rash will appear on the surfaces of skin due to insect bite and bee stings which will lead to formation of wounds and scars and can be treated with the help of various dermal and transdermal formulations.

Mosquito Bite; a red rashes and skin eruptions appears on the surface of the skin which will lead to itching and swelling of skin which can be treated with the help of mosquito repellent creams and sprays.

Figure 1: Diagrammatic representation of Skin structure and its Function (www.google.com)

Herbal Drugs for skin Disorders [1,10]:

Table 1: List of medicinal plants with its parts, Extraction Technique, and Therapeutic Uses

Herbal Drug Processing and Extraction Methods [7]:

The herbal extracts are highly accepted in its active form when it is extracted in the form of its phytoconstituents form. In recent years many conventional and modern methods are used to process the extraction of the herbal and medicinal plants to extract the active constituents from them. India is a rich source of Cultivation, Processing, and extraction of various useful and key phytoconstituents from plants in a very cheap and affordable technological way. A recent WHO report stated that 5.6 billion people or 80% of our world’s population use herbal products for their primary health care. Herbal medicines are used to treat various skin diseases and processed in the form of dermal and transdermal formulations. Before processing starts, the harvested herbal plants must be pre-processed. At this stage, the plants must be dried to remove moisture for preservation, preventing bacteria activity and restricting fungal growth. To increase the surface area of the dried herb plant materials for improved contact with the solvent during the subsequent extraction process, the dried plant must first be ground. An increased surface area of the dried plant materials will improve the performance of the extraction process. The particle size of the milled plants is one of the factors that can affect the extraction yield. In the extraction processes, the operating conditions can greatly affect the efficiency of a particular technique. For solid–liquid extractions, important parameters include an appropriate solvent system, the solvent-to-herbs ratio, the particle sizes of ground and dried herbs materials, the temperature, the duration, and the agitation rate. The scale up techniques to extract various herbal constituent from plants are tabulated in the [Table no:2]

Table 2: List of Herbal drug Extraction and Processing Methods [7,9]

Need of Novel Drug delivery Nanocarriers for “Herbal Remedies” [10,11]:

The excellent skin barrier of the stratum corneum make most of the drugs to cross the skin barriers challenging. In the realm of dermatology, the skin assumes a dual role as both an obstructive barrier and a conduit for the bequest of pharmaceutical agents. Conventional topical formulations often grapple with the intricate stratagems of transversing the multiple strata of the cutaneous mantle to attain their intended destinations. Nanotechnology[Table:3]. emerges as the intrepid answer to these challenges as it engineers nanoparticles vested with the capability to encapsulate, shield, and precisely administer therapeutic compounds to designated skin strata. The main purpose involves elevating the bioavailability and therapeutic effectiveness of these agents. As we know plant-based extracts have a varied physical and chemical properties which makes them very sensitive, hygroscopic, thermodegradable, and Fastly degrades in acidic medium. To minimize the excessive loss during the extraction, manufacturing and to make highy soluble in the blood stream and to enhance its permeability through skin at the site of action, dermal novel drug delivery systems emerged as a more reliable and profitable form of dosage form. “Nanoparticles” are nanosized colloidal constructions made from regular engineered or modified polymers with a size range between 1 to 1000 nm. The encapsulation, entrapment, or embedment of herbal phytoconstituents into the nanoparticles is defined as herbal nanoparticles. “Dermal drug delivery “of herbal extracts are used to prevent and cure skin disorders which are tabulated in [Figure:2]. The macromolecules and nanocarrier’s are the most promising drug delivery [8,10] means:

1. To increase the efficacy and safety of phytomedicines and herbal biomarkers.
2. Study infusion and stable blood levels
3. Self-administration
4. Improved patient compliance
5. Microscale needles are more safe and painless alteration to hypodermic needle injections especially for protein biotherapeutics and vaccines.
6. The convenience of embedding the formulations into different medical devices for application on the surface of the skin

Figure 2 : Flow Chart representing challenges arises through dermal and transdermal Routes

Table:3 Types of Herbal Nano pharmaceuticals used as skincare Products [5]

Transdermal drug delivery systems in herbal Medicine [9,4]:

Transdermal drug delivery systems (TDDS), also known as “patches,” are dosage forms designed to deliver a therapeutically effective amount of drug across a patient’s skin. TDD is a painless method of delivering drugs systemically by applying a drug formulation onto intact and healthy skin. The drug initially penetrates through the stratum corneum and then passes through the deeper epidermis and dermis without drug accumulation in the dermal layer. When drug reaches the dermal layer, it becomes available for systemic absorption via the dermal microcirculation. The below components are used to manufacture the TDDS Patches:

1. Polymer matrix: This mainly helps to release the drug from transdermal patches depend on or controlled by the polymer. As we increase the concentration of polymer then it forms a very dense matrix which results in slow-release rate of drug. Polymer forms the backbone of transdermal drug delivery system. The drug diffusion across the polymer matrix and release rate of drug depends on the concentration and various physiochemical properties of drug as well as polymer. Examples: Gelatine, Hydroxy propyl methyl cellulose, PVA (Polyvinyl Alcohol), PVC (Polyvinyl chloride), Starch, Polyvinyl Polyethylene etc.
2. Active ingredient: Drug reservoir is most important component of transdermal patches. The active ingredients are selected as herbal phytoconstituents and synthetic extracts suitable for formulating according to the disease conditions. 
3. Penetration enhancers: These are the substances which enhance the skin permeability by enhancing properties of skin to drug. Polar pathway can be altered by causing protein conformational changes. Non-polar pathway can be altered by altering the rigidity of the lipids.
4. Surfactants: These are added when drug used shows hydrophilic character. They enhance polar pathway transport of the drug. Example; Pluronic F127, SLS (sodium lauryl sulphate) Ethanol, Methanol, Glycerol, Propylene glycol.
5. Plasticizers: They are used to reduce or minimize the brittleness of polymer film. They provide or give Flexibility and elasticity to the polymeric film.
6. Drug reservoir component: This is a component that contains one polymer or the combination of polymers in various concentrations and ratios.
7. Backing laminates: This helps to give and provide support. They should prevent the release of drug surface which is not in contact with skin. It should be compatible with drug and excipients. While selection the flexibility, strength, elasticity should be considered. This imparts appearance, flexibility, occlusions to transdermal drug delivery system. While selecting backing laminates the excipients compatibility should be considered. The most suitable backing laminates is the one with high flexibility. Examples; Metallic Plastic Laminates, Polyurethane, Aluminum foil.
8. Adhesive layer: This layer adheres the transdermal device on surface of skin at proper site and position.
1. Release liners: These are the protective layer which are removed before the application of transdermal patches on skin. They are helpful to prevent drug loss during storage and transportation condition. examples- Teflon, Silicon, Polyester etc.

Evaluation Parameter’s for Transdermal drug delivery systems:

1. Interaction studies; are commonly carried out in Thermal analysis, FT-IR, Ultraviolet radiation analysis (UV), and chromatographic techniques by comparing their physicochemical characters such as assay, melting endotherms, characteristic wave numbers, absorption maxima.
2. Percentage Moisture content; The weighed films are to be kept in a desiccator at room temperature for 24 hrs containing saturated solution of potassium chloride. After 24 hrs the samples are reweighed.

Percentage moisture uptake = [Final weight- Initial weight/ initial weight] ×100

3. Drug content; A specified area of patch is to be dissolved in a suitable solvent in specific volume. Then the solution is to be filtered through a filter medium and analyse the drug contain with the suitable method (UV or HPLC technique).
4. Uniformity of dosage unit Test; An accurately weighed portion of the patch is to be cut into small pieces and transferred to a specific volume volumetric flask, dissolved in a suitable solvent, and sonicate for complete extraction of drug from the patch and made up to the mark with same. The resulting solution was allowed to settle for about an hour, and the supernatant was suitably diluted to give the desired concentration with suitable solvent. The solution was filtered using 0.2?m membrane filter and analysed by suitable analytical technique (UV or High-Performance Liquid Chromatography techniques) and the drug content per piece will be calculated.
5. Polariscope examination; This test is to be performed to examine the drug crystals from patch by polariscope. A specific surface area of the piece is to be kept on the object slide and observe for the drugs crystals to distinguish whether the drug is present as crystalline form or amorphous form in the patch.
6. Peel Adhesion test; In this test, the force required to remove an adhesive coating form a test substrate is referred to as peel adhesion. Molecular weight of adhesive polymer, the type and number of additives are the variables that determined the peel adhesion properties. A single tape is applied to a stainless-steel plate or a backing membrane of choice and then tape is pulled from the substrate at a 180º angle, and the force required for tape removed is measured.
7. Rolling ball tack test; This test measures the softness of a polymer that relates to talk. In this test, stainless steel ball of 7/16 inches in diameter is released on an inclined track so that it rolls down and meets horizontal, upward facing adhesive. The distance the ball travels along the adhesive provides the measurement of tack, which is expressed in inch.
8. Probe Tack test; In this test, the tip of a clean probe with a defined surface roughness is brought into contact with adhesive, and when a bond is formed between probe and adhesive. The subsequent removal of the probe mechanically breaks it. The force required to pull the probe away from the adhesive at fixed rate is recorded as tack and it is expressed in grams.
1. In vitro drug release studies; The paddle over disc method (US Pharmacopeia apparatus V) can be employed for assessment of the release of the drug from the prepared patches.
10. In vitro skin permeation studies.
11. Skin Irritation study
50. Stability Studies; Stability studies are to be conducted according to the International Conference on Harmonization (ICH guidelines).

New Approaches for Herbal transdermal drug delivery systems:

Transdermal delivery has the advantage allowing sustained release of the drug. However, the drug delivery is limited owing to the barrier created by the stratum corneum. Microneedles are a transdermal drug delivery system that is painless, less invasive, and easy to self-administer, with a high drug bioavailability. The methods employed for modifying the barrier properties of the stratum corneum to enhance the drug penetration (and absorption) through the skin can be categorized as chemical and physical methods of enhancement.

1. Chemical Enhancers; Chemical permeation enhancers can work by one or more of the following three principal mechanisms:

• • Relaxation of the extremely ordered lipid structure of the stratum corneum.
  • Interacting with aqueous domain of bilayer of lipid.
  • Enhanced partition of the drug, by addition of co-enhancer or solvent into the stratum corneum.
  • Promoting penetration and establishing drugs reservoir in the stratum corneum. Water in structure, at the lipid interface. Some of the most widely studied permeation enhancers are di-methyl sulfoxide (DMSO), di-methyl acetamide (DMA), and diethyltoluamide (DEET), propylene glycol (PG).

2. Physical Enhancers [Table:4]

• Electroporation [Figure:3]; The use of electro-permeabilization as a method of enhancing diffusion across biological barriers. Electroporation involves the application of high voltage pulses to induce skin perturbation. High voltages (≥100 V) and short treatment durations (milliseconds) are most frequently employed. The technology has been successfully used to enhance the skin permeability of molecules with differing lipophilicity and size (i.e., small molecules, proteins, peptides, and oligonucleotides).
• Iontophoresis; This method involves enhancing the permeation of a topically applied therapeutic agent by the application of a low-level electric current, either directly to the skin or indirectly via the dosage form. Increase in drug permeation because of this methodology can be attributed to either one or a combination of electro-repulsion (for charged solutes), electro-osmosis (for uncharged solutes), and electro-perturbation (for both charged and uncharged) mechanisms.
• Sonophoresis; The term sonophoresis is used to describe the effects of ultrasound on the movement of drugs through intact living skin and into the soft tissue. The mechanism of sonophoresis involves

• Cavitation: Generation and oscillation of gas bubbles
• Thermal Effects: Temperature Increase
• Mechanical Effects: Occurrence of stress due to pressure variation induced by ultrasound.

• Magnetophorosis; This method involves the application of a magnetic field that acts as an external driving force to enhance the diffusion of a diamagnetic solute across the skin. Skin exposure to a magnetic field might also induce structural alterations that could contribute to an increase in permeability.
• Thermophoresis; The skin surface temperature is usually maintained at 32°C in humans by a range of homeostatic controls.
• Microneedle-based devices; The first microneedles were discovered by “Robert Chambers” in year 1921.The drug delivery device for the percutaneous administration of drugs is based on this method. These micro-needles of length 50 to 110 mm will penetrate the stratum corneum and epidermis to deliver the drug from the reservoir. The Microneedles are classified into solid, hollow, coated, polymeric, and dissolvable types. The Several polymers are used to fabricate Microneedles polylactic acid, Polylactic co glycolic acid, Polycyanoacrylate etc.
• Application of Electrical induced Techniques Transdermal Patches in Pharmaceutical and Cosmetic Industries [16,17]: In dermatology the principles of electrically enhanced drug delivery in transdermal drug delivery systems are now a days are used in treating various skin diseases like wound healing, treatment of burns, keloid, hypertrophic scars, psoriasis, acne, and alopecia. The cosmetic implication of skin conditions can be a source of great importance to affect the skin texture, sturdiness, regenerative properties of skin and hyperpigmentation. Several active agents are incorporated such as ionic agents, lactic acid, glycolic acid, and salicylic acid which can be used to prevent skin ageing, retinoic acid used for acne and scars, vasoactive agents are used in the treatment of cellulite such as caffeic acid, lipoic acid, and nicotinic acid. Vitamins such as ascorbic acid and its salt derivatives which can be used to treat hyperpigmentation of skin. Amino acids and peptides provided that the pH is in the range to maintain them in an ionic state. Nonionic water-soluble agents such as noisomes containing caffeine, Sorbitan monoesters, Sorbitan trioleate, Polysorbates and cholesterol used to treat cellulite and polymer-based microneedles [Table:5]. At a high frequency electrical application leads to heat emission from ultrasound which can also be damaging to the surface of the skin, and cavitation can potentially lead to tissue damage. Nanoparticle toxicity is another potential risk. Transdermal drug delivery using electrically assisted microneedle systems has emerged as a promising alternative to traditional drug administration routes. Integrating iontophoresis, electrophoresis and sonophoresis with that of the microneedle technology has shown a significant improvement in treating various skin conditions [14,15]. Optimal and low frequency Sonophoresis will be painless and have minimal to no risk.

Figure 3: Diagrammatic Representation of various electrically assisted drug delivery through skin

Table 4: Electrically assisted synthetic microneedle through iontophoresis and Sonophoresis:

Table: 5 Herbal Transdermal drug delivery through polymer loaded microneedles [21,22]

Quantum dots incorporated Microneedle Transdermal drug delivery system [16,27]:

Quantum dots (QDs) are luminescent nanocrystals with rich surface chemistry and unique optical properties that make them useful as probes or carriers for traceable targeted delivery and therapy applications. Quantum dots can be functionalized to target specific cells or tissues by conjugating them with targeting ligands. Owing to their small size, excellent brightness, and long photostability, quantum dots have enormous medicinal potential. They have shown potential in drug delivery, bioimaging, and sensing to improve medication therapy, disease diagnostics, and monitoring. Quantum dots can be functionalized with biocompatible coatings and coupled with pharmaceuticals for targeted distribution, decreasing off-target effects and regulating drug release at the site of action. They can also improve the absorption of cellular drugs. Quantum dots great resolution and low background noise make them ideal contrast agents for bioimaging. Biodegradable microneedles are available in both solid and hollow forms. One technical challenge with biodegradable microneedles involves making devices with sufficient fracture resistance to pierce the stratum corneum without fracture. Polymeric microneedles are usually fabricated using the micro molding technique; water evaporation, UV light curing, and heat curing are often used to create the solid microneedle device from the liquid precursor. Carboxymethyl cellulose, Gantrez AN material, fibroin dextran, chitosan, poly methyl vinyl, polylactic-co-glycolic acid, and poly glycolic-acid are commonly solidified by drying or water evaporation. Polyethylene glycol diacrylate and polylactic acid resins containing a photo initiator are cured using UV light. Polyvinylpyrrolidone is mixed with thermoinitiator that is solidified by heating at 60 o C for 5 hours. Herbal medicine carbon dots derived from herbal medicine have become the most recent incomer in the family of carbon dots. carbon dots are discovered by Scrivens in early 2004, carbon dots (CDs) are emerging as a novel nanomaterial which is smaller than 10 nm in size. carbon dots not only possess the merits from traditional semiconductors (inorganic quantum dots, etc.) and small molecules (fluorophores, etc.) but also exhibit unique properties such as photobleaching resistance, photostability, good biocompatibility and stable physicochemical characteristics. Various herbal phytoconstituents are derived from the different plant parts and they are macerated or carbonized with hydrothermal technique and fabricated into Herbal carbon quantum dots which are represented in [Figure:4]

Figure 4: Schematic representation of Quantum dots microneedle through the Iontophoresis technique through the skin