Keywords

3.1 Introduction

Over the years, drug delivery systems are the epitome of the medicines arena. A drug delivery system is defined as a formulation that, after administration, enables the therapeutic agent to reach its site of action and shows therapeutic response without affecting the non-target tissues, cells, and organs. The area of drug delivery systems is broad, which also covers the targeted systems and controlled release systems of drug delivery [1]. Since ancient times, the drug is delivered by the most common traditional route of administration, i.e., oral and parenteral. For more than two decades, the research was preceding to overcome the problems (such as poor absorption, difficulty in dose intake, first pass metabolism) related to the traditional route of drug administration, which leads to the researchers to use the skin as a carrier for the drug delivery. The researchers found that the delivery of drugs via skin can successfully reduce the side effects related to other routes of administrations [2]. The delivery of therapeutic agents via skin was very challenging for the researchers. Since the late 1970s, intensive research has been carried out to understand the mechanism of drug penetration through the skin to enable the design of delivery systems for topical absorption [3]. Skin delivery can be of different types, such as transdermal delivery, dermal delivery, and topical delivery. In recent times, the demand for topical administration of drugs is increasing because of the high patient compliance and topical or systemic delivery of drugs whenever possible [4]. The topical delivery system is defined as the method of application of the formulation to the uppermost layer of the skin for the treatment of local skin diseases. In topical drug delivery, the skin penetration of drugs into the deeper tissues or systemic circulation is unnecessary for the therapeutic response of the drug [3]. There are several conventional topical formulations available in the market, as ascribed in Table 3.1. The topical administration of the active therapeutic agent mostly depends upon its physicochemical properties, its ability to permeate the skin, the structure of carrier, and skin conditions [5]. The formulation's role is also very relevant in the topical delivery as the therapeutic efficacy of the drug can be altered by the interaction between the vehicle and the skin. The formulation assists the drug substance to reach the target site and also helps in the management of drug transport, active duration, and dosage integrity. To prepare and design an optimized formulation with the above characteristics, understanding the skin’s structural features is also necessary along with the vehicle, which has a crucial role in the drug delivery mechanism. The vehicle super-saturation can improve the thermodynamic activity of the formulation for drug penetration through the skin. Therefore, topical delivery came out as a ray of hope in the treatment of various skin conditions like fungal infections, inflammation, bacterial infections, and skin cancers [6].

Table 3.1 Marketed conventional topical formulations
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However, the conventional topical formulations were unable to provide the desired therapeutic efficacy and showed high degradation of the drug at the skin surface, making scientists research on nanocarriers based topical drug delivery. The nano-drug delivery systems have various applications over conventional drug delivery, such as allowing the drug to cross the skin membrane easily at a shallow dose and enhancing therapeutic efficacy. The nanocarriers encapsulate the drug, which can also retrain its degradation on the skin. Table 3.2 shows various types of nanocarriers designed by the researchers for the topical delivery of active molecules and their specific treatment [7]. Nanodelivery systems are suitable strategies for enhancing the rate and extent of drug penetration and improving drug absorption [8]. These all properties of nanocarriers such as potency, targeting ability, and permeability are depended upon its characterization studies, i.e., size, polydispersity index, percentage of components, and surface charge. Therefore, newer analytical and characterization techniques are necessary to determine these properties related to nanoformulations and control them during production and synthesis. The new drug delivery systems which can be utilized for the skin penetration of therapeutic agent were largely unexplored even after the numerous scientific research in this field. However, to overcome these hurdles, the scientific and systematic approaches were employed by the researchers to optimize the process design and nanoformulations [9, 10]. In topical delivery, the incomplete understanding of process production is the main reason for the researchers to implement Quality by Design (QbD) approach to optimize nano-drug delivery systems, which can lead to the development of optimized nano-based formulations. The main advantage of the QbD concept is the development of high-quality drug products without extensive regulatory oversight [11]. QbD has implemented one of its components called the design of experiment (DoE) to develop an optimized pharmaceutical product. Besides product development, QbD has applications in the process design, robust manufacturing process, quality attributes, regulatory flexibility, and also improves the regulatory communication between industry and regulators. In the topical drug delivery, DoE provides real-time quality control with real-time release and decreases post-approval changes in the formulation. The various topical delivery formulations developed with the implementation of DoE have been discussed further in this chapter [12].

Table 3.2 Topical drugs encapsulated in nanocarriers
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3.2 Advantages of Topical Drug Delivery

The approach related to drug delivery’s topical route provides various advantages over other drug administration routes. Figure 3.1 represents the advantages of topical delivery. These include no systemic absorption of drugs, high concentration of drug at the target site, avoidance of the first pass metabolism sustained delivery of the drug, ease of administration, enhancement in bioavailability, a continuation of dosage regimen with short biological half-lives, non-invasive application of formulation, and patient compliance. Topical drug delivery also decreased the peak-associated side effects and ensured a steady-state profile, which helps in the increment of drug level above minimum therapeutic concentration [13]. The topical administration of drugs provides relief in a broad range of therapeutic indications. According to the researchers, various studies showed that the topical delivery of drugs exhibited sustained pharmacological action due to the high rate and extent of drug delivery in a reasonable time to achieve local therapeutic concentration [14]. This can enhance the absorption of drugs compared to other routes of administration [15]. The delivery of nanocarriers via topical administration also possesses several advantages: sustained release of the drug, skin targeting, increased uptake, a base to the bioactive compounds, avoiding direct contact of harmful drugs with the skin, and reduces adverse effects related to the drug. The most crucial application of topical delivery is to treat contagious skin diseases such as atopic dermatitis, skin cancer, and psoriasis. The encapsulation of bioactive agents into nanocarriers also reduces its degradation when coming in contact with the skin at the time of topical delivery [16].

Fig. 3.1
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Advantages of topical delivery

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3.3 Challenges in Topical Delivery

The skin is the largest organ of the body and consists of three main layers called the outermost epidermis layer (50–150 μm thick), inner dermis layer (about 250 μm thick), subcutaneous fat tissues. The nutrients maintain their vitality by crossing through dermal–epidermal junction because of the lack of blood vessels in the epidermis. The epidermis is further divided into five different layers named as stratum corneum (SC), stratum lucidum, stratum granulosum (granular layer), stratum spinosum (spinous layer), and stratum germinativum (basal layer). The epidermis with the absence of SC is called the viable epidermis, which is responsible for the barrier nature of skin for topical drug delivery, and most of the cutaneous disorders are also found in it [17]. The transportation of substances from the skin occurs mainly via three important routes by the passive diffusion process. The transportation routes are transcellular, intercellular, and trans-appendageal route [18]. As suggested by various researchers, the most effective penetration route for topical drug delivery is the trans-appendageal route, and some drugs also used the intercellular pathway between the corneocytes to cross the skin membrane [19].

The penetration of substances from hair follicles is also possible. It was demonstrated that skin penetration through the hair follicle pathway could enhance the penetration and absorption of bioactive agents for drug delivery. Hair follicles also work as a center for the depot formation, which can multiply the SC storage capabilities [20]. Figure 3.2 represents all the major routes of topical delivery of drugs. Instead of these routes of drug transportation via the skin, the protein transporters present in the skin also play a vital role in the transportation of substances. Therefore, the localization and expression of various tight junction proteins present at the epithelial layers and hair follicles have been altered during the disease stage of the skin, such as psoriasis. The main barrier in topical delivery is the SC, which causes the challenge that the drug has to cross the SC and reach viable epidermis or systemic circulation to show the therapeutic efficacy.

Fig. 3.2
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Pictorial representation of topical drug delivery

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Then another challenge is the balance between the drug penetration through SC and its buildup in the skin to ensure the therapeutic concentration of the drug [2]. Though molecules are unable to fully and readily pass through the SC membrane, they can only cross the membrane. The rate of percutaneous absorption of molecules is determined by the state of SC hydration [7]. Andrea and co-workers suggested that the skin permeability was increased by the removal of SC, whereas after removing the full epidermis, the magnitude of skin permeation enhanced by 1–2 orders of magnitude. The results manifested that skin permeability can be altered with different magnitudes by the influence of different skin layers [21]. The researchers also investigated that the permeation of active molecules through the SC is more in the diseased skin (e.g., psoriasis, dermatitis, and fungal infection) compared to the intact skin. The challenges in the penetration of the drugs across the skin are not only derived of the skin structure, but also depend on the nature of the topical delivery vehicle. One such factor in the resistance of permeability is the exogenous substances [22]. The characteristics of drugs/substances play an essential role in topical delivery. Although most of the substances have different physicochemical properties such as poor aqueous solubility, high permeability, and different molecular structures, which make them rigid to cross skin membrane, to penetrate the skin layer effectively, the drug should possess log partition coefficient value (log P) between 1 and 4 and a molecular weight less than 500 Da [23]. Despite all the above challenges, the permeation of drugs across the skin can be enhanced by various chemical and physical techniques such as radiofrequency, iontophoresis, electroporation, microneedles, magnetophoresis, and ultrasound. These techniques are very advantageous in topical delivery, but their utilization is limited due to their toxicity and lack of feasibility. However, the delivery of peptides, proteins, nucleotide fragments, vaccines, or cancer therapeutics does not still fit in the criteria of the attractive prospects of topical delivery via different techniques and strategies [24]. These challenges or drawbacks are the significant factors for the implementation of nanodelivery in the topical application, which could also expand the range of drugs available for the topical administration [25].

3.4 Role of Nanocarriers in Topical Delivery

As discussed above, conventional topical formulations have numerous side effects and limitations related to efficacy, safety, and patient compliance. The skin is a comprehensive and easy route for drug application, but it is not evident that drug is reaching its site of action. To overcome these side effects, the researchers’ designed novel drug delivery systems without reducing the efficacy of therapeutic agents. The therapeutic strategies for the management of skin disease have also been available due to the new delivery of a drug [26, 27]. The nanocarriers provide satisfactory pharmacokinetic data such as low absorption of the drug as compared to conventional formulations. In general, the use of nanocarriers offers several advantages over other drug delivery systems. These are used to enhance the stability of drugs by chemical or physical means, increase the solubility of hydrophobic drugs, deliver higher concentrations of drugs to the target site due to and enhanced permeation and retention (EPR) effect, and offers sustained and controlled release of the drug. The nanocarriers are modified with cell-specific ligands and anticipate the targeted treatment of drugs. Nanocarriers enhance the permeation of therapeutic agents through the SC by accumulating in the hair follicles and provide drug release into the deeper skin layers. There are various nanoparticles for topical delivery, such as lipid-based nanocarriers, polymeric nanoparticles, emulsified nanocarriers, dendrimers, and organic nanoparticles [28]. When the nanocarriers are formulated with the implementation of DoE, the above applications become more dominant, which have been discussed further in this chapter.

3.5 Applications of DoE in the Development of Topical Drug Delivery Systems

3.5.1 Lipid-Based Systems

3.5.1.1 Liposomes

Liposomes are the lipid-based vesicular drug delivery systems made up of one or more lipid bilayers. They are biocompatible and biodegradable and prepared by natural phospholipids with mixed lipid chains. They can entrap hydrophilic, lipophilic, and amphiphilic drugs. The drug can be either intercalated into a lipid bilayer or encapsulated in the aqueous phase. The physicochemical properties of the drug and lipid composition decide the location of drugs in liposomes [29]. The topical administration of drugs using liposomal formulation is a promising area for the concern of skin diseases. At the International Pharmaceutical Federation Congress (1979), the first report enlightened the evidence of altered drug deposition during the topical delivery of drugs via liposomal formulation was presented [30]. This report provided the base for further research articles indicating that entrapment of drugs into liposomes enhanced the skin deposition of drugs and reduced its absorption into the blood. The liposomes might reduce the percutaneous absorption of the drug by increased local action. These studies claim that liposomes can be useful for drug delivery by topical route [31]. Therefore, it is necessary to prepare the liposomes with optimized liposomal composition, high-throughput production, and better in vivo kinetics. QbD implementation in liposomal design and production can be suitable for optimizing elements, identifying suitable composition, and high screening methodologies. The various type of designs is utilized for the same [32]. All lipid-based nanocarriers with their DoE are enlisted in Table 3.3. The first optimization studies for liposomes were reported in 1988 by the well-known researcher Gregoriadis [58].

Table 3.3 List of experimental designs used in lipid-based nanocarriers
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Singh et al. in 2005 reported that the optimization of formulation with traditional approaches is unpredictable, uneconomical, invariable, and time-consuming. Therefore, the use of systemic optimization techniques with response surface methodology (RSM) can overcome these side effects and help choose the best formulation. The researchers employed full factorial design with two independent variables at three levels, and the response variables selected were percent drug diffused, percent drug entrapment (EE%), and percent drug leakage for the optimization of the liposomal formulation. The other process variables were kept constant throughout the study. The Design-Expert software was selected, and a response surface plot was formed to fit the second-order polynomial equation. The results concluded that the full factorial design was successful in optimizing liposomal formulation, and liposomes showed high potential carriers for the topical delivery of nimesulide. The linearity with negligible deviation was shown between the observed and predicted values. The prophecy of the best liposomal formulation can be achieved using optimization methods [33]. Mura et al. [34] implemented a D-optimal design for the development of benzocaine liposomes. The primary purpose of the design implementation was to identify the essential factors influencing the response. Afterward, Doehlert design was applied for the response surface plot of the preliminary phase factors. Then, ANOVA was used for the validation of the regression model. Then, the formulation factors and their best level for response optimization were identified by the graphical analysis of the effects. The results suggested that benzocaine’s permeability to the skin layer was increased because of the formulation optimization. The in vivo studies of optimized formulation indicated the enhancement of the benzocaine therapeutic efficacy compared to the previous best benzocaine liposomal formulation. This confirmed the relationship between the intensity of anesthetic effect and the in vitro drug permeation rate of benzocaine and demonstrating the actual effectiveness of the proposed approach for the liposomal formulation optimization [34].

Li et al. [59] prepared daptomycin loaded flexible nanoliposomes for topical skin therapy. They applied the Design-Expert version 8.1 to design the experimental groups and analyze the results. The results depicted that the optimized nanoliposomes exhibited narrow particle size distribution, high stability, and increase drug loading capacity. The in vitro skin permeability data showed a high diffusion capacity of the daptomycin into the skin compared with the naive drug. Further, the optimized nanoliposomes indicated that the drug could rapidly achieve high-level concentration in the skin and underlying tissues. Hence, they may serve the potential clinical applications of daptomycin using a topical delivery approach to current intravenous delivery [59]. Hussain et al., in 2015, designed 5-fluorouracil loaded elastic liposomes for the management of skin cancer. The elastic liposomes were optimized by employing full factorial experimental design using Design-Expert software. A total of nine liposomal formulations were prepared using dependable variables against independent variables. The study results indicated that the optimized 5-fluorouracil loaded elastic liposomes had desired quality and pharmacokinetics compared to the plain drug. The liposomes manifested optimum permeation ability and overcame therapeutic limitations for the drug's topical delivery, which was absent in the conventional dosage form [35].

Bavarsad et al. in 2016 reported penetration-enhancer vesicles (PEV) for topical delivery of tretinoin using Transcutol, and a full factorial design was employed for the formulation optimization. The three-dimensional response surface plot stated that by increasing the phospholipid concentration up to 20%, the drug release was decreased in 15 min. The optimized PEV exhibited high incorporation efficiency and enhanced skin drug penetration compared to the conventional cream formulation due to the solubilizing properties of the Transcutol. The optimum PEV showed less hyperkeratosis without hyperplasia, high patient compliance, and caused mild adverse effects such as peeling, erythema, and burning in comparison to the conventional cream formulation [36]. Çelik et al. [37] developed coenzyme Q10 (CoQ10) and D-panthenyl triacetate (PTA)-loaded liposomes for topical delivery. For this purpose, a central composite design was used with three factors studied at five different levels. The experimental design aided in the preparation of CoQ10 and PTA liposomes successfully for the first time, and the results showed enhancement in EE% and drug loading capacity. Hence, the report concluded that the encapsulation of these two compounds could be a promising way for the effective delivery of both compounds simultaneously [37]. Chen et al. [38] prepared novel elastic liposomes for skin delivery of papain by implementing three-level three-variable Box–Behnken Design. The Design-Expert software was used for data prediction. The results showed an increment in EE%, high drug deposition in skin, and higher cumulative amounts and fluxes of optimized papain loaded liposomes compared to the papain solution. The topical delivery of liposomes could inhibit the hypertrophic scar in rabbit ears, including decreased collagen fibers, decreased microvessels, and signaling pathway regulation. The optimized elastic liposomes were found to be effective in topical scar treatment [38].

Naguib et al. [40] prepared acetazolamide loaded penetration-enhancing hybridized vesicles for the topical delivery of glaucoma. The formulation was optimized by applying a highly accurate and robust D-optimal mixture statistical design. The prepared and optimized liposomes are evident in high physicochemical and biological properties, large particle size, high drug release, increased EE%, excellent stability, and high drug bioavailability compared to conventional liposomes. The optimized acetazolamide loaded liposomes also showed lesser side effects as compared to the conventional acetazolamide loaded liposomes [40].

Joseph et al. [39] developed lornoxicam loaded liposomes for topical delivery. The central composite design was implemented for formulation optimization. The two variables (phospholipid and cholesterol content) and two responses (EE% and in vitro drug release) and the Design-Expert software were selected for the optimization. The response surface plot was formed to evaluate the independent variables’ response to the dependent variables (Fig. 3.3). The plot showed that the EE% was the same at the medium content of phospholipid and cholesterol, but as their concentration decreases, the EE% was also decreased and vice versa. The highest and lowest levels increased the drug release, while at other levels, the drug release was decreased. Thus, the study concluded that there was an enhancement in the half-life of the optimized formulation. The optimized liposomes had stability limitation for 3–4 weeks and large particle size compared to the naïve drug. The ex vivo permeation studies and in vitro skin toxicity studies showed that the low doses could be efficient for the topical delivery of the optimized formulation, which can enhance the efficacy of the drug with fewer side effects [39].

Fig. 3.3
figure 3figure 3

(a) Relationship between the predicted and actual values, (b) interaction between the variables and responses, (c) 3D surface plots for variables of (a) entrapment values (b) drug release. Figure source: Joseph et al. [39]

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Aziz et al., in 2018, developed diacerein loaded elastosomes for topical delivery to treat osteoarthritis. The full factorial design was implemented for the optimization of elastosomes, and the results showed high skin permeation and retention capacity of diacerein compared to the plain drug. Thus, it is concluded that the topical delivery of the drug can reduce its adverse effects on oral administration [41].

Dar et al., in 2018, developed sodium stibogluconate loaded nano-deformable liposomes for topical treatment of leishmaniasis. A Box–Behnken design was selected with three levels and three factors to optimize the formulation using RSM. The study’s result stated that the surface area of the optimized formulation decreased with an increase in the vesicle size of the liposomes. The optimized topical liposomes reduced the side effects, which were seen in the intralesional or intravenous delivery of the sodium stibogluconate [60]. Dawoud et al. [61] prepared insulin chitosan mucoadhesive liposomal gel for wound healing. The Box–Behnken design was implemented with three critical process parameters for liposomes optimization. The optimization of liposomes availed high stability and sustained release profile to the insulin for the wound healing. Chen et al. in 2020 prepared zedoary turmeric oil and tretinoin encapsulated liposomal gel for topical delivery of psoriasis using orthogonal design. The results suggested that the optimized formulation allowed high EE%, solubility, and stability. The optimized liposomal gel increased the tretinoin content in the skin and reduced administration frequency compared to the naïve drug [61].

3.5.1.2 Ethosomes

Touitou and his co-workers have discovered a new type of vesicular system named ethosomes. Ethosomes differ from the liposomes in their structure, mechanism of action, and mode of application [62]. Ethosomes are defined as the non-invasive vesicular drug delivery systems and carry drugs deep into the skin layers or systemic circulation. These are malleable, soft lipid vesicles containing phospholipids, alcohol (in high concentration), and water. Ethosomes may vary in the size range from 10 nm to few microns. Ethosomes are well-established nanocarriers for local delivery of drugs and can penetrate the skin layers with high transdermal flux. The high concentration of ethanol (20–50%) in the ethosomes is the reason for its high permeation ability because the ethanol can disturb the organization of skin lipid bilayer. The ethosomes also possess the same stability as conventional vesicles [63]. Akhtar et al., in 2012, designed Cavamax W7 composite ethosomal gel for topical delivery of clotrimazole using factorial design. The design was validated by the Design-Expert software version 8.0.5. The response surface plot showed that EE% was increased with an increase in the concentration of soya lecithin and Cavamax W7. The optimized ethosomes were found to be stable, offered better efficiency, and higher steady-state flux than the reference ethosomes. The in vivo study of optimized ethosomes indicated the high permeability of the vesicles into the skin layers. Thus, the Cavamax W7 ethosomes were discovered as the superior nanocarrier systems for the clotrimazole topical delivery [42].

Pathak and Kumari [43] prepared Cavamax W7 composite ethosomal gel of psoralen by implementing the factorial design, and the design was validated by the Design-Expert software version 8.0.5. The response surface plot found that EE% was increased at the highest level of soy lecithin and Cavamax W7. The results concluded that the permeation of optimized formulation was more than that of the reference ethosomes, even without using a permeation enhancer. The optimized psoralen ethosomes had the potential to minimize the adverse effects associated with photosensitivity of psoralen and were the efficient carrier for the topical delivery of the drug [43]. Ahmed et al. developed tramadol loaded nanoethosomal transgel for the topical delivery. The three-factor three-level Box–Behnken design was used for formulation optimization. It was observed that the vesicle size had a direct positive relationship with the phospholipid concentration, and the permeability of the system was increased with a decrease in particle size. Therefore, the optimized ethosomal gel possessed high analgesic efficacy, enhanced skin permeability and bioavailability, improved patient compliance, and avoided frequent dosing compared to the oral marketed formulation [44].

Garg et al. [64] prepared methoxsalen loaded ethosomes-based hydrogel for the treatment of vitiligo. The two-factor, three-level face-centered design was implemented for the optimization of ethosomes. The response surface plot showed that at a higher level of ethanol and lower phospholipid levels, the vesicles' size decreased. Hence, the QbD approach in the optimization of formulation provided detailed knowledge about the ethosomal gel quality and the formulation attributes. Overall, the optimized formulation imparted better therapeutic efficiency and higher flux in comparison to the commercial product and hydroethanolic formulation [64]. Akhtar et al. [45] designed nanoethosomal glycolic vesicles of triamcinolone acetonide to manage atopic dermatitis using a three-level Box–Behnken experimental design. The design was applied to investigate the sensitivity of the responses against the variations in the experimental variables. The research group concluded that the optimized formulation not only provided an enhancement in the drug entrapment, but also offered better stability, higher drug release flux, and uniform, and more profound permeability compared to the reference ethosomes. The carbomer gel of the optimized ethosomal glycolic vesicles was found to be clear and transparent at a pH range of skin. Overall, the nanoethosomal vesicles were suitable for the topical delivery of triamcinolone acetonide [45].

3.5.1.3 Niosomes

Niosomes are the vesicular systems composed of non-ionic surfactants and are multilamellar. They do not contain phospholipids, making them different from the liposomes [65]. Niosomes can be used as the drug delivery vehicles to deliver the drugs to the lung, ocular system, skin, brain, liver, tumor organ, etc. They are very successful in the topical delivery of drugs as they increase the SC properties by enhancing their smoothness or decreasing transepidermal water loss [66]. They have various topical delivery applications such as drug stability, sustained drug release, high skin penetration, and the ability to encapsulate both hydrophilic and lipophilic drugs [67]. DoE implementation in the niosomes can avoid excessive effort, time, and materials for the formulation development and provide significance of different variables.

Solanki et al. formulated piroxicam loaded proniosomes for the topical delivery employing the three-factor three-level Box–Behnken statistical design. The response surface plots depicted that on decreasing the concentration of cholesterol, EE% of the niosomes was also decreased. The optimized niosomes were found to be less leaky and small particle size. The study concluded that the optimized proniosomes exhibited all the desired properties for topical delivery [46]. Abdelbery et al., in 2015, prepared methotrexate loaded niosomes for the treatment of psoriasis. A three-level three-factor Box–Behnken design was applied for the formulation optimization, and the independent variables selected were the methotrexate concentration in hydration medium, the total weight of niosomal components, and surfactant. The dependent variables selected were EE% and particle size. The EE% was reported to decrease along with an increase in hydration volume. The reason for increment in particle size was the excessive amount of drugs inside the niosomes. The in vivo study displayed that the niosomes were non-irritant after applied topically. Thus, it is concluded that the methotrexate loaded niosomes had the therapeutic efficacy to treat psoriasis, and preclinical applications on animal models were necessary to establish the safety of the niosomes [47].

El-say et al. developed niosomal gel for the topical delivery of diacerein using the Box–Behnken design model. The three independent variables selected were the percentage of stearylamine, hydrophilic–lipophilic balance, and concentration of surfactant and sonication time. The dependent variables selected were the EE%, particle size, and percentage of diacerein release. The stat graphics model with two ANOVA was used for the statistical analysis. It was found that the increase in sonication time can reduce the vesicle size, and a decrease in both percentages of stearylamine and surfactant might decrease the EE%. Also, the optimized formulation resulted in a suitable topical application and exhibited high pharmacodynamic and anti-inflammatory activity compared to the conventional gel of diacerein [48]. Jacob et al. developed acyclovir loaded niosomal gel for the topical application, and 32 full factorial design was implemented for the niosomes optimization. From the contour response plot, it was observed that there is no significant difference in the particle size between various formulations. The vesicle size and EE% were changed while changing the concentration of surfactant and cholesterol. The study’s result stated that the formulation manifested high bioavailability and sustained release of drugs compared to conventional dosage form [50].

Moghddam et al. prepared diacerein loaded niosomes for the treatment of psoriasis by applying 33 Box–Behnken design. The optimized niosomal formulation exhibited high EE%, reasonable particle size, and homogenous surface morphology. The developed formulation was found to be the better carrier for diacerein delivery [51]. Arafa et al. in 2017 employed Minitab software for the optimization of pregabalin loaded niosomes. The data exhibited that EE% of the formulation had a direct relationship with the cholesterol ratio. However, the in vitro release study showed that the drug delivery followed Higuchi and Fickian release model. The ex vivo permeation of niosomes was increased as compared to the hydrogels. Therefore, it is investigated that the hydrogels loaded niosomes are the only capable carrier for the controlled release of pregabalin [68].

Al-Mahallawi et al. [69] designed methotrexate loaded ultra-permeable niosomes by implementing a 33 Box–Behnken experimental design with Design-Expert software to evaluate the design. The independent variables selected were the Cremophor RH 40 percentage, stabilizer percentage, and sonication time, and the dependent variables evaluated were the EE%, vesicle size, and polydispersity index. The response surface plot depicted that the sonication time and stabilizer percentage had a negative impact on the EE%, and increasing stabilizer percentage might enhance the vesicle size of the niosomes. Thus, the results concluded that the optimized formulation showed better stability profile and in vivo data evaluated the high amount of drug deposition in the rat skin compared to the methotrexate solution [69].

3.5.1.4 Solid Lipid Nanocarriers (SLNs)

SLNs were first discovered in 1991; they are the lipid nanocarriers containing the properties of both lipid-based systems and the polymeric nanoparticle systems. SLNs encapsulate drugs at the lipid matrix composed of a solid lipid or mixture of solid lipids. They have various advantages such as drug protection against enzymes or degradation, controlled drug release, biocompatibility. SLNs are better nanocarriers for skin delivery as they enhanced the permeation of encapsulated drug [70].

Bikkad et al. developed halobetasol propionate loaded SLNs for the topical delivery using 32 full factorial design. Amount of Tween 80 and the content of glycerol monostearate were selected as the independent factors, and particle size and EE% were the dependent factors. The Design-Expert software was used for data analysis, and the response surface plot showed that the particle size was increased with decrease in the ratio of Tween 80: glyceryl monostearate and EE% increased with increase in the ratio of Tween 80 and glyceryl monostearate. The optimized formulation exhibited controlled drug release, less skin irritation, better stability, lower side effects, and high therapeutic efficacy compared with the conventional gel [53]. Raza et al. in 2013 developed isotretinoin loaded SLNs for the treatment of skin acne. They employed a face-centered cubic design for the optimization of response variables. From the response surface plot, it resulted that the phospholipid had a positive relationship with the particle size and zeta potential. The optimized SLNs were highly stable, with a low aggregation tendency of the dispersed phase. The SLNs could be the better option for the isotretinoin topical delivery [55].

Daneshmand et al. [56] prepared SLNs for the auraptene delivery as an anti-inflammatory agent. A central composite design with a five-factor three responses was used for the analysis of variables. The optimized formulation exhibited high EE% and low skin irritancy. The in vitro data provided controlled release of the drug as compared to the conventional cream. The optimized SLNs revealed high anti-inflammatory efficacy and maximum local effect of the drug at the skin with no sensitization [56].

3.5.2 Emulsion Based Systems

3.5.2.1 Microemulsion

Hoar and Schulman, in 1943, prepared a single-phase, uniform, non-conducting, transparent solution called microemulsions. Microemulsions are thermodynamically stable, transparent mixtures of oil, water, and surfactant. These have various advantages such as high stability, better bioavailability, deliver both hydrophilic and lipophilic drugs. Microemulsions are different from other nanocarriers in such a way that they are amphiphilic and have multi-drug delivery [71]. Table 3.4 shows the list of designs used in these systems. Microemulsion can enhance drug penetration from the skin, making it a suitable option for the topical delivery of drugs. In recent times, researchers take an interest in the in silico techniques such as DoE for the prediction of microemulsion phase behavior.

Table 3.4 List of experimental designs used in emulsified systems and polymeric systems
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Patel et al. designed clobetasol propionate loaded microemulsion gel for the treatment of vitiligo. The D-optimal design was implemented for the optimization of formulation, and the independent variables selected were the amount of mixture of surfactant and co-surfactant (Smix) and oil concentration. The dependent variables selected were the globule size and solubility of the drug. The response surface plot evaluated that the decrease in globule size is directly proportional to the amount of oil proportion. The results concluded that the optimized microemulsion exhibited better skin permeation, high skin retention, and less skin irritation than the marketed formulation. Therefore, the optimized topical microemulsion gel could be a useful option for the treatment of vitiligo [74]. Negi et al. in 2014 developed lidocaine and prilocaine loaded microemulsion hydrogel for topical delivery using D-optimal mixture design. The formulation variables selected were the Smix, water, and oil, and the response variables chosen were skin retention, globule size, percent cumulative permeation, and skin permeation flux. The response analysis was carried out, and the response surface plot (Fig. 3.4) evaluated that with an increase in the water level, the globule size was decreased, and percent skin retention of both the drugs increased. The thermodynamic activity of the drug decreased with an increase in the surfactant concentration and the drug’s affinity.

Fig. 3.4
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3D response surface plots and 2D contour plots of (a) globule size of microemulsion, (b) skin retention of lidocaine, (c) skin retention of prilocaine, (d) permeation flux of lidocaine, (e) permeation flux of prilocaine, (f) skin permeation of lidocaine, (g) skin permeation of prilocaine (adapted from [73])

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The response surface plot proved that the maximum water content and minimum Smix could enhance the globule size, % skin retention, and permeation flux and percutaneous absorption. The results concluded that the optimized microemulsion hydrogel showed high skin absorption and skin permeation compared to the commercial cream [73]. Radwan et al. prepared sertaconazole loaded microemulsion gel as an antifungal agent. The formulation was optimized, employing factorial design and Design-Expert software. The results indicated that the optimized microemulsions impart a high skin retention activity and better antimycotic activity as compared to the marketed formulation. Moreover, optimized microemulsion did not show any histopathological changes on rat skin as proved to be non-irritant as compared to marketed cream. Thus, the dermal delivery could be achieved by the sertaconazole loaded optimized microemulsion [75].

3.5.2.2 Nanoemulsions

Nanoemulsions are the emulsified carriers composed of oil, surfactant, and water. Their droplet size is smaller than the microemulsion. They are usually opaque to blue-white [71]. Nanoemulsions have various advantages compared to a conventional emulsion, such as increased interfacial area, better stability, high solubility, and improved bioavailability.

Yu et al. applied a D-optimal design for the optimization of metronidazole loaded nanoemulsions with anti-rosacea properties. The droplet size, % skin retention, and metronidazole content were selected as the independent variables, and the oil content, water, and Smix were used as the dependent variables. The response surface plot evaluated that at the moderate levels, optimized nanoemulsion showed less droplet size and better skin retention properties and high therapeutic efficacy [79]. Afifah et al., in 2018, prepared kojic monooleate loaded nanoemulsions for the treatment of hyperpigmentation in the skin. A five-factor one-response D-optimal design was used for the evaluation of nanoemulsions. The three-dimensional surface plot stated that the droplet size was decreased with increasing xanthan gum and surfactant content. The in vitro data of the optimized formulation suggested that the microemulsion was less toxic and safe for the cosmeceutical applications [78].

3.5.3 Polymer-Based Systems

3.5.3.1 Polymeric Nanoparticles (PNPs)

PNPs are biomaterials with immense potential, simple design, higher biocompatibility and biomimetic properties. In the drug delivery systems, the main characteristics of PNPs are controlled particle size and excellent solubility and elasticity. PNPs are of two types, i.e., nanocapsules and nanospheres. They can reach its target site at the desired time and level [85]. Topical delivery of PNPs can provide protection to drugs from degradation, reduce side effects of toxic drugs via controlled disease, and increase skin penetration [86].

Marto et al., in 2018, developed novel human neutrophil elastase loaded starch nanocapsule implementing the central composite design. The design was used to evaluate the response of independent variables on the dependent variables. The independent variables selected were the amount of drug, Tween 80 content, lipid content, and percentage of surfactant and drug loading, particle size, zeta potential, and EE% are the dependent variables. The MODDE software was used for the data evaluation, and it was found that the enhancement in the content of lipid and Tween 80 led to a decrease in the particle size and increase in the EE%. The result of the study concluded that the optimized nanocapsules exhibited high efficacy and skin permeation activity and showed a promising carrier for topical delivery [81]. Pizarro et al. prepared fluorometholone loaded nanoparticles for inflammatory disorders. The formulation was optimized using three factors (concentration of drug, surfactant, and polymer) and three levels (particle size, zeta potential, polydispersity index, and EE%) central composite design matrix. The response surface plot showed that the particle size increased with an increase in polymer concentration and decreased with an increase in the surfactant concentration. The zeta potential and polydispersity index exhibited that the formulation is monodispersed in nature with very high EE%. The skin permeation and anti-inflammatory studies showed that the optimized formulation had better skin permeability than commercial formulation. However, the optimized fluorometholone NPs were found to be successful for the topical delivery to reach the posterior segment of the eye [87].

3.5.3.2 Micelles

Micelles are polymeric nanocarriers composed of mainly surfactants and polymers. The micelles are formed only, when the surfactant concentration reaches to or above the critical micellar concentration and these nanostructures consist of a solvophobic core and solvophilic shell. The hydrophobic drugs can encapsulate in the core of polymeric micelles, and shell provides stability to the micelles. The micelles shell has the ability to prevent protein adsorption and opsonization. However, micelles have various advantages in topical drug delivery, such as high biocompatibility, better drug deposition, and high skin permeation [88]. The polymeric micelles have been reported to increase the drug's solubility at the skin site, sustained drug release.

Varshosaz et al. developed chitosan gel containing nanomicelles for the effective topical delivery of simvastatin for wound healing using irregular full factorial design. The independent variables chosen for the optimization were the hydration temperature, polymer/drug ratio, organic solvent and hydration time, and the responses were the EE%, particle size, zeta potential, drug release, and drug loading efficiency. From the response surface plot (Figs. 3.5 and 3.6), the drug release was found to be rapid at first and then followed by sustained release. The hydration temperature affected the particle size. The EE% was increased with an increase in the drug content. Thus, the study concluded that the optimized formulation showed high wound healing ability and could be a promising carrier for topical delivery [83]. Zhou et al. prepared vinpocetine loaded micelles with the implementation of orthogonal design to optimize micelles. They found that the optimized micelles exhibited a sustained release profile and prolonged plasma concentration compared to the vinpocetine injection [84].

Fig. 3.5
figure 5

The effects of different studied parameters on loading efficiency of nanomicelles

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Fig. 3.6
figure 6

The effects of different studied parameters on the particle size of nanomicelles. Figures source: Varshosaz et al. [83]

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3.6 Conclusion

The skin is the easiest, non-invasive, and an appealing route for the drug administration, which initiates the research towards the topical drug delivery system. The main drawback of the topical delivery is the SC barrier which resists the entry of bioactive agents. Henceforth, nanosystems such as liposomes, niosomes, SLNs, microemulsions, PNPs become the widely formulated technologies to overcome these side effects and improve the efficacy of drugs at the skin. It requires many efforts to prepare a nanosystem, either raw materials, preparation methods, or characterization techniques. At the duration of the formulation of nanosystems, various process and formulation parameters are required, and their optimization is crucial for the desired quality of the product for topical delivery. Thus, the DoE is a vital optimization technique for the optimization of nanosystems and ensures desirable product design with a highly potential final topical product.