**7. Nanocarriers**

414 Pharmacology

rotary drilling and mechanical vibration as methods to enhance insertion of hollow

The methods that have been adopted for microneedle fabrication include wet etching, deep reactive ion etching (DRIE) (Teo et al., 2005), microinjection moulding (Sammoura et al., 2007), isotropic etching, isotropic etching in combination with deep etching and wet etching respectively, dry etching, isotropic and anisotropic, photolithography, thin film deposition (Moon & Lee, 2003), laser cutting (Martanto et al., 2004), and inclined LIGA process (Perennes et al., 2006). Studies have shown that factors such as microneedle geometry, coating depth on solid microneedle and skin thickness affect the drug delivery efficiency using microneedles (Al-Qallaf et al., 2009a; 2009b). To ensure that both the insertion and delivery occur at the right location, they should be sharp enough and at least 100μm in

*Vaccination against virus:* Researchers have recently presented microneedle patches as a better alternative for immunization. The vaccine can be coated unto microneedle array and presented as a simple patch which can allow patients to immunize themselves without the necessity for intense medical training (Stoeber & Liepmann, 2005). *Cutaneous fluid extraction and glucose monitoring:* A prototype of a disposable microneedle based glucose monitoring devices has been designed in which, the fluid extraction chamber attached to the microneedle can be connected to a sensing device which measures and indicates the glucose concentration in the body (Zimmermann et al., 2003). *Acne treatment:* The treatment is limited by the low rate of penetration of drugs through the stratum corneum. So, experiments have been carried out by applying the TheraJectMATTM dissolving microneedles containing API in a GRAS matrix to the surface of human skin with acne (Kwon, 2006). *Delivery of nanoparticles:* It was showed that the delivery of particles of 1μm in

microneedles and the fluid infusion flow rate (Wang et al., 2006).

Fig. 3. Two dimensional view of hollow and solid microneedle.

**6.2 Microneedles manufacturing** 

length (Stoeber & Liepmann, 2000).

**6.3 Microneedles applications** 

Nanocarriers are so small to be detected by immune system and they can deliver the drug in the target organ using lower drug doses in order to reduce side effects. Nanocarriers can be administrated into the organisms by all the routes; one of them is the dermal route. The nanocarriers most used and investigated for topic/transdermal drug delivery in the pharmaceutical field are liposomes, dendrimers, nanoparticles and nanoemulsions (Table 7).


Chemical and Physical Enhancers for Transdermal Drug Delivery 417

Liposomes are hollow lipid bilayer structures that can transport hydrophilic drugs inside the core and hydrophobic drugs between the bilayer (Bangham, 1993). They are structures made of cholesterol and phospholipids. They can have different properties depending on the excipients included and the process of their elaboration. The nature of liposomes makes them one of the best alternatives for drug delivery because they are non-toxic and remain inside the bloodstream for a long time. Liposomes can be surface-charged as neutral, negative or positive, depending on the functional groups and pH medium. Liposomes can encapsulate both lipophilic and hydrophilic drugs in a stable manner, depending on the polymer added to the surface (Rodriguez-Justo & Morae et al., 2011). There are small unilamellar vesicles (25 nm to 100nm), medium-sized unilamellar vesicles (100 nm and 500nm), large unilamellar vesicles, giant unilamellar vesicles, oligolamellar vesicles, large multilamellar vesicles and multivesicular vesicles (500 nm to microns). The thickness of the membrane measures approximately 5 to 6 nm. These shapes and sizes depend of the preparation technique, the lipids used and process variables. Depending on these parameters, the behavior both *in vivo* and *in vitro* can change and opsonization processes, leakage profiles, disposition in the body and shelf life are different due to the type of

Liposomes preparation techniques follow three basic steps with particular features depending on safety, potential scale up and simplicity: 1) Lipid must be hydrated, 2) Liposomes have to be sized and 3) Nonencapsulated drug has to be removed. The degree of transdermal drug penetration is affected by the lamellarity, lipid composition, charge on the liposomal surface, mode of application and the total lipid concentrations (Cevc & Blume, 1992). Some examples of drugs delivered throughout the skin by using liposomes are melatonin (Dubey et al., 2007b), indinavir (Dubey et al., 2010), amphotericin B (Manosroi et al., 2004), methotrexate (Dubey et al., 2007a), ketoprofen (Maestrelli et al., 2005), estradiol (Essa et al., 2004), clindamicyn hydrochloride and lignocaine (Sharma et

Dendrimers are monodisperse populations that are structurally and chemically uniform. They allow conjugation with numerous functional groups due to the nature of their branches. The amount of branches increases exponentially and dendrimers growth is typically about 1 nm per generation (Svenson & Tomalia, 2005). The dendrimers classification is based on the number of generations. After the creation of a core, the stepwise synthesis is called first generation; after that, every stepwise addition of monomers creates the next generation. This approach allows an iterative synthesis, providing the

The kind of polymer chosen to construct the dendrimer by polimerization is very important with regard to the final architecture and features. In addition, the use of branched monomers has the peculiarity of providing tailored loci for site-specific molecular recognition and encapsulation. Notably, 3D and fractal architecture, as well as the peripheral functional groups, provide dendrimers with important characteristic physical

**7.1 Liposomes** 

al., 1994).

**7.2 Dendrimers** 

liposome (Rodriguez-Justo & Morae et al., 2011).

ability to control both molecular weight and architecture.


Table 7. Examples of Nanocarriers used for transdermal drug delivery

## **7.1 Liposomes**

416 Pharmacology

<1 μm Self emulsification Self organizing system

<2 μm Sonication Gas filled microspheres

in which surfactants crystallize into tightly packed bilayers that spontaneously form

Dodla & Bellamkonda , 2008

Tartis et al., 2008

Fang et al., 2007

Elsayed et al., 2006

Rojas-Oviedo et al., 2007

Jin et al., 2006

Rossier-Miranda et al., 2009

Hong et al., 2009

Elnaggar et al., 2009

cylinders

stabilized by phospholipids, polymers or low density proteins.

Solid lipid core stabilized by a monolayer of phospholipids molecules embedded in the particle surface.

Non invasive delivery carriers that enable drugs to reach the deep skin layers and/or the systemic circulation.

The particle core is composed of noncrystalline calcium phosphate or ceramic diamond, and it is covered by a polyhydroxyl oligomeric film.

Pure drug vesicles

amphiphilic drugs

Hollow capsules with

Bilayered structures made of non-ionic surfactant vesicles.

Submicron emulsions

formed by

elastic shells.

o/w or w/o

**Nanocarrier Size Preparation Methods Characteristics References** 

Lipid

Lipid

microcylinders

microbubbles

Lipospheres 0.2-100 μm Melt method, multiple

Ethosomes <400 nm Cold method, hot

Aquasomes 60-300 nm Self-assembling of

Pharmacosomes <200 nm Hand-shaking method,

1.5 µm

Niosomes 10-1000 nm Self-assembly of

Nanoemulsions 20-200 nm High-pressure,

Colloidosomes 200 nm –

microemulsion, cosolvent method

hydroxyapatite by coprecipitation method

Ether-injection method

Self-assembly of colloidal particles at the interface of emulsion droplets

nonionic surfactant

homogenization, microfluidization, phase inversion Temperature.

Table 7. Examples of Nanocarriers used for transdermal drug delivery

method

Liposomes are hollow lipid bilayer structures that can transport hydrophilic drugs inside the core and hydrophobic drugs between the bilayer (Bangham, 1993). They are structures made of cholesterol and phospholipids. They can have different properties depending on the excipients included and the process of their elaboration. The nature of liposomes makes them one of the best alternatives for drug delivery because they are non-toxic and remain inside the bloodstream for a long time. Liposomes can be surface-charged as neutral, negative or positive, depending on the functional groups and pH medium. Liposomes can encapsulate both lipophilic and hydrophilic drugs in a stable manner, depending on the polymer added to the surface (Rodriguez-Justo & Morae et al., 2011). There are small unilamellar vesicles (25 nm to 100nm), medium-sized unilamellar vesicles (100 nm and 500nm), large unilamellar vesicles, giant unilamellar vesicles, oligolamellar vesicles, large multilamellar vesicles and multivesicular vesicles (500 nm to microns). The thickness of the membrane measures approximately 5 to 6 nm. These shapes and sizes depend of the preparation technique, the lipids used and process variables. Depending on these parameters, the behavior both *in vivo* and *in vitro* can change and opsonization processes, leakage profiles, disposition in the body and shelf life are different due to the type of liposome (Rodriguez-Justo & Morae et al., 2011).

Liposomes preparation techniques follow three basic steps with particular features depending on safety, potential scale up and simplicity: 1) Lipid must be hydrated, 2) Liposomes have to be sized and 3) Nonencapsulated drug has to be removed. The degree of transdermal drug penetration is affected by the lamellarity, lipid composition, charge on the liposomal surface, mode of application and the total lipid concentrations (Cevc & Blume, 1992). Some examples of drugs delivered throughout the skin by using liposomes are melatonin (Dubey et al., 2007b), indinavir (Dubey et al., 2010), amphotericin B (Manosroi et al., 2004), methotrexate (Dubey et al., 2007a), ketoprofen (Maestrelli et al., 2005), estradiol (Essa et al., 2004), clindamicyn hydrochloride and lignocaine (Sharma et al., 1994).

#### **7.2 Dendrimers**

Dendrimers are monodisperse populations that are structurally and chemically uniform. They allow conjugation with numerous functional groups due to the nature of their branches. The amount of branches increases exponentially and dendrimers growth is typically about 1 nm per generation (Svenson & Tomalia, 2005). The dendrimers classification is based on the number of generations. After the creation of a core, the stepwise synthesis is called first generation; after that, every stepwise addition of monomers creates the next generation. This approach allows an iterative synthesis, providing the ability to control both molecular weight and architecture.

The kind of polymer chosen to construct the dendrimer by polimerization is very important with regard to the final architecture and features. In addition, the use of branched monomers has the peculiarity of providing tailored loci for site-specific molecular recognition and encapsulation. Notably, 3D and fractal architecture, as well as the peripheral functional groups, provide dendrimers with important characteristic physical

Chemical and Physical Enhancers for Transdermal Drug Delivery 419

a) b)

Nanoemulsion are isotropic dispersed systems of two non miscible liquids, normally consisting of an oily system dispersed in an aqueous system (o/w nanoemulsion), or an aqueous system dispersed in an oily system but forming droplets or other oily phases of nanometric sizes (100 nm). They can be stable (methastable) for long times due to the extremely small sizes and the use of adequate surfactants. Nanoemulsions can use hydrophobic and hydrophilic drugs because it is possible to make both w/o or o/w nanoemulsions (Sonneville-Aubrun, et al. 2004). They are non-toxic and non-irritant systems and they can be used for skin or mucous membranes, parenteral and non parenteral administration in general and they have been used in the cosmetic field. Nanoemulsions can be prepared by three methods mainly: high-pressure homogenization, microfluidization and phase inversion temperature. Transdermal delivery using nanoemulsions has been reduced due to the stability problems inherent to this dosage form. Some examples of drugs using nanoemulsions to transdermal drug delivery are gamma tocopherol, caffeine, plasmid DNA,

aspirin, methyl salicylate, insulin and nimesulide (Shakeel & Ramadan, 2010).

Transdermal drug delivery has several potential advantages over other parenteral delivery methods. Apart from the convenience and noninvasiveness, the skin also provides a "reservoir" that sustains delivery over a period of days. Furthermore, it offers multiple sites to avoid local irritation and toxicity, yet it can also offer the option to concentrate drugs at local areas to avoid undesirable systemic effects. However, at present, the clinical use of transdermal delivery is limited by the fact that very few drugs can be delivered transdermally at a viable rate. This difficulty is because the skin forms an efficient barrier for most molecules, and few noninvasive methods are known to significantly enhance the

In order to increase the range of drugs available for transdermal delivery the use of chemical and physical enhancement techniques have been developed in an attempt to compromise skin barrier function in a reversible manner without concomitant skin irritation. Recently, several alternative physical methods have emerged to transiently break the stratum corneum barrier and also the use of chemical enhancers continues expanding. The projectile methods use propelled microparticles and nanoparticles to penetrate the skin barrier. Microneedle arrays are inserted through the skin to create pores. "Microporation" creates arrays of pores in the skin by heat and radio frequency ablation. Also, ultrasound has been employed to disrupt the skin barrier. All these methods have their own advantages

Fig. 4. a) Nanospheres and b) nanocapsules.

**7.4 Nanoemulsions** 

**8. Conclusions** 

penetration of this barrier.

and chemical properties. In comparison with linear polymers, dendritic structures have "dendritic voids" that give these molecules important and useful features. These spaces inside dendrimers can mimic the molecular recognition performed by natural proteins. Furthermore, dendrimers have a high surface-charge density due to ionizable groups that help them to attach drugs by electrostatic forces, regardless of the stoichimetry. This dendrimer-drug association provides drugs with better solubility, increasing their transport through biological membranes and sometimes increasing drug stability. The number of molecules that can be incorporated into dendrimers is related to the number of surface functional groups; therefore, later-generation dendrimers are more easily incorporated into dendritic structure. However, not all the functional groups are available for interaction due to steric volume, molecule rotation or stereochemistry effects. Dendrimers can have positive and negative charges, which allows them to complex different types of drugs (Kabanov et al., 1998). The main problems with this kind of transdermal carrier are poor biodegradation and inherent cytotoxicity (Parekh, 2007). In order to reduce their toxicity, dendrimers have been linked to peptides and which are formed from amino acids linked via peptide-amide bonds to the branches of dendrimers in the core or on the surface. When they are biotransformed, dendrimer-peptide systems produce amino-acid derivatives. Finally, the synthesis of these structures is less expensive and purification does not present any difficulty (Niederhafner et al., 2005). Due to their form and size, these molecules can carry drugs, imaging agents, etc. Dendrimers interact with lipids present in membranes, and they show better permeation in cell cultures and intestinal membranes (Cheng et al., 2008). Dendrimers also act like solubility enhancers, increasing the permeation of lipophilic drugs; nevertheless, they are not good carriers for and hydrophilic drugs.

#### **7.3 Nanoparticles**

Nanoparticles are smaller than 1,000 nm. Nowadays, it is possible to insert many types of materials such as drugs, proteins, peptides, DNA, etc. into the nanoparticles. They are constructed from materials designed to resist pH, temperature, enzymatic attack, or other problems (Huang L. et al., 2010; Wei et al., 2010). The nanoparticle technology can be divided into three stages: first generation (involves those nanoparticles that had only one component in their structure and these delivery systems are able to transport drugs in the blood until they reach the target), second generation (implies nanoparticles made of one main component and additional substances and these complexes are able to cross barriers and reach difficult targets such as the brain) and third generation is represented by nanoparticles that can be made of nanoparticles with one main component combined with a second component to reach a specific target (Cui et al., 2005; Herffernan & Murthy, 2005). Moreover, nanoparticles can be classified as nanospheres or nanocapsules (Figure 4). Nanospheres are solid-core structures and nanocapsules are hollow-core structures (Yoo et al., 2005). Nanoparticles can be composed of polymers, lipids, polysaccharides and proteins (Goswami et al., 2010; Li et al., 2009). Nanoparticles preparation techniques are based on their physicochemical properties. They are made by emulsification-diffusion by solvent displacement, emulsification-polymerization, in situ-polymerization, gelation, nanoprecipitation, solvent evaporation/extraction, inverse salting out, dispersion polymerization and other derived from these one.

Fig. 4. a) Nanospheres and b) nanocapsules.

## **7.4 Nanoemulsions**

418 Pharmacology

and chemical properties. In comparison with linear polymers, dendritic structures have "dendritic voids" that give these molecules important and useful features. These spaces inside dendrimers can mimic the molecular recognition performed by natural proteins. Furthermore, dendrimers have a high surface-charge density due to ionizable groups that help them to attach drugs by electrostatic forces, regardless of the stoichimetry. This dendrimer-drug association provides drugs with better solubility, increasing their transport through biological membranes and sometimes increasing drug stability. The number of molecules that can be incorporated into dendrimers is related to the number of surface functional groups; therefore, later-generation dendrimers are more easily incorporated into dendritic structure. However, not all the functional groups are available for interaction due to steric volume, molecule rotation or stereochemistry effects. Dendrimers can have positive and negative charges, which allows them to complex different types of drugs (Kabanov et al., 1998). The main problems with this kind of transdermal carrier are poor biodegradation and inherent cytotoxicity (Parekh, 2007). In order to reduce their toxicity, dendrimers have been linked to peptides and which are formed from amino acids linked via peptide-amide bonds to the branches of dendrimers in the core or on the surface. When they are biotransformed, dendrimer-peptide systems produce amino-acid derivatives. Finally, the synthesis of these structures is less expensive and purification does not present any difficulty (Niederhafner et al., 2005). Due to their form and size, these molecules can carry drugs, imaging agents, etc. Dendrimers interact with lipids present in membranes, and they show better permeation in cell cultures and intestinal membranes (Cheng et al., 2008). Dendrimers also act like solubility enhancers, increasing the permeation of lipophilic drugs;

nevertheless, they are not good carriers for and hydrophilic drugs.

polymerization and other derived from these one.

Nanoparticles are smaller than 1,000 nm. Nowadays, it is possible to insert many types of materials such as drugs, proteins, peptides, DNA, etc. into the nanoparticles. They are constructed from materials designed to resist pH, temperature, enzymatic attack, or other problems (Huang L. et al., 2010; Wei et al., 2010). The nanoparticle technology can be divided into three stages: first generation (involves those nanoparticles that had only one component in their structure and these delivery systems are able to transport drugs in the blood until they reach the target), second generation (implies nanoparticles made of one main component and additional substances and these complexes are able to cross barriers and reach difficult targets such as the brain) and third generation is represented by nanoparticles that can be made of nanoparticles with one main component combined with a second component to reach a specific target (Cui et al., 2005; Herffernan & Murthy, 2005). Moreover, nanoparticles can be classified as nanospheres or nanocapsules (Figure 4). Nanospheres are solid-core structures and nanocapsules are hollow-core structures (Yoo et al., 2005). Nanoparticles can be composed of polymers, lipids, polysaccharides and proteins (Goswami et al., 2010; Li et al., 2009). Nanoparticles preparation techniques are based on their physicochemical properties. They are made by emulsification-diffusion by solvent displacement, emulsification-polymerization, in situ-polymerization, gelation, nanoprecipitation, solvent evaporation/extraction, inverse salting out, dispersion

**7.3 Nanoparticles** 

Nanoemulsion are isotropic dispersed systems of two non miscible liquids, normally consisting of an oily system dispersed in an aqueous system (o/w nanoemulsion), or an aqueous system dispersed in an oily system but forming droplets or other oily phases of nanometric sizes (100 nm). They can be stable (methastable) for long times due to the extremely small sizes and the use of adequate surfactants. Nanoemulsions can use hydrophobic and hydrophilic drugs because it is possible to make both w/o or o/w nanoemulsions (Sonneville-Aubrun, et al. 2004). They are non-toxic and non-irritant systems and they can be used for skin or mucous membranes, parenteral and non parenteral administration in general and they have been used in the cosmetic field. Nanoemulsions can be prepared by three methods mainly: high-pressure homogenization, microfluidization and phase inversion temperature. Transdermal delivery using nanoemulsions has been reduced due to the stability problems inherent to this dosage form. Some examples of drugs using nanoemulsions to transdermal drug delivery are gamma tocopherol, caffeine, plasmid DNA, aspirin, methyl salicylate, insulin and nimesulide (Shakeel & Ramadan, 2010).
