**5. Electroporation**

Electroporation is the phenomenon in which cell membrane permeability to ions and macromolecules is increased by exposing the cell to short high electric field pulses. The increase in permeability is attributed to the electric field induced "breakdown" of the cell membrane and the formation of nano- scale defects or "pores" in the membrane – and hence electro-"poration". Electroporation can be of two types - reversible and irreversible. In irreversible electroporation the electric field is such that the membrane permeabilization leads to cell death. This may be caused by either permanent permeabilization of the membrane and cell lysis (necrosis) or by temporary permeabilization of a magnitude which can cause a severe disruption of the cell homeostasis that can finally results in cell death, either necrotic or apoptotic. In reversible electroporation the electric pulse causes only a temporary increase in permeability and the cell survives. The reversible electroporation mode has numerous applications in biotechnology and medicine both, *in vitro* and *in vivo*. Irreversible electroporation has applications in the food industry, for sterilization and in medicine for tissue ablation (Ball et al., 2010).

#### **5.1 Mechanisms of transdermal electroporation**

The theory postulates two paths for electroporation induced transdermal transport, through pores formed in the multiple lipid bilayers connecting corneocytes and through appendage cells. Small lipid-soluble molecules can partition into the SC, and then diffuse across the lipid bilayer membranes, but water soluble molecules, particularly charged molecules, cannot penetrate significantly by this route. High voltage pulsing (> 50V) creates aqueous pathways ("pore") through stratum corneum (SC) lipid bilayer membranes, and short pathway segments are formed across 5--6 lipid bilayer membranes which connect adjacent corneocyte interiors forming transcellular straight-through pathways. Moderate voltage (= 5

Chemical and Physical Enhancers for Transdermal Drug Delivery 413

upper layer of the skin without reaching the dermis, to be an efficient method to deliver drugs transdermally in an almost painless method. The drug diffuses across the rest of the epidermis into the dermis where it is absorbed into the blood circulation. Nowadays different types of microneedles have been designed by other researchers as well, varying in their materials of

Microneedles are available as both solid and hollow microneedles made of various materials (Figure 3). Till date, five methods of transdermal delivery mediated by microneedles have been attempted (Gill & Prausnitz, 2007): *Poke with patch approach*: It can be inserted into the skin to pierce the stratum corneum and create micro conduits through which drug can enter into the lower layers of the epidermis (Henry et al., 1998). *Coat and poke approach:* It involves coating the drug to be delivered around the surface of the microneedle. By inserting the microneedles through the skin, the drug coating dissolves off in the skin fluid and the dissolved drug diffuses through the skin into the blood microcirculation. The coating methods are used to roll coating, spray coating and dip coating (Gill & Prausnitz, 2006). *Dip and scrape:* The dip and scrape method involves placing the array in contact with the drug solution and then scraping multiple times across the skin to create microabbrassions (Mikszta et al., 2002). *Dissolving microneedles:* It is referred to microneedles made from a biodegradable polymeric material with the drugs encapsulated inside them. In this method, the drug is released in a controlled manner as the microneedle dissolves off when inserted into the skin (Lee W. J et al., 2007). *Injection through hollow microneedles:* This occurs where the microneedles are designed with holes at the centre or with side openings through which drugs are microinjected into the lower layers of the skin and then diffuses across the viable

fabrication, shapes, dimensions, modes of application, etc. (Chabri et al., 2009).

skin until it reaches the blood vessels in the dermis (Griss & Stemme, 2003).

cheap to produce.

Solid microneedles: These are easier to fabricate, have better mechanical strength and sharper tips as compared to hollow microneedles (Rhoxed et al., 2008a). Solid silicon microneedles have been widely used for the transdermal drug delivery studies (Donnelly et al., 2009; Haq et al., 2009). However, silicon is expensive, not biocompatible and brittle. Therefore it breaks easily during the penetration across skin (Chen et al., 2008). Polymer has been used as an alternative material because it is a cheaper and stronger material which could reduce tissue damage (Fernandez et al., 2009). Polymer increases the bluntness of the microneedle tip due to the low modulus and yield strength of polymer. Polymer microneedles have a main limitation with its mechanical properties which could cause needle failure during the penetration across skin (Park et al., 2007). Bevelled tip microneedles have been fabricated using biodegradable polymers (Park, 2004). Metal is the third material used to manufacture microneedles. It is mechanically strong and relatively

Hollow microneedles*:* The purpose of this type of microneedles is to deliver drugs through the bore at the needle tip. This reduces the sharpness of needle tip which affect the penetration of this needle into skin. These issues have been resolved recently including openings at the side in the microneedles rather than at the bottom (Roxhed et al., 2008). These microneedles have their tip closed initially; however they can be opened on insertion into the skin where the tip dissolves in the high saline solution in the interstitial fluid. The tips can also be opened as a result of applied pressure. It has been proposed the use of

**6.1 Microneedle types and their methods of transdermal delivery** 

to 50V) pulses appear to electroporate cell linings of the appendages. Temperature is considered to play a role in the permeabilization.

#### **5.2 Advantages and disadvantages of electroporation for transdermal drug delivery**

The advantages and disadvantages that the electroporation technique offers are summarized in Table 6.


Table 6. Advantages and disadvantages of using electroporation as a physical penetration enhancer.

#### **5.3 Applications of electroporation**

The field of skin electroporation is made of two aspects. The first deal with electroporation in a conventional sense in relation to the cells of the skin and the second is unique and relates to transdermal effects. The concept of transdermal electroporation may be traced to fundamental research on the breakdown of flat lipid bilayer membranes. Prausnitz et al., (1993) addresses the fact that transdermal transport normally occurs primarily through the intracellular lipids organized in bilayers. Small molecular weight lipophilic drugs can be effectively delivered by passive transdermal delivery. However, the stratum corneum does not permit passage of polar/hydrophilic molecules and macromolecules. The paper suggests that microsecond to millisecond electroporation type pulsed electric fields applied across the skin produce, in a manner similar to that found in studies on flat lipid bilayers, trans bilayer aqueous pores. It reports that electroporation produces transient structural changes in the skin resulting in an up to four orders of magnitude increase in transdermal mass transfer flux of polar molecules in human skin *in vitro* and animal skin *in vivo*.

#### **6. Microneedles**

The use of microneedles is another method for bypassing the stratum corneum barrier, which have been introduced as a form of transdermal drug delivery. They can penetrate the

to 50V) pulses appear to electroporate cell linings of the appendages. Temperature is

**5.2 Advantages and disadvantages of electroporation for transdermal drug delivery**  The advantages and disadvantages that the electroporation technique offers are summarized

Table 6. Advantages and disadvantages of using electroporation as a physical penetration

The field of skin electroporation is made of two aspects. The first deal with electroporation in a conventional sense in relation to the cells of the skin and the second is unique and relates to transdermal effects. The concept of transdermal electroporation may be traced to fundamental research on the breakdown of flat lipid bilayer membranes. Prausnitz et al., (1993) addresses the fact that transdermal transport normally occurs primarily through the intracellular lipids organized in bilayers. Small molecular weight lipophilic drugs can be effectively delivered by passive transdermal delivery. However, the stratum corneum does not permit passage of polar/hydrophilic molecules and macromolecules. The paper suggests that microsecond to millisecond electroporation type pulsed electric fields applied across the skin produce, in a manner similar to that found in studies on flat lipid bilayers, trans bilayer aqueous pores. It reports that electroporation produces transient structural changes in the skin resulting in an up to four orders of magnitude increase in transdermal

mass transfer flux of polar molecules in human skin *in vitro* and animal skin *in vivo*.

The use of microneedles is another method for bypassing the stratum corneum barrier, which have been introduced as a form of transdermal drug delivery. They can penetrate the

**Advantages Disadvantages** 

Cell damage: If the pulses are of the wrong length or intensity, some pores may become too large or fail to close after membrane discharge causing cell damage or rupture

The transport of material into and out of the cell during the time of electropermeability is relatively nonspecific (Murthy et al., 2004).

(Murthy et al., 2004).

considered to play a role in the permeabilization.

Enhanced drug penetration (of selected drugs) over

Allows strict control of transdermal penetration rates. Versatility: electroporation is effective nearly with all

Efficiency: a large majority of cells take in the target

Permits rapid termination of drug delivery through

The procedure may be performed with intact tissue

Less anxiety provoking or painful than injection. In many cases, greater patient satisfaction.

cells and species types (Sung et al., 2003).

DNA or molecule (Huang et al., 2005).

termination of electroporation.

Not immunologically sensitizing.

**5.3 Applications of electroporation** 

in Table 6.

passive transport.

(Heller et al., 1996).

**6. Microneedles** 

enhancer.

upper layer of the skin without reaching the dermis, to be an efficient method to deliver drugs transdermally in an almost painless method. The drug diffuses across the rest of the epidermis into the dermis where it is absorbed into the blood circulation. Nowadays different types of microneedles have been designed by other researchers as well, varying in their materials of
