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1 Vaccination

The importance of vaccination in limiting morbidity from infectious disease cannot be overestimated and, according to the World Health Organisation, saves the lives of over 2.5 million children per year. The aim of vaccination is to build individuals’ immunity against a specific disease. Vaccines traditionally correspond to one of four types: those containing a dead microorganism, those with live-attenuated microorganisms, those with protein subunits and those with inactivated toxic components (toxoid). A number of innovative vaccines are in development such as recombinant vector and DNA vaccines. These agents resemble a disease-causing microorganism and stimulate the body’s immune system to recognise the agent as foreign, destroy it and ‘remember’ it, so that the immune system can more easily challenge these microorganisms upon subsequent encounters.

The critical component in guaranteeing successful vaccination is appropriate vaccine administration. While intramuscular (M) and subcutaneous (SC) routes are used for the majority of vaccines, this method is not without its problems. These routes require highly trained personnel for administration and are associated with pain and distress which might lead to reduced patient compliance. Additionally, in developing countries, hypodermic injections are associated with a high risk of cross-contamination between patients due to the possibility of needlestick injuries or reuse of contaminated needles. When mass vaccination is necessary, issues of production and/or supply may also arise [1, 2].

The majority of vaccines are delivered either into subcutaneous fat or muscle. Delivery vaccines into the dermis are rare [3], and even topical or transcutaneous application to the surface of the skin [4, 5], also termed epicutaneous application [5], are rarer still. The above routes of application are each effective only because of the ability of dendritic cells (DCs) to uptake, process and present the antigen to T lymphocytes in the draining lymphoid organs. Whereas subcutaneous fat and muscle tissue contain relatively few DCs, the dermis and the epidermis are densely populated by different subsets of DCs. Consequently, antigen delivery by hypodermic injection will bypass the skin’s immune cells leading to less efficient vaccination. For this reason, the skin represents an ideal site for vaccine delivery, as vaccination at this site will evoke strong immune responses at much lower doses of antigen than intramuscular vaccines [6]. The potential of skin immunisation was observed in a clinical trial where epidermal influenza vaccination induced influenza-specific CD8 T cell response, while classical intramuscular route did not [7]. Dose-sparing approaches are critical to ensuring a sufficient supply of certain vaccines, especially in pandemic diseases [8].

2 Skin Structure and Function

As the largest and one of the most complex organs in the human body, the skin (Fig. 43.1) is responsible for a varied range of functions [9, 10]. The barrier properties of the skin afford protection against physical, microbial or chemical invasions. The skin is made up of three layers: the epidermis, dermis and subcutaneous tissue. The epidermis consists of the viable epidermis and the stratum corneum. The viable epidermis consists of four histologically distinct layers: the stratum germinativum, stratum spinosum, stratum granulosum and stratum lucidum. The epidermis is not of uniform thickness, varying from 60 μm on the eyelids to 800 μm on the palms [11]. The layers of the epidermis are a vascular and receive nutrients by diffusion of substances from the underlying dermal capillaries.

Fig. 43.1
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Diagrammatic representation of the major features of skin anatomy

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The dermis (or corium) resides atop the subcutaneous fat layer and is approximately 3–5 mm thick [12] and consists of a mucopolysaccharide matrix within which exists a network of elastin and collagen fibres, providing both elasticity and structure to the skin [13, 14]. The dermis is maintained physiologically by a network of nerve endings, lymphatics and blood vessels [15]. The cutaneous blood supply delivers nutrients and oxygen to the skin and allows waste products to be removed. Beneath the dermis lies the subcutaneous fat layer, subcutis, subdermis or hypodermis [13]. The hypodermis, consisting mainly of adipose tissue, acts as an insulator (due to the high content of adipose tissue), as well as supporting the dermis and epidermis physically and nutritionally. The hypodermis also carries the main blood vessels and nerves to the skin and may contain sensory organs [16]. Resting beneath the vascular dermis, the role of the hypodermis in drug delivery is not considered to be major [17].

The stratum corneum, or horny layer, is the outermost layer of the epidermis and thus the skin. It has now well accepted that this layer constitutes the principal barrier for penetration of most drugs. The horny layer represents the final stage of epidermal cell differentiation. The thickness of this layer is typically 10 μm, but a number of factors, including the degree of hydration and skin location, influence this. For example, the stratum corneum on the palms and soles can be, on average, 400–600 μm thick while hydration can result in a four-fold increase in thickness.

The stratum corneum consists of 10–25 rows of dead keratinocytes, now called corneocytes, embedded in the secreted lipids from lamellar bodies. The corneocytes are flattened, elongated, dead cells, lacking nuclei and other organelles. The cells are joined together by desmosomes, maintaining the cohesiveness of this layer. The heterogeneous structure of the stratum corneum is composed of approximately 75–80 % protein, 5–15 % lipid and 5–10 % other substances on a dry weight basis.

The majority of protein present in the stratum corneum is keratin and is located within the corneocytes. The keratins are a family of α-helical polypeptides. Individual molecules aggregate to form filaments (7–10 nm diameter and many microns in length) that are stabilised by insoluble disulphide bridges. These filaments are thought to be responsible for the hexagonal shape of the corneocyte and provide mechanical strength for the stratum corneum [12]. Corneocytes possess a protein-rich envelope around the periphery of the cell, formed from precursors, such as involucrin, loricrin and cornifin. Transglutaminases catalyse the formation of γ-glutamyl cross-links between the envelope proteins that render the envelope resistant and highly insoluble. The protein envelope links the corneocyte to the surrounding lipid-enriched matrix.

The main lipids located in the stratum corneum are ceramides, fatty acids, cholesterol, cholesterol sulphate and sterol/wax esters. These lipids are arranged in multiple bilayers called lamellae (Fig. 43.2). Phospholipids are largely absent, a unique feature for a mammalian membrane. The ceramides are the largest group of lipids in the stratum corneum, accounting for approximately half of the total lipid mass, and are crucial to the lipid organisation of the stratum corneum.

Fig. 43.2
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Arrangement of lipids in the stratum corneum

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The bricks and mortar model of the stratum corneum are a common representation of this layer. The bricks correspond to parallel plates of dead keratinised corneocytes, and the mortar represents the continuous interstitial lipid matrix. It is important to note that the corneocytes are not actually brick shaped but rather are polygonal, elongated and flat (0.2–1.5 μm thick and 34.0–46.0 μm in diameter). The ‘mortar’ is not a homogenous matrix. Rather, lipids are arranged in the lamellar phase (alternating layers of water and lipid bilayers), with some of the lipid bilayers in the gel or crystalline state. The extracellular matrix is further complicated by the presence of intrinsic and extrinsic proteins, such as enzymes. The barrier properties of the stratum corneum have been assigned to the multiple lipid bilayers residing in the intercellular space. These bilayers prevent desiccation of the underlying tissues by inhibiting water loss and limit the penetration of substances from the external environment.

3 Immune Function of Skin

Vaccination development remains an important field in both research and pharma, whereby in addition to extending the spectrum of antigens for novel vaccines, developing improved administration strategies to ameliorate vaccine efficacy remains a challenge. The concept of delivery of vaccines through or into the skin has been gathering momentum in the past decade, largely due to the increasing recognition that a tight semi-contiguous network of immunoregulatory cells that reside in the different skin layers is an ideal target for vaccine administration. Dendritic cells (DCs), macrophages and neutrophilic granulocytes are the principal phagocytes in the skin (Fig. 43.3), while numerous cells of the adaptive immune system, such are CD8+ T cells and the full spectrum of CD4+ T cells, can be found in normal skin [3–7, 18–44]. A detailed description of dendritic cells is provided in Chap. 2 of this book.

Fig. 43.3
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Immune structure of skin

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4 Vaccination via the Skin Using Microneedles

Intradermal (ID) vaccination using MN is one of the most attractive approaches for delivering an antigen to the dermal layer of the skin without using hypodermic injections, which are associated with transmission of infection and inappropriate disposal in the developing world. MN arrays (Fig. 43.4) consist of a multiplicity of microprojections ranging from 25 to 2,000 μm in height, attached to a base support [45]. These microprojections can create aqueous transport pathways at the micron scale, painlessly breaking through the stratum corneum barrier when the microneedles are applied to the skin surface [46]. The micropores created by MNs readily permit transport of a wide range of micromolecules and macromolecules, such as immunotherapeutic agents, including vaccines and proteins [47]. Importantly, MN insertion does not cause bleeding.

Fig. 43.4
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Laser engineered, micromoulded, microneedle array prepared from aqueous blends of the structure-forming copolymer poly(methyl vinyl ether-co-maleic acid)

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MNs were first described by Henry et al. in 1998 and have since been the subject of continuous research [48]. MNs are fabricated from various materials such as metals, glass, silicon and FDA-approved polymers [49]. Donnelly et al. detail a range of methods for the development and fabrication of various MNs [50]. There are four main modes of action of MNs, which are poke and patch, coat and poke, poke and release and poke and flow [51] (Fig. 43.5).

Fig. 43.5
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Schematic representation of methods of MN application to the skin to achieve enhanced transdermal drug delivery. (a) (Poke and patch) shows solid MN that are applied and removed to create transient micropores, followed by application of a traditional transdermal patch. (b) (Coat and poke) shows solid MN coated with drug for instant delivery. MN is removed after the coating material dissolves. (c) (Dissolving microneedles) show soluble polymeric/carbohydrate MN-containing drug that dissolves in skin interstitial fluid over time, thereby delivering the drug. (d) (Poke and flow) shows hollow MN for delivery of fluids containing drug

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4.1 Poke and Patch

The poke and patch approach is based on using solid MNs to puncture the skin followed by applying antigen to the treated area in order to diffuse antigen into the skin. Substantial work using this approach has been carried out by the Bouwstra Group, where microneedles are inserted into the skin in order to increase its permeability, after which the vaccine is applied. The Bouwstra Group, in one report, studied mouse immune responses after transcutaneous immunisation (TCI) using two model antigens: diphtheria toxoid (DT) and influenza subunit vaccine [52]. Stainless steel MN arrays (16 × 300 μm MNs) were used to perforate the mouse skin followed by application of a DT formulation with or without cholera toxin (CT). The application of DT to MN-treated skin resulted in significantly higher serum IgG and toxin-neutralising antibody titres than in unperforated skin. The presence of CT increased the immune response to similar levels as observed after subcutaneous injection of AlPO4-adsorbed DT (DT-alum). Unlike in the DT case, MN array pretreatment showed no effect on the immune response to the influenza vaccine alone. The addition of CT strongly improved the immune response, independent of MN treatment. The conclusion drawn by the authors of the study was that that TCI of DT in the presence of CT after MN treatment results in similar protection to injection of DT-alum.

A study investigated the effect of co-administration of various adjuvants with DT in the modulation of the immune response in TCI mice after the application of MN arrays [53]. Mice were treated with DT co-administrated with lipopolysaccharide (LPS), Quil-A, CpG oligodeoxynucleotide (CpG) or CT as adjuvants. The MN array pretreatment group displayed high serum IgG levels, and these were remarkably improved by co-administration of adjuvants. The group treated with DT co-administered with CT showed similar IgG levels to those treated subcutaneously with DT-alum. N-trimethyl chitosan also proved beneficial in boosting the immune response to DT following MN pretreatment when in solution with the antigen, although no improvement in immune response was seen from DT-loaded nanoparticles of N-trimethyl chitosan [54].

The impact of transdermal vaccination on the development of melanoma was reported by Bhowmik et al. (2011), who delivered a novel microparticulate vaccine to the skin following puncture using of MN-based Dermaroller® [55]. Mice were grouped into four groups; group one was treated using Dermaroller® microneedles, followed by application of microparticles containing encapsulated antigen obtained from S-91 melanoma cancer cells. Group two was injected subcutaneously with the same dose of microparticles containing encapsulated vaccine. Group three was given blank microparticles administered in the same way as for the transdermal group. The last group was injected with saline subcutaneously. Eight weeks following vaccination, the mice were challenged with live melanoma cells. In the group which were treated using Dermaroller® MNs and SC injection of vaccine, there was no measurable tumour growth 35 days after tumour injection. A significant increase in IgG antibody levels was seen for both transdermal and subcutaneous vaccinated groups when compared with control groups. However, slightly increased IgG antibody levels were seen in the transdermally vaccinated group when compared to the SC group. This can be attributed to the presence of Langerhans cells (LCs) in the epidermis layers which are activated when exposed to antigen. The authors concluded that the developed formulation for melanoma cancer which can be administered using MNs technology gives rise to new approaches to the prevention of melanoma cancer.

4.2 Coat and Poke

The coat and poke approach involves the coating of solid MNs with an antigen of choice, which can be delivered in a one-step process. This could be a very attractive approach for mediating ID vaccine delivery since the smaller amount of antigen which coats the microneedles should be sufficient to induce a strong immune response. Since the antigen coating on the MNs is in a solid form [56], long-term stability should be improved, ensuring optimal shelf life [57].

The Prausnitz Group at the Georgia Institute of Technology has carried out immunisation studies using stainless steel monument-shaped arrays of 5 MNs dip-coated in vaccine. The MNs used were approximately 700 μm in length, and their manufacture was achieved by laser-cutting stainless steel sheets. Plasmid encoding hepatitis C virus and seasonal influenza: H1N1, H3N2, inactivated virus, influenza virus-like particles and recently BCG were engaged successfully in MN-mediated ID immunisation. After optimisation of the coating formulation that led to the inclusion of trehalose as an antigen stabiliser [58], MNs were coated and inserted into the skin of mice. The results showed that the coated MNs stimulated a robust immune response, providing complete protection against the lethal influenza virus challenge similar to conventional intramuscular injection. The study concluded that effective vaccination was achieved due to the inclusion of the stabilising agent in the coating formulation, as this acted to protect antigen activity. In light of this, in subsequent studies, trehalose was included in the coating formulation.

In a study by Koutsonanos et al., a single MN immunisation with inactivated H3N2 influenza virus induced significantly higher hemagglutination inhibition (HI) titres in comparison with that observed by IM injection [2]. Solid metal MNs coated with inactivated influenza virus were found to be at least as effective as the conventional IM route in inducing similar levels of functional antibodies at low or high antigen concentrations, in clearing the virus from the lungs of infected mice, in conferring protection and in inducing short lived as well as memory B immune responses. While serum IgG responses in IM injection were seen to be dependent upon dose, this was not the case with MN delivery, which produced similar responses at low or high antigen loadings. This finding suggests that the skin has a higher capacity to produce an immunologic response. The same system was used to evaluate the potential of BCG-coated MN vaccine patches [59]. The viability of BCG was maintained by adding 15 % trehalose to the coating solution, which improved the stability of the live BCG vaccine. BCG-coated 10 microneedle patches (5 × 104 CFUs of BCG) were applied to the skin of guinea pigs. Another group of guinea pigs were injected intradermally with BCG (5 × 104 CFUs) using a 26 gauge needle and 1 ml tuberculin syringe. The results of this study indicated that BCG vaccine-coated MNs can induce a strong antigen-specific cellular immune response in both the lung and spleen of guinea pigs, comparable to that induced by using a 26 gauge needle. It was found that MN BCG vaccination elicited similar frequencies of TNF-α secreting or both IFN-γ and TNF-α cytokine secreting bifunctional CD4+ T cells to that induced by hypodermic injection. A strong IgG response was produced by both vaccination methods.

The capacity to produce a protective immune response of modified recombinant trimeric soluble influenza virus hemagglutinin (sHA GCN4pII)-coated MNs was assessed by the group [60]. Comparison was made between results from the modified and unmodified protein (sHA). Mice vaccinated with MN-coated sHA trimeric induced fully protective immune response against influenza virus challenge. Both sHA- and sHA GCN4pII-coated MNs induced improved clearance of replicating virus compared to the SC route. The MNs coated with sHA GCN4pII induced a stronger Th1 response in mice suggested by the ratio of IFN-γ + CD4+ T cell to IL- 4+. The study concluded that MNs coated with stabilised sHA trimers were as effective in inducing a protective immune response and afforded equal level of protection as conventional subcutaneous administration.

Professor Mark Kendall’s research has pioneered the development of Nanopatch™ technology. Nanopatch™ devices are fabricated from silicon using a process of deep reactive-ion etching. The projections are solid silicon, sputter coated with a thin (~100 nm) layer of gold, and contain 3,364 densely packed projections. These arrays are smaller, by two orders of magnitude, than standard needles, and are also much smaller than typical microneedle arrays. The Nanopatches™ have been used to target ID vaccination against West Nile virus and chikungunya virus in mice. The efficiency of Nanopatch™ immunisation was demonstrated using an inactivated whole chikungunya virus vaccine and a DNA-delivered attenuated West Nile virus vaccine [61]. Nanopatch™ technology was also used as an alternative delivery system to the IM prophylactic vaccine against cervical cancer Gardasil® and succeeded in delivering up to 300 ng of vaccine to the ears of mice. Moreover, in terms of virus-neutralising response, results were found equal to those of a control group of IM-vaccinated mice in mouse serum samples from mice vaccinated using Nanopatch™ technology [62]. Similarly impressive results have also been reported when Nanopatch™ coating was used with the influenza vaccine, Fluvax® [63].

Research at Zosano Pharma (formerly ALZA Corporation) assessed the performance of the Macroflux®, another device containing an array of microprojections, for intracutaneous delivery of a model antigen, ovalbumin. The findings revealed that at all antigen doses, administration of OVA-coated Macroflux® resulted in immune response comparable to that observed after intradermal administration of the same doses of antigen. In addition, it was observed that application of 1 and 5 μg of antigen via Macroflux® resulted in 10- and 50-fold increases in anti-ovalbumin levels in comparison to delivery of equivalent doses via intramuscular or subcutaneous routes [64]. Follow-up mechanistic studies revealed that the immunologic response was unaffected by MN height (225–600 μm) and density (140 and 657/cm2), but was dependent on the dose of antigen delivered [65].

4.3 Poke and Flow

The poke and flow approach is based on diffusion of vaccine through conduits of solid MNs. The antigen can be delivered either by passive diffusion, pressure or electrical-driven flow [45, 66]. However, this approach is vulnerable to some of the same problems as conventional vaccination techniques including the requirement of a cold chain and possible need for trained personnel [62]. The narrow bore of MNs and the dense elastic nature of the skin may also limit fluid flow. Partial retraction of the MN array before the introduction of fluid was found by Wang et al. to partially compensate for this limitation; however [67], Frost suggested that the co-administration of the enzyme hyaluronidase, which degrades hyaluronic acid in the extracellular matrix of the skin, might reduce skin resistance [68]. Martanto et al. provided many explanations of the factors affecting the flow rate through hollow MNs. Hollow MNs are the subject of increasing interest because of their potential for use for TCI or ID vaccination [69].

MicronJet® is a novel device developed by NanoPass specifically for intradermal delivery, consisting of an array of four MNs, each 450 μm in length. The needles are of silicon crystal bonded to a plastic adapter which can be mounted on any standard syringe. Van Damme et al. investigated the safety and efficacy of this device for dose-sparing intradermal influenza vaccination in healthy adults [70]. α-RIX® (GSK Biologicals) influenza vaccines were delivered using a hollow MN device (MicronJet®) and their safety and immunogenicity assessed. In the trial, which was carried out with a group of 180 healthy adult subjects, low-dose influenza vaccines delivered by MicronJet® elicited immune responses similar to those elicited by 15 μg HA per strain injected IM in human volunteers. The prevalence of local reaction presented a limitation however, although such reactions were transient in nature. Similar developments have been made at BD Technologies. A 34G stainless steel MN with an inner diameter of 76 μm, an outer diameter of 178 μm and an exposed length of 1 mm was used to deliver three distinct influenza vaccines. Results suggested that this mode of delivery was capable of producing the full immunological response at a dose at least ten-fold lower than with IM administration and up to 100-fold more potent, depending on the nature of the antigen [71]. The same researchers went on to uncover dose-sparing benefits of MN-delivered anthrax vaccinations over intramuscular administration in a subsequent study [72].

4.4 Dissolving/Soluble Microneedles

Dissolving MNs may present an innovative approach for vaccine delivery. Such MNs are based upon water-soluble polymers or carbohydrate material which encapsulates the drug within the needle matrix. On piercing the skin, the MNs dissolve completely to deliver their contents. Dissolvable MNs show promise in vaccine delivery breakthrough for many reasons. Since the MNs will dissolve after insertion into the patient’s skin, the possibility of cross infection is eliminated. Also, the approach eliminates the necessity of any special disposal mechanism, as no sharp biohazardous waste is produced. The solid state nature of the contained/encapsulated vaccine should also reduce the need for cold chain storage and transport. In addition, MN patches offer the possibility of self-administration of vaccine during pandemics as well as rendering mass immunisation programs in developing countries. As these self-disabling MNs lack many of the disadvantages of conventional vaccination techniques and also some of those associated with the MN strategies mentioned to this point, poke and patch MNs are receiving increasing attention for their value in vaccination applications.

Dissolvable MN patches were first taken up for the administration of vaccines by the Prausnitz group. Sullivan et al. manufactured MN patches and investigated possibilities for influenza vaccination using a simple patch-based system. Patches based on 650 μm poly(vinylpyrrolidone) MNs containing 3 μg lyophilised inactivated influenza virus vaccine generated robust antibody and cellular immune responses which gave complete protection against lethal influenza challenge [73]. In fact, in comparison with IM vaccination, MN delivery displayed better results following vaccination in terms of lung virus clearance and cellular recall responses.

The Kendall group described the micromoulding of dissolving MN arrays from master templates of one of their Nanopatch™ designs [74]. Replica MNs were formed from carboxymethylcellulose by means of multiple castings into poly(dimethylsiloxane) moulds. Dual-layer MNs containing both the model antigen ovalbumin and the adjuvant Quil-A elicited post-immunisation schedule antibody levels in mice which were similar to an IM ovalbumin/Quil-A immunisation group at day 28 and superior to the IM group at day 102, despite using a lower antigen dose in the MNs. Research using an influenza vaccine gave similar findings.

4.5 Intradermal Gene Delivery

Gene therapy may be defined as the insertion, alteration or removal of genes from an individual’s cells and biological tissues to treat disease. Replacement of a mutated gene via the insertion of functional genes into an unspecified genomic location is the most frequently used gene therapy. Other approaches exist however which may involve either directly correcting the mutation or modifying normal genes. While the technology has yet to be developed to its full potential, it has been used with some success. The most common form of genetic engineering involves insertion of a functional gene into host cells. This is accomplished by isolating and copying the gene of interest, generating a construct containing all the genetic elements for correct expression and then inserting this construct into the host organism. Localised delivery and expression of gene therapeutics within the skin may provide novel treatment options for a number of pathological conditions, including skin disorders of genetic origin as well as nonsurgical management of malignancy. If genetic manipulation could be used to influence epidermal cells to produce and secrete antigenic molecules, then the potent immunostimulatory properties of the skin could be harnessed to provide efficient protection from the disease concerned.

The Birchall Group at Cardiff University in Wales have pioneered epidermal gene delivery using various microneedle arrays to penetrate the stratum corneum, which would form a barrier to intradermal delivery of genetic constructs. The group initially set out to establish whether silicon-based microneedles (150 μm in height, base width 45–50 μm), microfabricated via an isotropic etching/BOSCH reaction process, were capable of creating microchannels in the skin large enough to permit the passage of lipid/polycation/plasmid DNA (LPD) nonviral gene therapy vectors [75]. Scanning electron microscopy was used to visualise the microconduits created in heat-separated human epidermal sheets after application of the microneedles.

Following confirmation of particle size and particle surface charge by photon correlation spectroscopy and microelectrophoresis, respectively, the diffusion of fluorescent polystyrene nanospheres and LPD complexes through heat-separated human epidermal sheets was established in vitro using a Franz-type diffusion cell. In vitro cell culture with quantification by flow cytometry was used to determine gene expression in human keratinocytes (HaCaT cells). Membrane treatment with microneedles was found to notably enhance the diffusion through epidermal sheets of 100 nm diameter fluorescent polystyrene nanospheres, used as a readily quantifiable predictive model for LPD complexes. The delivery of LPD complexes either into or through the membrane microchannels was also demonstrated. In both cases, considerable interaction between the particles and the epidermal sheet was observed. LPD complexes were shown through in vitro cell culture to mediate efficient reporter gene expression in human keratinocytes in culture when formulated at the appropriate surface charge.

The group’s next study made use of platinum-coated silicon microneedles. These were used to produce microconduits, approximately 50 μm in diameter and extending through the stratum corneum and viable epidermis [76]. Following optimisation of skin explant culturing techniques and confirmation of tissue viability, it was demonstrated that gene expression could be mediated through use of the microneedles to transmit the beta-galactosidase reporter gene. Preliminary studies confirmed localised delivery, cellular internalisation and subsequent gene expression of pDNA following microneedle disruption of skin.

Following this result, it was decided to establish whether microfabricated silicon microneedle arrays could effectively achieve localised delivery of charged macromolecules and plasmid DNA (pDNA) [77]. Microconduits of 10–20 μm in diameter were found in human epidermal membrane following treatment with the microneedles. The delivery of the marker biomolecule beta-galactosidase and of a ‘nonviral gene vector mimicking’ charged fluorescent nanoparticle to the viable epidermis of microneedle-treated tissue was demonstrated using light and fluorescent microscopy.

Track-etched permeation profiles generated using ‘Franz-type’ diffusion cell methodology and a model synthetic membrane showed that >50 % of a colloidal particle suspension permeated through membrane pores in approximately 2 h. These findings were taken by the group to indicate cutaneous delivery of lipid/polycation/pDNA (LPD) gene vectors, and other related vectors, to the viable epidermis, could be achieved via microneedle treatment. Preliminary gene expression studies confirmed that naked pDNA can be expressed in excised human skin following microneedle disruption of the SC barrier. The presence of a limited number of microchannels, positive for gene expression, points to the value of further investigation aimed at optimisation of the morphology of the microneedle device, its method of application and the pDNA formulation.

Aqueous solutions loaded with gene delivery vehicles would not remain in situ on the surface of the skin and therefore would be of limited use in clinical practice. For this reason, the group then examined the possibility of using sustained release pDNA hydrogel formulations with their microneedle delivery system to improve delivery of plasmid DNA (pDNA) in skin [78]. Microneedles were again fabricated by wet etching silicon in potassium hydroxide. Hydrogels based on Carbopol® polymers and thermosensitive PLGA-PEG-PLGA triblock copolymers were prepared. Freshly excised human skin was used to characterise microneedle penetration (microscopy and skin water loss), gel residence in microchannels, pDNA diffusion and reporter gene (beta-galactosidase) expression. Upon application of the microneedles, channels of approximately 150–200 μm depth increased transepidermal water loss in skin. pDNA hydrogels were shown to harbour and gradually release pDNA. Following microneedle-assisted delivery of pDNA hydrogels to human skin, expression of the pCMV beta reporter gene was displayed in the viable epidermis proximal to the microchannels. It was concluded that targeted pDNA hydrogels can potentially provide sustained gene expression in the viable epidermis.

Microneedle-mediated intradermal gene delivery has also been recently explored by the Prausnitz Group [79]. In this study, specific cytotoxic T lymphocytes (CTLs) were effectively primed through vaccination with a plasmid encoding hepatitis C virus nonstructural 3/4A protein using coated microneedles. Importantly, the minimally invasive microneedles were as efficient in priming CTLs as more complicated or invasive delivery techniques, such as gene guns and hypodermic needles. The Kendall Group has also investigated gene delivery using coated Nanopatches™ [80], as described above.

Although Prausnitz had proposed the use of microneedles in combination with electroporation in 2005 [81], it was in 2007 that this was first achieved, when it was reported by Hooper et al., the efficient delivery of an experimental smallpox DNA vaccine consisting of four vaccinia virus genes (4pox) by means of a novel method involving skin electroporation using plasmid DNA-coated microneedle arrays [82]. Electroporation is a process where cells are transiently permeabilised using high-intensity electric field pulses. The Easy Vax® delivery system employed consisted of 80 electrically conducting microneedles each around 1 mm in height coated with dried vaccine DNA.

The pulse protocol consisted of six pulses of 100 V, 100 mS pulse duration and 125 mS pulse interval. Four arrays, each coated with 30 μg of a separate plasmid, were used to deliver the 4pox DNA vaccine to mice. A separate site was used for each array was administered to a separate site (inner and outer right and left thigh). Mice vaccinated with the negative control plasmids were administered using one array to an inner thigh. The smallpox DNA vaccine stimulated robust antibody responses against the four immunogens of interest, including neutralising antibody titres which were greater than were produced by the conventional live virus vaccine administered by scarification. Furthermore, complete protection was found in the vaccinated mice against a lethal intranasal challenge with vaccinia virus strain IHD-J.

This study represented the first demonstration of a protective immune response being elicited by microneedle-mediated skin electroporation and suggests that there is scope for further exploration of this area. Indeed, Daugimont et al [83]. followed up on this by investigating the potential of hollow conductive microneedles for needle-free intradermal injection and electric pulse application in order to generate an electric field in the superficial layers of the skin sufficient for electroporation. Microneedle arrays along with a vibratory inserter were employed to disrupt the stratum corneum, thus piercing the skin. Effective injection of proteins into the skin was achieved, resulting in an immune response directed to the model antigen ovalbumin. However, the dual function of microneedle electrode seemed to pose certain drawbacks for DNA electrotransfer. This could be due to the distribution of the electric field in the skin as shown by numerical calculations and/or the low dose of DNA injected. The authors concluded that these parameters require further study in order to optimise minimally invasive DNA electrotransfer in the skin.

5 Microneedle-Mediated Delivery of Nanoparticles as a Vehicle for Improved Antigen Targeting to Skin DCs

In the past few years, particle-based vaccines have been proposed for successful immunisation. They have been used to protect antigen stability in vivo and to deliver it in a controlled and sustained manner to the site of action [84].

Drug-loaded nanoparticles (NPs) are colloidal systems, typically 10–1,000 nm in diameter, with a therapeutic payload entrapped, adsorbed or chemically coupled to an orbicular matrix [85, 86]. Nanoparticles are widely used for controlled delivery of small molecule drugs, oligonucleotides and protein antigens to a variety cell types, including dendritic cells [87]. Among the different parameters that need to be considered in design of particle-based vaccines, the particle size and their physicochemical properties are particularly important for skin vaccination. It has been demonstrated that polymeric nanoparticles <500 nm in diameter have high rate of intracellular uptake by variety of APC [88].

Several groups have demonstrated that nanoparticles have adjuvant effects comparable to those of CFA or ALUM and, as synthetic adjuvants, can activate DCs to induce T cell immune responses against encapsulated antigens [89–91]. An important advance was the demonstration that nanoparticles as the adjuvants promote activation of the NLRP3 inflammasome [92].

Nanoparticles have been extensively studied for oral and parenteral administration owing to their sustained drug release [93, 94]. This property of nanoparticles could also be utilised for topical antigen administration to target skin DCs with antigen over a prolonged period. Researchers had attempted to use nanoparticles for topical drug delivery, and they found that the drug permeation was enhanced by gradual drug release from the nanoparticles on the skin surface, but did not optimise way to deliver nanoparticles inside the skin [95–97]. This suggested that as a drug delivery vehicle, the nanoparticle could sustain drug release, but, if it was applied as a drug reservoir to treat the skin disease, it must be delivered into the skin layers instead of remaining on the skin surface.

Some other researchers tried to verify the penetration of nanoparticles across the skin, but found that only small numbers of NPs were able to permeate into the skin passively through the hair follicles while most NPs were restricted by stratum corneum and unable to penetrate the skin [98, 99]. In the penetration experiments in vitro using the full-thickness skin, it was found that NPs could diffuse into the dermis as well as into the epidermis [100]. To investigate if the microconduits on the epidermis produced by microneedles could be the channels for NPs to penetrate the skin, the researches in vitro have been designed and proved that nanoparticles could pass through the human epidermal membrane and get into skin layers [101, 102]. Moreover, Bal and colleagues showed that in intradermal antigen delivery in vivo to skin pretreated by metal MNs antigen was more efficiently taken up by skin DCs when it was encapsulated into polymeric NPs, comparing when delivered in a soluble form [103]. All of the studies above indicated that microneedles may be an effective vehicle for the intradermal delivery of antigen encapsulated nanoparticles in vivo.

6 Conclusions

Dendritic cells, as key regulators of immune responses, play a critical role in the design of modern vaccines [24, 99]. The skin harbours a network of these cells and for that reason is recognised as an attractive target for immunisation. However, one important element that has not been dealt with successfully yet is the functional heterogeneity of DCs subsets. Researchers are now trying to improve intracutaneous vaccination by harnessing specific properties of particular DCs subsets, as they became known. In the future, it might be possible to deliver antigen alongside a specific adjuvant to a particular DC subpopulation, while avoiding others with opposite effects. Targeting of antigen to the specific, functionally defined subsets of skin DCs is a promising strategy to further develop not only protective but also therapeutic vaccines.

Given the ever-increasing evidence available within the academic and patent literature that MN of a wide variety of designs are capable of achieving successful intradermal and transdermal delivery of vaccines, it is envisaged that the already concerted industrial effort into development of MN devices will now intensify. Furthermore, novel applications of MN technology are likely to come to the forefront. The ability of MN arrays to extract bodily fluids for determination of efficacy of vaccination is particularly interesting. As technological advances continue, MN arrays may well become the pharmaceutical dosage forms and monitoring devices of the near future. However, there are a number of barriers that will firstly need to be addressed in order for microneedle technology to progress.

The ultimate commercial success of microneedle-based delivery and monitoring devices will depend upon not only on the ability of the devices to perform their intended function but also on their overall acceptability by both healthcare professionals (e.g. doctors, nurses and pharmacists) and patients. Accordingly, efforts to ascertain the views of these end users will be essential moving forward. The seminal study by the Birchall Group [104] in this regard was highly informative. The majority of healthcare professionals and members of the public recruited into this focus-group-centred study were able to appreciate the potential advantage of using microneedles, including reduced pain, tissue damage, risk of transmitting infections and needle stick injuries, feasibility for self-administration and use in children, needlephobes and/or diabetics. However, some concerns regarding effectiveness mean to confirm successful drug delivery (such as a visual dose indicator), delayed onset of action, cost of the delivery system, possible accidental use, misuse or abuse were also raised. Healthcare professionals were also concerned about interindividual variation in skin thickness, problems associated with injecting small volumes and risk of infection. Several other possible issues (accidental or errors based) and interesting doubts regarding microneedle use were discussed in this study. Overall, the group reported that 100 % of the public participants and 74 % of the healthcare professional participants were optimistic about the future of microneedle technology [104]. Such studies, when appropriately planned to capture the necessary demographics, will undoubtedly aid industry in taking necessary action to address concerns and develop informative labelling and patient counselling strategies to ensure safe and effective use of microneedle-based devices. Marketing strategies will, obviously, also be vitally important in achieving maximum market shares relative to existing and widely used conventional delivery systems.

In order to gain acceptance from healthcare professionals, patients and, importantly, regulatory authorities (e.g. the US FDA and the MHRA in the UK), it appears a strong possibility that an applicator aid and a ‘dosing indicator’ be included within the overall microneedle ‘package’, with the microneedle array itself being disposable and the applicator/dosing indicator reusable. While a wide variety of applicator designs have been disclosed within the patient literature, only a few, relatively crude, designs based upon high impact/velocity insertion, or rotary devices, have been described. Application force has a significant role to play in microneedle insertion depth. Clearly, patients cannot ‘calibrate’ their hands and, so, will apply microneedles with different forces. Unless a large-scale study can be done showing consistent rates and extents of microneedle-mediated drug delivery when the microneedles have been inserted by hand, then, for consistent dosing across the population, applicator devices will need to be supplied. Moreover, patients will need a level of assurance that the microneedle device has actually been inserted properly into their skin. This would be especially true in cases of global pandemics or bioterrorism incidents where self-administration of microneedle-based vaccines becomes a necessity. Accordingly, a suitable means of confirming that skin puncture has taken place may need to be included within an applicator device or the microneedle product itself.

From a regulatory point of view, currently little is known about the safety aspects that would be involved with long-term usage of microneedle devices. In particular, studies will need to be conducted to assess the effect that repeated microporation has upon recovery of skin barrier function. However, given the minimally invasive nature of the micropores created within the skin following microneedle application, especially in comparison to the use of a hypodermic needle and the fact that statistically it is highly unlikely that microneedles would be inserted at exactly the same sites more than once in a patient’s lifetime, it is envisaged that microneedle technology will be shown to have a favourable safety profile. Indeed, skin barrier function is known to completely recover within a few hours of microneedle removal, regardless of how long the microneedles were in place. Local irritation or erythema (reddening) of the skin may be an issue for some patients. Since the skin is a potent immunostimulatory organ, it would be interesting to know whether repeated microneedle use would ever cause an immune reaction to the drug, excipients of microneedle materials and whether such an effect would be so significant as to cause problems for patients.

Infection is an issue that has long been discussed in relation to use of microneedle-based systems, since they, by necessity, puncture the skin’s protective stratum corneum barrier. However, as we have shown [105], microbial penetration through microneedle-induced holes is minimal. Indeed, there have never been any reports of microneedles causing skin or systemic infections. This may be because of the above-mentioned immune component of the skin or the skin’s inherent nonimmune, enzyme-based, defences. Alternatively, since the micropores are aqueous in nature, microorganisms may be more inclined to remain on the more hydrophobic stratum corneum.

Whether skin cleansing before microneedle application is necessary remains to be seen and is a vital question. Ideally, this would not have to be done, so as to avoid unnecessarily inconveniencing patients and making the use of the product in the domiciliary setting appear more akin to a self-administered injection than application of a conventional transdermal patch. Regulators will ultimately make the key decisions based on the weight of available evidence. Depending upon the application (e.g. drug/vaccine/active cosmeceutical ingredient delivery or minimally invasive monitoring), microneedle-based devices may be classed as drug delivery stems, consumer products or medical devices. From a delivery perspective, it will be important if microneedles are considered as injections rather than topical/transdermal/intradermal delivery systems, since this will determine whether the final product will need to be sterilised, prepared under aseptic conditions or simply host a low bioburden. Any contained microorganisms may need to be identified and quantified, as may the pyrogen content. Should sterilisation be required, then the method chosen will be crucial, since the most commonly employed approaches (moist heat, gamma or microwave radiation, ethylene oxide) may adversely affect the microneedles themselves and/or any contained active ingredient (e.g. biomolecules).