muscle. Skin is a particularly attractive target for vaccine delivery as
it is enriched with resident immune cells, the Langerhans cells
[17]. These cells are efficient antigen presenting cells which help
in mounting a robust immune response against any antigen [18].
1.1.2 Particle
Bombardment of the Skin
Ballistic methods or particle-based methods accelerate powdered
vaccines to high velocities that drive them into the skin. Usually,
stratum corneum and layers of dermis are the sites of their deposition. Such a method of delivering vaccines into the epithelium is
called epidermal powder immunization (EPI) [19]. This technique
was developed in 1986, for the delivery of DNA-coated metal
particles of $1 μm in diameter into plants to genetically modify
them, and it was known as the gene gun [20]. The ballistic method
was further developed to deliver conventional and DNA vaccines to
humans [21]. Ballistic methods typically deliver to the superficial
layer of the skin thereby targeting Langerhans cells [22].
1.1.3 Topical Application
to the Skin
Application of medicine to the skin is the easiest method of drug
delivery. This method has been used for thousands of years. Systemic drug delivery through the skin developed with the use of
transdermal scopolamine patches for treating motion sickness
[23]. Administration of vaccines via the dermal route dates to
more than 1000 years. Immunization against smallpox was
practiced in India by scratching dry scabs from smallpox lesions
onto the skin of healthy individuals [24]. Even today, the skin
remains the site for immunization against smallpox using the bifurcated needle [25]. Although this method has a historical role, only
recent times have drawn attention of the healthcare community to
the use of topical vaccine application as a general mode of
immunization [26].
1.1.4 Topical Adjuvants
Topical application of adjuvants such as cholera toxin
(CT) together with the vaccine on the skin is known to generate a
robust mucosal and systemic immune response [27]. This is an
extensively studied method of all topical-immunization methods.
Topical application of CT provides the required activation signal for
Langerhans cells to mature and thereby transforming them into
potent antigen-presenting cells [28]. These cells then prime the
immune response to co-administered vaccines. However, the role
of CT, a relatively large protein (86 kDa), which diffuses across the
stratum corneum, remains unclear [29]. However, using emery
paper (abrasive paper) to disrupt stratum corneum has been
demonstrated to add an adjuvant effect to the application of vaccine
and achieve efficient immune responses [30].
Methods of Needle-Free Injections
439
it is enriched with resident immune cells, the Langerhans cells
[17]. These cells are efficient antigen presenting cells which help
in mounting a robust immune response against any antigen [18].
1.1.2 Particle
Bombardment of the Skin
Ballistic methods or particle-based methods accelerate powdered
vaccines to high velocities that drive them into the skin. Usually,
stratum corneum and layers of dermis are the sites of their deposition. Such a method of delivering vaccines into the epithelium is
called epidermal powder immunization (EPI) [19]. This technique
was developed in 1986, for the delivery of DNA-coated metal
particles of $1 μm in diameter into plants to genetically modify
them, and it was known as the gene gun [20]. The ballistic method
was further developed to deliver conventional and DNA vaccines to
humans [21]. Ballistic methods typically deliver to the superficial
layer of the skin thereby targeting Langerhans cells [22].
1.1.3 Topical Application
to the Skin
Application of medicine to the skin is the easiest method of drug
delivery. This method has been used for thousands of years. Systemic drug delivery through the skin developed with the use of
transdermal scopolamine patches for treating motion sickness
[23]. Administration of vaccines via the dermal route dates to
more than 1000 years. Immunization against smallpox was
practiced in India by scratching dry scabs from smallpox lesions
onto the skin of healthy individuals [24]. Even today, the skin
remains the site for immunization against smallpox using the bifurcated needle [25]. Although this method has a historical role, only
recent times have drawn attention of the healthcare community to
the use of topical vaccine application as a general mode of
immunization [26].
1.1.4 Topical Adjuvants
Topical application of adjuvants such as cholera toxin
(CT) together with the vaccine on the skin is known to generate a
robust mucosal and systemic immune response [27]. This is an
extensively studied method of all topical-immunization methods.
Topical application of CT provides the required activation signal for
Langerhans cells to mature and thereby transforming them into
potent antigen-presenting cells [28]. These cells then prime the
immune response to co-administered vaccines. However, the role
of CT, a relatively large protein (86 kDa), which diffuses across the
stratum corneum, remains unclear [29]. However, using emery
paper (abrasive paper) to disrupt stratum corneum has been
demonstrated to add an adjuvant effect to the application of vaccine
and achieve efficient immune responses [30].
Methods of Needle-Free Injections
439