WO2009077859A1

WO2009077859A1 - Microneedle injecting apparatus

Info

Publication number: WO2009077859A1
Application number: PCT/IB2008/003734
Authority: WO
Inventors: Johanna Aaltje Bouwstra; Frederik Johannes Verbaan; Dirk-Jan Van Den Berg; Henk Verpoorten
Original assignee: Universiteit Leiden
Priority date: 2007-12-14
Filing date: 2008-12-12
Publication date: 2009-06-25
Also published as: GB0724402D0

Abstract

A method of delivering a microneedle array to a substrate including the steps of propelling the microneedle array towards the substrate utilising a microneedle applicator device which propels the microneedle array towards the substrate at a speed of 0.5 m/s to 5 m/s, the device subsequently withdrawing the microneedle array from the substrate.

Description

MICRONEEDLE INJECTING APPARATUS

This invention relates to a method of injecting a composition, for example into a mammal. It also relates to an apparatus suitable for performing such injections.

It is known to try to inject compositions into a substrate such as the skin of mammals, for example, humans, using a microneedle array. An example of this is WO-A-2006/055771 (3M Innovative Properties Company). This describes an applicator comprising an array of microneedles which is designed to propel a microneedle array at the skin when the applicator is at a predetermined distance from and orientation to the skin. A suitable propulsion velocity in this document is said to be between 2m/s and 20m/s. This document is predominantly concerned with patch devices.

One of the objects of such applicators is to pierce the stratum corneum (which is typically only 10 to 20 μm thick) and reach viable epidermis to allow therapeutic agents to pass through the layer into the tissues below; the mass and the length of the microneedles may be controlled to reduce the likelihood of the stimulation of nerve tissue at greater depths in the dermis, which can result in the sensation of pain.

WO 2666/108185 (3M Innovative Properties Company) describes a similar applicator which is particularly configured to sense the pushback force from the target when the applicator is placed against the target area (e.g. on the skin). The applicator is said to be particularly suitable for determining sites on the body for use of the microneedle array, especially for the application of patch devices carrying microneedle arrays.

Further, WO 2005/044333 (Alza Corporation) describes an applicator for applying a microprojection member at the stratum corneum. The microprojection member can be in the form of a patch having microprojections attached thereto, or a microprojection array. Suitable piercing elements are said to include needles, and may typically have a microprojection length of less than 500 microns, preferably less than 250 microns, and a microprojection thickness of 5 to 50 microns. Active agents may inter alia be coated on the microprojections, or otherwise administered from a physical reservoir. The applicator may deliver the microprojections to the skin preferably at an impact velocity of 1.0 -10 m/sec.

The listing or discussion of an apparent prior-published document in the specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

A problem identified by the inventors with previous microneedle array systems lies in the difficulty in getting the microneedle array to overcome the natural elasticity of skin and penetrate the skin to provide satisfactory delivery of topical agents through the skin. In addition, it has been found that the use of relatively short (e.g. 300μm or less) microneedle arrays may not satisfactorily and reproducibly pierce the skin, as there may be a tendency for skin to fold around the microneedle, resulting at best in only partial piercing of the skin.

The invention provides an improved method and apparatus for delivering a microneedle array to a substrate such as a mammal's skin, especially human skin.

Thus, according to a first aspect of the invention, there is provided a method of delivering a microneedle array to a substrate including the steps of propelling the microneedle array towards the substrate utilizing a microneedle applicator device which propels the microneedle array towards the substrate at a speed of between 0.5m/s and 5m/s. Preferablythe device also subsequently withdraws the microneedle array from the substrate.

In a further preferred aspect, there is provided a method of delivering a microneedle array to a substrate including the steps of propelling the microneedle array towards the substrate utilizing a microneedle applicator device which propels the microneedle array towards the substrate at a speed of between 0.5m/s and 5m/s, wherein the microneedles have an outer diameter of 150μm to 350μm.

In the embodiment, the substrate is mammalian skin, conveniently human skin.

In an embodiment, the microneedle array is retained in the substrate, typically for a period of 1 second to 100 seconds, conveniently 1 to 20 seconds, prior to being actively withdrawn from the substrate by the microneedle applicator device.

The method according to the invention has been found to be particularly suitable for delivery to the skin of compounds such as therapeutic agents through the stratum corneum. The application of the microneedle array to the stratum corneum at a controlled speed, using a microneedle applicator device which subsequently actively withdraws the device from the substrate, provides for a more satisfactory and uniform piercing of the stratum corneum, thereby overcoming the natural elasticity of skin, which has a tendency to resist piercing by microneedle arrays. Suitable therapeutic agents include biologically active agents of the type described in WO 2005/044333, the content of which is specifically included herein by reference, and vaccines.

Use of the microneedle arrays according to the invention reproducibly results in high transport rates of active compounds across the stratum corneum.

Conveniently, the method of delivering a microneedle array to the substrate can be used to cause a composition to be delivered to the substrate, in particular the interior of the substrate (e.g. more than 50μm, conveniently more than 200μm below the surface of the substrate). Conveniently, where the substrate is mammalian skin such as human skin, the composition may comprise a therapeutic agent, which therapeutic agent may be caused to be delivered to the interior of the substrate, i.e. beneath the stratum corneum. The composition or therapeutic agent so delivered can conveniently be applied to the surface at the same time as or subsequent to the delivery of the microneedle array to the surface, whereby the lacerations caused by the microneedle array may facilitate passage of the composition or thereapeutic agent from the surface of the substrate to beneath the surface. Where the substrate is mammalian skin, such as human skin, conveniently the composition or therapeutic agent is delivered beneath the stratum corneum in a procedure which is substantially painless.

According to a further aspect of the invention there is provided a microneedle applicator for inserting one or more microneedles into a substrate comprising:

- a housing engagable with the substrate; and

- an actuator coupled with a support member supporting the or each microneedle, and movable within the housing towards a first position to insert the or each microneedle into the substrate, and preferably towards a second position to remove the or each microneedle from the substrate.

Conveniently, the substrate is mammalian skin, for example, human skin.

The provision of an actuator that is movable between first and second positions to selectively insert and remove one or more microneedles from a substrate allows for the delivery of a metered dose of a compound such as a medicament, for example a hydrophilic drug, to a patient or other subject via the subject's skin.

Preferably the actuator is movable towards the first position at a variable predetermined velocity so as to insert the or each microneedle into the substrate at a given predetermined velocity.

The ability to vary the velocity at which the actuator moves towards the first position permits variation in the dose of medicament delivered to the subject, with higher velocities providing a greater transfer of, e.g. hydrophilic drug, than lower velocities. Optionally the actuator is able to maintain the first position for a predetermined ^' period of time to hold the or each microneedle in the substrate for the predetermined period of time. Such an arrangement will allow for increased transfer of compound across the substrate, for example, to the subject via his/her, or its, skin.

In a preferred embodiment of the invention the predetermined period of time is variable so as to hold the or each microneedle in the substrate for a given predetermined period of time.

The ability to vary the period of time for which the actuator is able to hold the or each microneedle in the substrate such as skin allows for tailoring of the microneedle insertion and removal regime according to, for example, the type of medicament being administered or the subject's skin type.

Preferably the actuator includes a solenoid having an iron core movable within a current-carrying coil. This arrangement is relatively compact and is able to move the actuator towards the first position in an essentially linear manner, i.e. with almost instantaneous acceleration up to a desired insertion velocity.

The iron core may be biased towards the second position. Biasing the iron core towards the second position permits automatic removal of the or each microneedle from the substrate. Furthermore, the inclusion of such biasing allows the applicator to adopt a fail safe configuration, i.e. a configuration in which the or each microneedle is removed from the skin portion, in the event of, e.g. electrical failure occurring.

In another preferred embodiment of the invention the coil is selectively electrically couplable across a capacitor. The ability to couple the coil across a capacitor provides the coil with a large burst of current which causes essentially linear movement of the iron core. In a further preferred embodiment of the invention the capacitor is further electrically couplable with a first voltage supply to selectively charge the capacitor. Such an arrangement provides a convenient way of charging the capacitor, thereby allowing repeated use of the applicator.

Optionally the magnitude of the first voltage supply is variable so as to vary the magnitude of charge applied to the capacitor. The ability to vary the magnitude of charge applied to the capacitor allows for variation in the current applied to the coil which, in turn, varies the strength of the magnetic field generated by the coil. This varies the force applied to the iron core, and so varies the velocity at which the iron core moves. Accordingly it is possible to vary the velocity at which the or each microneedle is inserted into the skin portion.

In a further preferred embodiment of the invention the capacitor is electrically coupled in parallel with a second voltage supply. The second voltage supply maintains a desired voltage across the coil and hence a desired minimum current flowing through the coil. The desired minimum current flowing through the coil generates a magnetic field which applies a sufficient electromagnetic force to the iron core to balance the effect of the biasing applied to the iron core, and thereby maintain the iron core, and the or each microneedle coupled therewith, in a stationary position relative to the housing. Such an arrangement, therefore, allows the actuator to hold the or each microneedle in the substrate such as the skin portion for a predetermined period of time.

Preferably the actuator further includes an electrical switch selectively movable between a first position to electrically couple the capacitor with the coil and a second position to electrically couple the capacitor with the first voltage supply. The inclusion of an electrical switch provides for a convenient way of switching between charging the capacitor and inserting the or each microneedle into the substrate. Optionally the switch is configurable to delay movement between the first and second positions. As a result movement of the actuator towards the second position is delayed, thereby allowing for a predetermined delay between inserting the or each microneedle into the substrate and removing the or each microneedle from the substrate.

By "array" in the context of this description is meant structures which are capable of piercing the stratum corneum to facilitate the transdermal delivery of therapeutic agents or the sampling of fluids through or to the skin. Conveniently, an array will comprise at least one, and conveniently two or more microneedles. In the context of this invention though, "microneedle arrays" excludes patches comprising microneedles, which are adhesive substrates which are applied to and retained on the skin as the stratum corneum is pierced, the microneedles facing and being retained in the stratum corneum.

However, methods according to the invention may include the use of adhesive patches which do not have microneedle arrays, for example similar to sticking plasters, which act simply to retain a composition or therapeutic agent in the vicinity of lacerations in the substrate caused by the microneedle array, which adhesive plasters may be applied subsequent to piercing of the skin.

Also within the context of this description "microneedles" refer to the specific microscopic structures associated with the array that are capable of piercing the stratum corneum to facilitate the transdermal delivery of therapeutic agents or the sampling of fluids through the skin. By way of example, microneedle structures can include needle or needle-like structures, as well as other structures capable of piercing the stratum corneum.

Conveniently the microneedles used in the context of this invention will have a length less than 450μm, conveniently less than 350μm, preferably less than

300μm. They will typically have a length greater than lOOμm, typically more than 150μm, typically more than 200μm. All the microneedles in the microneedle array may typically have these lengths. They may typically have diameters ranging from 150μm to 350μm, preferably from 200μm to 300μm. They will typically be sharpened, i.e. they will have a bevelled point, which bevelled point may be symmetrical or unsymrnetrical.

The microneedles used according to the invention may have any desirable cross- section, but they may typically be circular in cross-section. The microneedles may be solid, or they may have some form of channel in them, which channel may be on the exterior surface of the microneedle, or it may be in the form of a central hollow lumen. Where the microneedles have channels such as hollow central lumens, they may conveniently be provided in fluid communication with a reservoir which is capable of storing a composition or therapeutic agent for delivery to below the surface of the substrate. In certain embodiments, the microneedles may be solid. The microneedles may be coated or uncoated.

The microneedles are arranged in the form of an array. The array will comprise at least two microneedles, preferably at least four microneedles; in certain preferred embodiments it may comprise up to about 150 microneedles.

Conveniently, in the array the microneedles are substantially equally spaced, and may be arranged in geometric patterns, for example, as spheres, rectangles, diamonds or circles. Certain preferred arrays include arrays of 3 x 4, 4 x 4, 6 x 6 and 9 x 9 microneedles; these may be arranged in equally spaced square-shaped arrays.

In the invention the microneedles are conveniently delivered to the substrate (e.g. mammalian skin, conveniently human skin) at speeds in the range of 0.5m/s to 5m/s, conveniently lm/s to 4m/s. In some circumstances, the use of a higher delivery speed may result in a higher delivery rate of active compounds across the stratum corneum. Preferred usages of the method and device according to the invention include the use of the device to make lacerations in the stratum corneum, in the vicinity of which are applied a therapeutic agent which may optionally be covered with a dressing, the therapeutic agent having its absorption into the dermis enhanced by the lacerations. Alternatively, the device may be equipped with a microneedle array which is attached to a reservoir containing the therapeutic agent, which once the microneedle array has punctured the stratum corneum may be caused to deliver the therapeutic agent through channels in the microneedles. A further preferred use may involve coating the therapeutic agent onto the microneedles, which in the method of the invention are briefly retained in the substrate prior to their withdrawal.

The invention will now be described by way of example only with reference to the accompanying Figures, in which:

- Figure 1 shows a schematic view of a microneedle applicator for use according to the invention;

- Figure 2 shows the various microneedle arrays used in this study are i) commercially available (assembled) needles in a 4x4 array (a) and a higher magnification of a single microneedle (b), solid microneedle arrays in a 4x4 array (c) and a higher magnification of a single solid microneedle

- Figure 3 shows the velocity of the microneedles using the electrically driven applicator: The velocity is linearly related to the applied voltage;

- Figure 4 shows piercing of assembled 4x4 assembled microneedle arrays and 4x4 solid microneedle arrays across full thickness human skin when applied manually with a velocity of 1 m/s , 3 m/s or manually. Piercing was visualized with the Trypan blue assay; - Figure 5 shows piercing of assembled 4x4 assembled microneedle arrays and 4x4 solid microneedle arrays across dermatomed human skin when applied manually with a velocity of 1 m/s, 3 m/s or manually. Piercing was visualized with the Trypan blue assay; and - Figure 6 shows transport of Cascade Blue (CB) after pre-treatment of dermatomed human skin with 4x4 solid microneedle arrays (a) and 4x4 assembled microneedle arrays (b) pierced through the skin with a velocity of 3 m/s, 1 m/s or manually. Data are presented as averages ± SEM, Each curve is at least 6 replicas of at least three donors.

Examples

Example 1

The microneedle applicator according to a first embodiment of the invention is designated generally by the reference numeral 100.

The applicator 100 includes a housing 102, only a nose portion 104 of which is shown, and an actuator 106.

The actuator 106 is coupled with a support member 108 which supports sixteen 0.3mm diameter microneedles 110 arranged in a 4x4 array. The microneedles 110 are spaced from one another by 1.25mm. Other embodiments of the invention may include a different number, size and/or arrangement of microneedles.

The actuator 106 is movable within the housing 102 towards a first position (not shown) to insert the microneedles 110 into a skin portion 112, and towards a second position, as shown, to remove the microneedles 110 from the skin portion 112.

To allow such movement of the actuator 106 the support member 108 is movably received within the nose portion 104. The nose portion 104 is able to space the microneedles 110 from the skin portion 112 by a desired amount while the actuator 106 is in the second position.

In the embodiment shown the actuator 106 is movable towards the first position at a variable predetermined velocity.

In addition, the actuator 106 shown is able to maintain the first position for a variable predetermined time.

The actuator 106 includes a solenoid 114 having an iron core 116 that is movable within a current-carrying coil 118. In the embodiment shown the iron core 116 is biased by a biasing member 120, in the form of a spring 122, towards the second position. Other embodiments of the invention may employ a differing form of biasing member.

The coil 118 is selectively electrically couplable across a capacitor 124.

The capacitor 124 is selectively electrically couplable with a variable first voltage supply 126 to charge the capacitor 124. In addition the capacitor 124 is electrically coupled in parallel with a second voltage supply 128. A diode 140 prevents current flowing from the variable first voltage supply 126 to the second voltage supply 128.

The actuator 106 further includes an electrical switch 130 which selectively connects the capacitor 124 with the coil 118 and the first voltage supply 126. In particular, the electrical switch 130 is selectively movable between a first position

132 to electrically couple the capacitor 124 with the coil 118 and a second position 134, as shown, to electrically couple the capacitor 124 with the first voltage supply 126.

The electrical switch 130 is configurable to delay movement between the first and second positions 132, 134.

In use of the applicator 100, a user adjusts the magnitude of the first voltage supply 126 to a desired level and adjusts the delay time of switch 130 to a desired time period, and ensures that the electrical switch is in the second position 134 so as to apply a desired magnitude of charge to the capacitor 124.

With the electrical switch 130 in the second position 134 no current flows through the coil 118, and so the iron core 116 is biased towards the second position.

When desired, the user moves the electrical switch 130 into the first position 132. Preferably the user does this using his or her foot.

When the switch 130 is in the first position the charge applied to the capacitor 124 is discharged through the coil 118, thereby causing a current to flow through the coil 118. Flow of such a current generates an electromagnetic field which attracts the iron core 116 and moves the iron core 116 towards the first position to insert the microneedles 110 into the skin portion 112.

The magnitude of the current flowing through the coil 118 determines the force applied to the iron core 116, and hence the velocity at which the iron core 116 moves towards the first position.

Once the charge stored by the capacitor 124 has dissipated, the second voltage supply 128 maintains sufficient current flow in the coil 118 to generate an electromagnetic holding force on the iron core 116 which is at least equal to the biasing force applied to the iron core 116 by the biasing member 120. This holds the iron core 116 in position relative to the housing 102, and thereby holds the microneedles 110 in the skin portion 112.

Following a predetermined delay, as chosen by the user by configuring the switch 130, the switch 130 moves from the first position 132 back to the second position 134 to disconnect the second voltage supply 128 from the coil 118. This removes the holding force on the iron core 116, and so allows the biasing member 120 to move the iron core 116 towards the second position, thereby removing the microneedles 110 from the skin portion 112.

Such movement of the switch 130 to the second position reconnects the first voltage supply 126 and recharges the capacitor 124, thereby permitting re-use of the applicator 100.

The embodiment also comprises a control box not shown) which is connected to the devices in which the delay and the voltage (=speed) can be set or varied.

Example 2

In this Example, the transdermal transport of the dyestuff Cascade Blue (MW 538 Da) was investigated using a delivery device delivering a microneedle array at a velocity of between InVs and 3m/s.

Materials and Methods

Materials

Cascade Blue (CB, MW 538) was purchased from In Vitrogen (Breda, The Netherlands). All other chemicals were from Sigma-Aldrich (Zwijndrecht, The Netherlands). The solid microneedle arrays were kindly provided by Transferium (Almelo, The Netherlands). Microneedle array fabrication

In these examples we used two types of microneedle arrays:

1. Assembled microneedle arrays were manufacture from commercially available 3OG hypodermic needles. As such, the individual microneedles have a diameter of about 300 μm and a length tailored to 300 μm. The microneedles are hollow (1). The pitch of the microneedles is 1.25 mm, resulting in a microneedle array with a surface of 16.4 mm². The tapered shaft length (bevelled edge) of these hypodermic needles is approximately 1.2 mm, which results in a very sharp tip. The microneedles were fixed in a polyetherketone mo Id ( 1 ) .

2. Solid microneedle arrays were assembled from stainless steel wire with a diameter of 200 μm and a length of 300 μm. The individual microneedles were cut tangentially to obtain sharp tips. The microneedles were fixed in a Polyetheretherketone mold, similar as the assembled microneedle arrays. The pitch between the solid microneedles is also 1.25 mm. The tapered shaft of the solid needles has a length of approximately 280 μm, resulting in a less sharp tip than those of the hypodermic needles.

The microneedle number array used was 4x4.

An electrically driven microneedle applicator as described in Example 1 was utilised. An array of microneedles is positioned at the end of the applicator and held in place by a metal holder shaped in the form of the mold of the microneedle arrays, which is protected by a Perspex cover. The device contains a coil through which current can be passed, resulting in a magnetic driving force allowing a metal rod to be pushed out of the coil, moving the, attached microneedle array. The applied voltage was varied between 24 and 60V, varying the magnetic driving force (and thus insertion speed) accordingly. A high speed camera with a known frame rate was utilised to visualize the movement of the microneedle array from which the speed of the microneedle array could be calculated. With regard to the manual piercing of skin using microneedle arrays, dermatomed human skin (DHS) was stretched on parafilm to counteract the elasticity of the skin after which disks of DHS (0 22 mm) was punched. Subsequently, the skin was supported by Styrofoam to protect the microneedles from damaged and pierced for 1 minute during which the microneedles are clamped with the polyetherketone mold in a conventional manual applicator. The applied pressure is approximately 50 N.

Preparation of human skin Abdominal or breast skin was obtained from local hospitals following cosmetic surgery and was used within 24 hours after surgery. Full thickness skin was prepared by removing residual subcutaneous fat using a surgical scalpel. The stratum corneum side was carefully wiped with 70% ethanol and demiwater.

Dermatomed human skin was prepared by fixing full thickness skin on a styrofoam support and the skin was dermatomed to a thickness of 300 - 400 μm using a Padgett Electro Dermatome Model B (Kansas City, USA) as described by Nugroho et al (2) The skin was stored at -80 °C until use, except for transport studies where only fresh human skin was used.

Piercing of human skin and visual inspection in vitro

Full thickness skin or dermatomed human skin was stretched on parafilm to counteract the elasticity of the skin. Subsequently, the skin was supported by styrofoam to protect the microneedles from damage and pierced using the electrically driven applicator. Visualization studies were performed macro- and microscopically after the skin had been pierced with the applicator using either a velocity of 3 m/s or by manual piercing of the skin. To evaluate the uniformity of piercing, the stratum corneum (SC) side of the pierced skin was covered with 0.4% Trypan Blue dye in phosphate buffer saline (PBS: NaCl: 8 g/1, KCl : 0.4 g/1, KH2PO4 : 0.4 g/1, Na2HPO4 : 2.86 g/1) for 1 hour. Full thickness skin was first dermatomed to a thickness of 300 - 400 μm before application of the Trypan Blue dye. The excess dye was then removed and the skin or epidermal sheet was evaluated for the appearance of blue dots on the dermal side of the skin or epidermal sheets.

Diffusion studies

In vitro transport studies were performed in order to examine the effect of microneedle pretreatment on the permeation of CB across dermatomed human skin. Transport studies were performed after the following pretreatments: a. Variation in microneedle arrays: 4x4 solid microneedle arrays or 4x4 assembled microneedle arrays (assembled from commercially available 3OG hypodermic needles). The microneedles were pierced into the skin with a velocity of 3 m/s. b. Variation in velocity of the microneedles when piercing the skin. Manual application, 1 m/s velocity and 3 m/s velocity of the microneedles when piercing the skin was compared. c. Stripped dermatomed skin: Dermatomed human skin was clamped between two circular plates fixed together with 4 screws. In the central part of upper plate, facing the stratum corneum side of the skin, a circular hole with a diameter of 1 cm was made to perform tape-stripping. Tape-stripping was carried out using an adhesive tape (Scotch® Magic™ Tape 810, 3M France,

France), which was pressed onto the skin for 5 seconds. Tape-strips were then rapidly pulled off with 1 fluent stroke. The direction of tape-stripping was alternated. Tape-stripping was continued until the viable epidermis was reached. In vitro transport studies were performed as described by Grams et al. (3) using continuous flow-through diffusion cells (PermeGear, Inc., Bethlehem, USA). The diffusion area was 1.16 cm². After pretreatment with microneedle arrays, dermatomed human skin or stripped dermatomed human skin was placed between the donor compartment and receptor compartment with the SC side facing the donor compartment. A disk of wire gauze (0 18 mm) was used to support the DHS. The acceptor phase was PBS (NaCl: 8 g/1, KCl : 0.4 g/1, KH₂PO₄ : 0.4 g/1,

Na₂HPO₄ : 2.86 g/1) and was kept at 37⁰C resulting in a skin surface temperature of approximately 32 °C in the diffusion cell. The donor solutions contained 0.2 mM cascade blue in PBS. The volume applied was 1 ml. The flow rate of the PBS in the acceptor chamber was maintained at approximately 1 ml/h by a peristaltic pump (Ismatec SA, Glattbrugg, Switzerland). The donor compartment was covered with 3M scotch tape to establish occlusive conditions. Hourly fractions of the acceptor phase were collected over a period of 20 h and analyzed by HPLC.

Analysis by HPLC

The fluorescent intensity of CB was determined by injecting 10 μl of sample on a Waters HPLC system consisting of a Waters 600 controller coupled to a Waters

717 plus auto sampler and a Waters 474 Scanning Fluorescence Detector (Waters

Chromatography B.V., Etten-Leur, The Netherlands) and a column Inertsil, 5 μm,

ODS-3, 250 mm x 3 mm (Varian B.V., Middelburg, The Netherlands). Excitation and emission wavelengths of 401 and 431 nm were used. The mobile phase consisted of acetonitrile and phosphate buffer (1OmM) with 1OmM tert-butyl amine at pH 7.5 in a ratio of 1 : 1.33 v/v.

Microneedles

A microneedle array assembled from commercially available 30G hypodermic needles, is depicted in Figures 2A and B These microneedle arrays are referred to as assembled microneedle arrays having a length of 300 μm. The microneedle arrays from stainless steel wire (300 μm in length) are shown in Figures 2C and D and will be referred to as solid microneedles.

Applicator design: speed variation

The speed with which the microneedles can be moved by the applicator was measured using a high speed camera with a known frame rate to visualize the movement of the microneedle array from which the speed of the microneedle array could be calculated. Figure 3 shows that the speed can be varied linearly from about 1 to 3 m/s corresponding to 24V to 60V, respectively. In vitro evaluation of piercing human dermatomed skin

Dermatomed skin: To study the effect of the electrically driven applicator on microneedle piercing was applied to the SC side of dermatomed skin was pierced. Then Trypan Blue Dye was applied on the pierced site. The microneedles were pierced into the skin with a speed of 3 m/s or 1 m/s. As a comparison, microneedles were applied manually with the previously described hand-held applicator (1). The presence of conduits in the skin was visualized by the appearance of blue dots at the dermal side. Figure 4 shows that for all types of microneedle arrays, blue dots are formed when they are applied with the electrically driven applicator. Furthermore, not only 4x4, but also 6x6 and 9x9 microneedle arrays were able to pierce dermatomed human skin. The appearance of blue dots was almost absent when dermatomed human skin was pierced manually with the same microneedle arrays. These results demonstrate that the electrically driven applicator has superior piercing properties compared to the manual applicator.

To study the effect of skin thickness on the piercing into the skin, we performed a similar experiment using full thickness human skin. Figure 5 shows piercing of the skin with the short microneedles, similarly to dermatomed human skin, which again indicates that the electrically driven applicator is able to overcome the flexibility of the skin and has superior piercing properties compared to the manual applicator.

Results and Discussion Transport studies were performed in order to study the effect of i) piercing velocity of the microneedles into the skin, ii) the microneedle shape and iii) the number of microneedles in the array. As model compound CB with a molecular weight of 538 was used.

Piercing velocity of the microneedles: The transport of CB was determined after pretreatment of DHS with assembled microneedle arrays and solid microneedle arrays, which were inserted into the skin with two different insertion velocities (1 and 3 m/s). Insertion of microneedles using the manual applicator was performed as a comparison. For all situations, the control skin (Le. no microneedle array pretreatment) resulted in non-detectable fluxes as shown in Figure 6. Manual pretreatment with microneedle arrays resulted in a slight but significant increase in the flux of CB, indicating that some of the short microneedles protruded the dermatomed human skin when applied manually. However, very interestingly, for all microneedle arrays, pretreatment with microneedle arrays applied by the electrically driven microneedle array applicator resulted in a drastic increase in transport of the compound across dermatomed human skin. The obtained CB fluxes (see table 1) were higher after insertion with a velocity at 3 m/s compared to an insertion velocity of 1 m/s. As far as the shape of the flux curves is concerned after an initial period of 2-4 hours there is still a gradual increase in transport, except for the pretreatment of the solid microneedles inserted at 3 m/s, where the transport rate was slightly reduced after 12 hours (see Figure 6).

Type of microneedle array: When comparing the 4x4 assembled microneedle arrays and the 4x4 solid microneedle arrays, it is clearly demonstrated that besides the insertion speed, the shape of the arrays also determines the CB transport rate across the dermatomed human skin. The solid microneedle arrays resulted in higher fluxes than the assembled microneedle arrays, see Figure 6.

In these Examples, we showed that the use of an impact insertion system greatly facilitates the insertion of microneedle arrays with short length (Le. 300 μm) into dermatomed human and full thickness human skin in vitro.

Microfabrication techniques for the production of silicon, metal, glass and polymer microneedle arrays with micrometer dimensions have been described. Production of microneedles often is highly specialized and includes complex multi-step processes (4, 5, 6, 7, 8), the contents of these references in as far as they relate to microneedle and preparation being specifically incorporated herein. Very recently several studies were reported in which coated microneedles were designed (9, 10, 11). In these Examples we used 4x4 microneedle arrays from commercially available 3OG hypodermic needles or stainless steel wire. Fabrication of microneedles prepared from stainless steel wire can also be modified for mass production.

The 30G hypodermic needles and the solid microneedles are fixed on a backplate forming arrays having a needle length of 300 μm. While the 30G hypodermic needles are very sharp with a long shaft of approximately 1.2 mm, the length of the shaft of the solid microneedles is around 280 μm. The tip shape of the latter is triangular with an angle of approximately 45°.

As is clear from Figures 4 and 5, using a velocity of only 3 m/s, microneedles with a length of 300 μm were inserted into the skin in a very reproducible manner, which contrasted with the results with the manual applicator. Therefore, the speed of insertion drastically affects piercing properties of the microneedle arrays. In these piercing studies we did not observe any difference in piercing ability between the differently shaped microneedles. In other words, piercing of the 30G hypodermic needles with a very sharp tip were very similar to that of the solid microneedles with a less sharp tip.

In order to study the effect of microneedle insertion velocity on the piercing ability in more detail transport studies were performed using the model drug CB. Two insertion velocities were used to pretreat dermatomed human skin, namely 1 and 3 m/s. From these studies it is clear that a higher insertion velocity results in a higher transport rate. This is observed for the two types of microneedle arrays used in this study. Furthermore, even an insertion velocity of only 1 m/s already resulted in a drastic increase in the transport rate compared to the manual application. When comparing the effect of the different microneedle arrays on the transport rate of CB, it is clear that the solid microneedle array pretreatment results in significant higher fluxes than the assembled microneedle array pretreatment. This shows that in addition to the insertion velocity, indeed, the shape of the microneedles also affects the transport across the skin. It is possible that, in case of the solid microneedle arrays, due to the less sharp tip, the protrusions in the skin are somewhat larger than in case of the assembled microneedle arrays, which might result in higher transport rates.

Table 1

CB mean fluxes±s.e.m. between 11 and 20 hrs ex ressed in mol/cm²/hr.

* M= manually piercing

References

(1) FJ. Verbaan, S.M. BaI, DJ. van den Berg, W.H. Groenink, H. Verpoorten, R. Luttge, J.A. Bouwstra, Assembled microneedle arrays enhance the transport of compounds varying over a large range of molecular weight across human dermatomed skin J Control Release. 117 (2007) 238-45.

(2) A.K. Nugroho, G.L. Li, M. Danhof, J.A. Bouwstra, Transdermal iontophoresis of rotigotine across human stratum corneum in vitro: influence of pH and NaCl concentration. Pharm Res 21 (2004) 844-850. (3) Y. Y. Grams, J.A. Bouwstra, Penetration and distribution of three lipophilic probes in vitro in human skin focusing on the hair follicle. J Control Release 83 (2002) 253-262.

(4) M.A. Teo, C. Shearwood, K.C. Ng, J. Lu, S. Moochhala, In vitro and in vivo characterization of MEMS microneedles. Biomed Microdevices 7(1) (2005) 47-52.

(5) J.H. Park, M.G. Allen, M.R. Prausnitz, Polymer microneedles for controlled-release drug delivery. Pharm Res 23(5) (2006) 1008-1019.

(6) S.P. Davis, W. Martanto, M.G. Allen, M.R. Prausnitz, Hollow metal microneedles for insulin delivery to diabetic rats. IEEE Trans Biomed Eng 52(5) (2005) 909-915.

(7) B. Ziaie, A. Baldi, M. Lei, Y. Gu, R.A. Siegel, Hard and soft micromachine for Bio MEMS: review of techniques and examples of applications in micro fluidics and drug delivery. Advanced Drug Delivery Reviews 56 (2004) 145. (8) Y. Ito, Yoshimitsu, J, Shiroyama K, Sugioka N, Takada K. Self-dissolving microneedles for the percutaneous absorption of EPO in mice, J. Drug

Target. 2006 (14) 255-261. (9) M. Cornier, B. Johnson, M. Ameri, Kofi Nyam, L. Libiran, D. D. Zhang, P.

Daddona J. Contr. ReI. 97 (2004) 503-511. (10) H.S. Gill, M.R. Prausnitz, Coated microneedles for transdermal delivery. J.

Contr. ReI. 117 (2007) 227-237. (11) H.S. Gill, M.R. Prausnitz, Coating Formulations for microneedles. Pharm. Res. 24 (2007 1369-1380.

Claims

1. A method of delivering a microneedle array to a substrate including the steps of propelling the microneedle array towards the substrate utilising a microneedle applicator device which propels the microneedle array towards the substrate at a speed of 0.5 m/s to 5 m/s, the device subsequently withdrawing the microneedle array from the substrate.

2. A method according to Claim 1, wherein the substrate is mammalian skin.

3. A method according to Claim 2, wherein the mammalian skin is human skin.

4. A method according to any one of the preceding claims, wherein prior to withdrawal of the microneedle array the array is retained in the substrate for a period of 1 to 100 seconds.

5. A method according to Claim 4, wherein prior to withdrawal of the microneedle array the array is retained in the substrate for a period of 1 to 20 seconds.

6. A method according to any preceding claim, wherein the delivery of the microneedle array to the substrate is used to deliver a compound to the substrate.

7. A method according to Claim 6, wherein the compound is a therapeutic agent or a vaccine.

8. A method according to any preceding claim, wherein the microneedle array comprises microneedles having a length less than 450μm, conveniently less than 350μm, conveniently less than 300μm.

9. A method according to any preceding claim, wherein the microneedle array comprises microneedles having a length greater than lOOμm, conveniently greater than 150μm, conveniently greater than 200μm.

10. A method according to any preceding claim, wherein the microneedle array comprises microneedles having a diameter in the range of 150 μm to 350 μm, preferably 200 to 300 μm.

11. A method according to any preceding claim, wherein the microneedle array comprises microneedles which have a bevelled point.

12. A method according to any preceding claim, wherein the microneedle array are circular in cross-section.

13. A method according to any preceding claim, wherein the microneedles in the microneedle array are solid.

14. A method according to claims 1 to 12, wherein the microneedles in the microneedle array comprise a channel.

15. A method according to Claim 14, wherein the channel is on the surface.

16. A method according to Claims 14 or 15, wherein the channel is a hollow central lumen.

17. A method according to any preceding claim, wherein the microneedle array comprises at least 2 microneedles.

18. A method according to claim 17, wherein the microneedle array comprises at least four microneedles.

19. A method according to any preceding claim, wherein the microneedle array comprises up to about 150_«microneedles.

20. A method according to any preceding claim, wherein the microneedles in the microneedle array are equally spaced.

21. A method according to any preceding claim, wherein the microneedles in the microneedle array are arranged in a geometric pattern.

22. A method according to any preceding claim, wherein the microneedle array is propelled towards the substrate by the microneedle applicator device at a speed of at least 1 m/s.

23. A method according to any preceding claim, wherein the microneedle array is propelled towards the substrate by the microneedle applicator device at a speed of up to 4 m/s.

24. A microneedle applicator, for inserting one or more microneedles into a substrate, comprising: a housing engagable with the substrate; and an actuator coupled . with a support member supporting the or each microneedle, and movable within the housing towards a first position to insert the or each microneedle into the substrate and towards a second position to remove the or each microneedle from the substrate.

25. A microneedle applicator according to Claim 24, wherein the actuator is movable towards the first position at a variable predetermined velocity so as to insert the or each microneedle into the substrate at a given predetermined velocity.

26. A microneedle applicator according to claim 24 or Claim 25, wherein the actuator is able to maintain the first position for a predetermined period of time to hold the or each microneedle in the substrate for the predetermined period of time.

27. A microneedle applicator according to claim 26, wherein the predetermined period of time is variable so as to hold the or each microneedle in the skin for a given predetermined period of time.

28. A microneedle applicator according to any one of claims 24 to 27, wherein the actuator includes a solenoid having an iron core movable within a current- carrying coil.

29. A microneedle applicator according to claim 28, wherein the iron core is biased towards the second position.

30. A microneedle applicator according to claim 29, wherein the coil is selectively electrically couplable across a capacitor.

31. A microneedle applicator according to Claim 30, wherein the capacitor is further electrically couplable with a first voltage supply to selectively charge the capacitor.

32. A microneedle applicator according to Claim 31 , wherein the magnitude of the first voltage supply is variable so as to vary the magnitude of charge applied to the capacitor.

33. A microneedle applicator according to any of claims 30 to 32, wherein the capacitor is electrically coupled in parallel with a second voltage supply.

34. A microneedle applicator according to any of Claims 31 to 33, wherein the actuator further includes an electrical switch selectively movable between a first position to electrically couple the capacitor with the coil and a second position to electrically couple the capacitor with the first voltage supply.

35. A microneedle applicator according to Claim 34, wherein the switch is configurable to delay movement between the first and second positions.

36. A microneedle applicator generally as herein described with reference to and/or as illustrated in the accompanying drawings.