Microneedles are micron sized needles typically being one micron in diameter and 1-100 microns long. DMNs confer a major advantage over traditional hypodermic needles in that they pierce the outermost layer of the epidermis, the stratum corneum without stimulating the underlying pain receptors .
The three permeation pathways through the skin include transappendagel routes (shunt routes), transcellular routes and intercellular routes . The transappendagel and transcellular routes are important for hydrophilic drug molecules while the intercellular route lends itself to the transport of small, lipophillic drug molecules. The stratum corneum is composed of anucleated keratinised cells surrounding a lipid bilayer .
Despite being only 10-20 microns thick and of 10-15 cell layers it possesses a remarkable ability as a barrier to conventional (trans)dermal drug delivery(4). The diffusion coefficient of drug molecules within the stratum corneum is usually at least 500-10,000 times smaller than within the viable skin. Piercing of the skin transiently produces microconduits thus optimizing transdermal drug delivery in a painless and non invasive manner.
Dissolvable microneedles have been fabricated in a multitude of geometries employing in-plane and out-of plane processes via state of the art microelectronics and micromachinary . For the purpose of this study, DMN's were manufactured based on the fundamentals of a novel spray coating technology.
Spray coating is commonly employed in the pharmaceutical industry as a means of coating tablets with a film in the millimetre range. However with respect to microneedles a coating of micron proportions suffices . Spraying involves delivery of formulation droplets into PDMS microneedle moulds and the convergence of these deposited droplets to form a stable film coat.The resulting patches of drug loaded microneedles were subjected to a series of assays to demonstrate drug delivery to the skin. A Franz cell assay was employed to assess the permeation of a Ketoprofen 0.25% w/v- 5% v/v Polyvinylalcohol(PVA) co-formulation into the skin. Franz cells are static diffusion cells . The apparatus can be divided into an upper portion- " donor chamber" and a lower portion- "receptor chamber". Chambers are separated by an artificial or biological membrane. Artificial membranes are less expensive and easier sourced than biological membranes however they do not model the lipid perturbation effects relevant to biological membranes .
Porcine skin was our membrane of choice. Porcine skin mimics the structural and organisational layout of the lipid components found in human tissue and is therefore an excellent surrogate tissue . The initial stages of the project focused on manufacturing needle loaded patches .Batches were examined for viability in testing and then subjected to recovery assays.
2. Materials and Methods
2.1 Materials
Ketoprofen (Sigma Aldrich, St.Louis, MO, USA) and Polyvinylalcohol -"PVA" (Sigma Aldrich) were the constituents of the co-formulation. Carboxymethylcellulose sodium salt-"CMC" (Fluka, Finland) and glycerol 86-88% (Riedel de Haen, Germany) were employed as a backing layer. PolyDiMethylSiloxane-"PDMS" moulds (Tyndall National Institute, Ireland) were prepared by a wet-etch processing method. Phosphate buffer solution (PBS) tablets were sourced from Sigma.
2.2 Preparation of Spray formulation
Ketoprofen is a class II drug (low solubility and high permeability) under the BCS system for classification .5mg of Ketoprofen was weighed out in a 2ml eppendorf on an electronic balance. Using a Gilson pipette (2-20ul), 10ul of PVA 5%v/v was added to the eppendorf and vortexed for three minutes.
10ul were added successively until the drug had gone fully into solution as indicated by the vial turning from a cloudy suspension to a transparent solution ( Figure 1 below
This assay was repeated for validation, The apparent solubility of Ketoprofen in 5% v/v PVA was 5mg per 2ml respectively (0.25% Ketoprofen w/v). After vortexing, vials were sonicated for 30 minutes to aid drug solubility. Vials of the formulation were stored at room temperature until required for spraying.
2.3 Spray coating process
Spray coating was carried out using a Dusen-Schlick 970 S8 nozzle (sourced from Germany) with a diameter of 0.5mm.The nozzle was fed by a compressed air source and was also linked to the drug co-formulation by means of a 1ml syringe (Figure 2). Spraying was carried out in a fume hood. An adjustable stage was used to accommodate the moulds. Double sided sticky tape was affixed to a clean petri dish. Three PDMS 280um moulds were lined side by side and firmly attached to the tape.
The nozzle was sonicated in water for 30 minutes to remove any contaminants. Key parameters for the spray coating procedure were previously determined by means of Taguchi factorial design of experiment based on its time saving methods .
Successful parameters for spraying are outlined in Figure 3. Once the parameters were optimal, the formulation was injected manually into the nozzle at approximately 4ml/min. Between sprays the nozzle was flushed with 20ml of purified water to remove any water soluble contaminants.
This assay was repeated for validation, The apparent solubility of Ketoprofen in 5% v/v PVA was 5mg per 2ml respectively (0.25% Ketoprofen w/v). After vortexing, vials were sonicated for 30 minutes to aid drug solubility. Vials of the formulation were stored at room temperature until required for spraying.
2.3 Spray coating process
Spray coating was carried out using a Dusen-Schlick 970 S8 nozzle (sourced from Germany) with a diameter of 0.5mm.The nozzle was fed by a compressed air source and was also linked to the drug co-formulation by means of a 1ml syringe (Figure 2). Spraying was carried out in a fume hood. An adjustable stage was used to accommodate the moulds. Double sided sticky tape was affixed to a clean petri dish. Three PDMS 280um moulds were lined side by side and firmly attached to the tape.
The nozzle was sonicated in water for 30 minutes to remove any contaminants. Key parameters for the spray coating procedure were previously determined by means of Taguchi factorial design of experiment based on its time saving methods .
Successful parameters for spraying are outlined in Figure 3. Once the parameters were optimal, the formulation was injected manually into the nozzle at approximately 4ml/min. Between sprays the nozzle was flushed with 20ml of purified water to remove any water soluble contaminants.
