Mucosal deformation from an impinging transonic gas jet and the ballistic impact of microparticles

Microneedle-assisted microparticle delivery by gene guns: experiments and modeling on the effects of particle characteristics

Microneedles (MNs) have been shown to enhance the penetration depths of microparticles delivered by gene gun. This study aims to investigate the penetration of model microparticle materials, namely, tungsten (<1 μm diameter) and stainless steel (18 and 30 μm diameters) into a skin mimicking agarose gel to determine the effects of particle characteristics (mainly particle size). A number of experiments have been processed to analyze the passage percentage and the penetration depth of these microparticles in relation to the operating pressures and MN lengths. A comparison between the stainless steel and tungsten microparticles has been discussed, e.g. passage percentage, penetration depth. The passage percentage of tungsten microparticles is found to be less than the stainless steel. It is worth mentioning that the tungsten microparticles present unfavourable results which show that they cannot penetrate into the skin mimicking agarose gel without the help of MN due to insufficient momentum due to the smaller particle size. This condition does not occur for stainless steel microparticles. In order to further understand the penetration of the microparticles, a mathematical model has been built based on the experimental set up. The penetration depth of the microparticles is analyzed in relation to the size, operating pressure and MN length for conditions that cannot be obtained in the experiments. In addition, the penetration depth difference between stainless steel and tungsten microparticles is studied using the developed model to further understand the effect of an increased particle density and size on the penetration depth.

Microneedle assisted micro-particle delivery from gene guns: experiments using skin-mimicking agarose gel

A set of laboratory experiments has been carried out to determine if micro-needles (MNs) can enhance penetration depths of high-speed micro-particles delivered by a type of gene gun. The micro-particles were fired into a model target material, agarose gel, which was prepared to mimic the viscoelastic properties of porcine skin. The agarose gel was chosen as a model target as it can be prepared as a homogeneous and transparent medium with controllable and reproducible properties allowing accurate determination of penetration depths. Insertions of various MNs into gels have been analysed to show that the length of the holes increases with an increase in the agarose concentration. The penetration depths of micro-particle were analysed in relation to a number of variables, namely the operating pressure, the particle size, the size of a mesh used for particle separation and the MN dimensions. The results suggest that the penetration depths increase with an increase of the mesh pore size, because of the passage of large agglomerates. As these particles seem to damage the target surface, then smaller mesh sizes are recommended; here, a mesh with a pore size of 178 μm was used for the majority of the experiments. The operating pressure provides a positive effect on the penetration depth, that is it increases as pressure is increased. Further, as expected, an application of MNs maximises the micro-particle penetration depth. The maximum penetration depth is found to increase as the lengths of the MNs increase, for example it is found to be 1272 ± 42, 1009 ± 49 and 656 ± 85 μm at 4.5 bar pressure for spherical micro-particles of 18 ± 7 μm diameter when we used MNs of 1500, 1200 and 750 μm length, respectively.

Potential of microneedle-assisted micro-particle delivery by gene guns: a review

CONTEXT

Gene guns have been used to deliver deoxyribonucleic acid (DNA) loaded micro-particle and breach the muscle tissue to target cells of interest to achieve gene transfection.

OBJECTIVE

This article aims to discuss the potential of microneedle (MN) assisted micro-particle delivery from gene guns, with a view to reducing tissue damage.

METHODS

Using a range of sources, the main gene guns for micro-particle delivery are reviewed along with the primary features of their technology, e.g. their design configurations, the material selection of the micro-particle, the driving gas type and pressure. Depending on the gene gun system, the achieved penetration depths in the skin are discussed as a function of the gas pressure, the type of the gene gun system and particle size, velocity and density. The concept of MN-assisted micro-particles delivery which consists of three stages (namely, acceleration, separation and decoration stage) is discussed. In this method, solid MNs are inserted into the skin to penetrate the epidermis/dermis layer and create holes for particle injection. Several designs of MN array are discussed and the insertion mechanism is explored, as it determines the feasibility of the MN-based system for particle transfer.

RESULTS

This review suggests that one of the problems of gene guns is that they need high operating pressures, which may result in direct or indirect tissue/cells damage. MNs seem to be a promising method which if combined with the gene guns may reduce the operating pressures for these devices and reduce tissue/cell damages.

CONCLUSIONS

There is sufficient potential for MN-assisted particle delivery systems.

Needle-free vaccine injection

Millions of people die each year from infectious disease, with a main stumbling block being our limited ability to deliver vaccines to optimal sites in the body. Specifically, effective methods to deliver vaccines into outer skin and mucosal layers--sites with immunological, physical and practical advantages that cannot be targeted via traditional delivery methods--are lacking. This chapter investigates the challenge for physical delivery approaches that are primarily needle-free. We examine the skin's structural and immunogenic properties in the context of the physical cell targeting requirements of the viable epidermis, and we review selected current physical cell targeting technologies engineered to meet these needs: needle and syringe, diffusion patches, liquid jet injectors, and microneedle arrays/patches. We then focus on biolistic particle delivery: we first analyze engineering these systems to meet demanding clinical needs, we then examine the interaction of biolistic devices with the skin, focusing on the mechanical interactions of ballistic impact and cell death, and finally we discuss the current clinical outcomes of one key application of engineered delivery devices--DNA vaccines.

Dextran or Hydroxyethyl Starch in Sprayfreeze-Dried Trehalose/Mannitol Microparticles Intended as Ballistic Particulate Carriers for Proteins

The goal of this study was to clarify the effects of dextran 10 kDa on the properties of spray-freeze-dried microparticles for use with ballistic injectors. A novel carrier of trehalose, mannitol, and the polymer is known to maximize particle density. Measurements of T'(g) showed that the dextran anti-plasticizes the trehalose/mannitol, but also undergoes phase separation. The product temperature exceeded T'(g) during primary drying. The collapsed particles can therefore be explained by plastic flow of the freeze concentrate. DSC of the powder showed T(g) at 45 degrees C and, in the first scan, a wide endothermic melting peak caused by mannitol recrystallization. Catalase showed 35% activity loss on rehydration of its spray freeze-drying (SFD) powder, which was improved in the TM/D (3:3:4) formulation, but not up to that level seen with either trehalose or mannitol alone. The dextran 10 kDa, which is vital to maximize particle density, was therefore detrimental to protein integrity during SFD, as also found with a 65-72 kDa dextran. Hydroxyethyl starch (HES) 200 kDa gave similar, limited stabilizing effects on the protein. The proportion of polymer in the formulation should be low to minimize protein damage, whilst high enough to give required particle morphology and density.

Characteristics of a micro-biolistic system for murine immunological studies

With an advanced computational fluid dynamics (CFD) technique, we have numerically developed and examined a micro-biolistic system for delivering particles to murine target sites. The micro-particles are accelerated by a high speed flow initiated by a traveling shock wave, so that they can attain a sufficient momentum to penetrate in to the cells of interest within murine skin (or mucosa). In immunization application, powdered vaccines are directly delivered into the antigen presenting cells (APCs) within the epidermis/dermis of the murine skin with a narrow and highly controllable velocity range (e.g., 699+/-5.6 m/s for 1.8 microm modeled gold particles) and a uniform spatial distribution over a diameter of approximately 4 mm target area. Key features of gas dynamics and gas-particle interaction are presented. Importantly, the particle impact velocity conditions are quantified as a function of: stand-off distance (2-15 mm), driver gas species (air/helium mixtures), particle density (1,050 kg/m3 and 19,320 kg/m3) and particle size (1-5 microm for gold particles and 10-50 microm for less dense particles, respectively). The influential parameters--representative of immunotherapeutic (e.g., DNA vaccination) and protein (e.g., lidocaine) biolistic applications--are studied in detail.

Numerical analysis of gas and micro-particle interactions in a hand-held shock-tube device

A unique hand-held gene gun is employed for ballistically delivering biomolecules to key cells in the skin and mucosa in the treatment of the major diseases. One of these types of devices, called the Contoured Shock Tube (CST), delivers powdered micro-particles to the skin with a narrow and highly controllable velocity distribution and a nominally uniform spatial distribution. In this paper, we apply a numerical approach to gain new insights in to the behavior of the CST prototype device. The drag correlations proposed by Henderson (1976), Igra and Takayama (1993) and Kurian and Das (1997) were applied to predict the micro-particle transport in a numerically simulated gas flow. Simulated pressure histories agree well with the corresponding static and Pitot pressure measurements, validating the CFD approach. The calculated velocity distributions show a good agreement, with the best prediction from Igra & Takayama correlation (maximum discrepancy of 5%). Key features of the gas dynamics and gas-particle interaction are discussed. Statistic analyses show a tight free-jet particle velocity distribution is achieved (570 +/- 14.7 m/s) for polystyrene particles (39 +/- 1 microm), representative of a drug payload.

Penetration of Crystalline Powder Particles into Excised Human Skin Membranes and Model Gels from a Supersonic Powder Injector

The penetration of crystalline ibuprofen particles into excised human skin membranes and model target gels on actuation with the PowderJect ND1 ballistic needle-free injector has been examined. The deliverable dose of powder exiting the device is approximately 50% of the total cassette loading; the rest is lost via a gap forced open between the injector base and the surface of the target gel. It could be shown that substantial comminution of two different ibuprofen particle size fraction (38-53 microm, 53-75 microm) occurs within the injector on actuation. This resulted in an equiparation of the two initially different size fractions. Reduced comminution occurs with an ibuprofen/PVP (95:5) particulate formed by spray-drying/compaction/milling. On actuation into excised human skin membranes approximately 37% of cassette dose is recovered from the stratum corneum by stripping, and only 3% from the epidermis. It is concluded that powder delivery is mainly intra-epidermal with few particles fully breaching the stratum corneum. This is a consequence of the relative magnitude of the particle diameter (approximately 48 microm) and the stratum corneum thickness (approximately 10 microm). A 'soft' polyethylene glycol monolayer target gel gives the closest total percentage recovery of powder to that seen with the excised human skin membranes. Bilayer gels differentiated between surface impact/retention and penetration, but were difficult to handle. Powder penetration studies should therefore preferentially to be performed with human skin-membranes.

Performance studies of particle acceleration for transdermal drug delivery

We have proposed a transdermal biolistic method to accelerate a powder formulation of drugs to penetrate human skin for the treatment of a range of diseases. One of the key issues for designing and evaluating transdermal biolistic system is ensuing that the powder drugs are delivered into the skin with a controllable velocity range and spatial distribution. The aerodynamics of supersonic nozzles and performance of the delivery system were initially studied, mainly analytically and experimentally. In this paper, computational fluid dynamics is utilized to characterize two existing prototype devices, in order to further investigate the transient gas and particle dynamics in their supersonic nozzles. To validate the implemented numerical approach, calculated pressure histories, two-dimensional flow structures and particle velocity distributions are made and compared with the reported experimental measurements. The key features of gas dynamics, gas-particle interaction and performance of the prototype transdermal biolistics are discussed and interpreted.