In vitro evaluation of microneedle strength: a comparison of test configurations and experimental insights

To ensure the safe and effective application of microneedles for drug delivery to the skin, the mechanical properties the microneedles and their ability to penetrate the skin are critical quality control parameters. While ex vivo and in vivo evaluations may be valuable to demonstrate actual skin penetration, they can be costly and difficult to accomplish consistently due to the inherent biological variability of the skin. On the other hand, in vitro approaches provide a facile means of characterising the intrinsic mechanical properties of the microneedles, independent of such biological variability. Thus, they can be used to predict and screen for the in vivo and ex vivo performance of new microneedle formulations.  A variety of experimental configurations has been reported in the literature focusing on mechanical evaluations including compression tests and in vitro microneedle insertion studies using a non-biological skin simulant, Parafilm® M. However, there has been a paucity of data that address the comparability of the various experimental configurations. Here, we evaluated several methods for assessing the mechanical properties of microneedles in vitro, including their ability to insert into a non-biological skin simulant under a defined axial force, and share some insights into the experimental design and data interpretation.

Microneedles (MNs) are an array of micron-sized needles with a length up to 2000 μm, which are sufficient to penetrate the stratum corneum of human skin without damaging blood capillaries or nerve endings. By creating micron-size pores in the skin, MN arrays offer a painless and active penetration enhancement strategy to increase skin permeability for drug delivery. This strategy allows drugs that do not meet the criteria for transdermal delivery, including a molecular weight <500 Da and a log P value of 1–3, to be delivered successfully across the skin. Prime drug candidates that can benefit from this penetration enhancement strategy include very lipophilic drugs or macromolecules such as proteins and DNA.

Skin penetration is a fundamental performance parameter of MN arrays. MN arrays should be able to withstand the external stress applied during their insertion into, or removal from, the skin. The mechanical strength of MN arrays is a quintessential criterion to avoid damage, including breaking and bending during application or handling that will greatly limit their clinical applications despite having a good drug dissolution and release profile. The types of materials used in MN fabrication and the MN geometry (including but not limited to the needle height, base diameter, inter-needle spacing and aspect ratio) are also crucial in determining the performance of MN arrays, to assure sufficient mechanical strength for skin insertion. The mechanical strength of polymeric MN arrays is usually attributable to the polymer base, whose mechanical properties are often well understood. However, the addition of drugs and excipients to the polymer base can drastically alter the mechanical strength of the MN formulation. For these reasons, the in vitro mechanical characterisation of MN arrays is usually the first-line investigation to select formulations for downstream performance evaluations, including ex vivo or in vivo mechanical tests.

To ascertain the mechanical characteristics of MN arrays, an arsenal of mechanical tests including mathematical simulations have been established. In particular, compression (percentage reduction of MN length), axial and transverse failure forces are widely measured. These tests not only mimic the insertion of MN arrays into skin but also provide a simple yet efficient method to quantify the mechanical properties of MNs. Using a texture analyser, MN arrays are typically driven towards a flat aluminium block whereupon a force is applied onto the MNs at the point of contact. With the aid of a microscope, the mode of MN failure (e.g., fracture and bending), together with the reduction in the MN height, can be observed. In addition, force vs. displacement data can be obtained from the axial compression test to quantify the MN failure force. These mechanical tests are pivotal to ensuring that the MN arrays can puncture the skin without failure (e.g. bending, buckling and fracturing).

Following these mechanical strength measurements, ex vivo evaluations are commonly carried out to assess the ability of the MN arrays to penetrate the skin. However, such ex vivo evaluations are subject to biological variability in the tissues, especially if they have been derived from different animal models, such as pigs, primates and rodents. Hence, an artificial membrane, usually Parafilm® M sheet, was proposed and validated by Larrañeta, et al. to mimic ex vivo skin in an in vitro MN insertion study. Although the penetration holes on the Parafilm® M sheet are commonly examined under a microscope, they can often be observed easily without one. Given that the thickness of the Parafilm® M sheet is known, the percentage of MNs penetrated as a function of the insertion depth can be estimated. This technique provides a fast and repeatable method to evaluate the skin insertion depth of MN arrays, which is important in MN formulation development.

Since the introduction of these methods for MN mechanical evaluation and insertion study, there have been several experimental configurations reported in the literature but these methodologies have not been validated against the original method by Larrañeta, et al. This study attempts to compare and validate the in vitro MN mechanical evaluation and insertion study using Parafilm® M sheet in several experimental configurations. We also share some intricate details on various mechanical tests for MN characterisation, with a view to overcoming the challenges that may be encountered in the analysis, including the experimental setup and key experimental parameters.

 

 

The MN array formulation was first prepared using a double casting technique. In the first casting step, 200 mg of 10% w/v of polyvinylpyrrolidone (PVP) K90 solution was cast into the polydimethylsiloxane mould and centrifuged at 4020g (RCF) for 15 min at room temperature to push the viscous solution into the mould’s cavities. The excess PVP solution on the mould surface was scraped off and the content of the mould was allowed to dry at room temperature for 1 h. Next, a second layer was cast to form the MN baseplate. In this step, 300 mg of PVP K90 solution (40% w/v) was cast over the first layer and centrifuged at 4020g (RCF) for 15 min at room temperature. Next, the mould was placed in an oven at 40 °C for 24 h. Finally, the MN array was peeled off and kept in a desiccator for further use. The current mould makes 34 conical needles of 500 μm in length and 175 μm in base diameter.

The axial compression test was performed using the TA-XT® Plus texture analyser (Stable Micro Systems, Haslemere, UK). The MN arrays were placed with the MNs facing up on a flat aluminium block of dimensions 9 × 10 cm and compressed against a metal cylindrical probe (diameter: 50 mm) of the texture analyser, as shown in Fig. 1A. The probe was set to move downwards at a speed of 0.5 mm s−1 until a fixed distance (0.5 mm for the current PVP MN) was reached. The maximum force obtained before MN failure was determined as the failure force.

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In vitro evaluation of microneedle strength: a comparison of test configurations and experimental insights, Bilal Harieth Alrimawi, Jing Yi Lee, Keng Wooi Ng and Choon Fu Goh, Cite this: DOI: 10.1039/ d4pm00024b, Received 29th January 2024, Accepted 26th April 2024, DOI: 10.1039/d4pm00024b rsc.li/RSCPharma