Loughborough University Institutional Repository In uence of array interspacing on the force required for successful microneedle skin penetration : theoretical and practical approaches

Insertion behaviour of microneedle (MN) arrays depends upon the mechanical properties of the skin and, MN geometry and distribution in an array. In addressing this issue, this paper studies MN array insertion mechanism into skin and provides a simple quantitative basis to relate the insertion force with distance between MNs. The presented framework is based on drawing an analogy between a beam on an elastic foundation and mechanism of needle insertion, where insertion force is separated into different components. Theoretical analysis indicates that insertion force decreases as interspacing increases. For a specified skin type, insertion force decreased from 0.029N/MN to 0.028N/MN when interspacing at MN tip was increased from 50μm (350μm at MN base) to 150μm (450μm at MN base). However, dependence of insertion force seems to decrease as the interspacing is increased beyond 150μm. To assess validity of the proposed model, a series of experiments were carried out to determine the force required for skin insertion of MN. Experiments performed at insertion speed of 0.5 mm/s and 1.0 mm/s yielded insertion force values of 0.030N and 0.0216N for 30μm interspacing at MN base (330μm interspacing at tip) and 0.028N and 0.0214N for 600μm interspacing at MN base (900μm interspacing at tip). Results from theoretical analysis and finite element modelling agree well with experimental results, which show MN interspacing only begins to affect insertion force at low interspacing (< 150μm interspacing at MN base). This model provides a framework for optimising MN devices, and should aid development of suitable application method and determination of force for reliable insertion into skin.