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To pierce through the skin and interact with the first biofluid available, microneedles should be mechanically strong. However, some polymers used to fabricate microneedles yield insufficient strength for the fabrication of arrays (PDMS, highly porous structures, etc.). To enhance mechanical properties, piercing materials can be used. They aim to pierce the skin evenly and dissolve quickly, clearing the way for underlying microneedles to interact with the interstitial fluid (ISF). Three materials-carboxymethyl cellulose (CMC), alginate, and hyaluronic acid (HA)-are discussed in this article. Low concentrations, for a quick dissolution while keeping enhancing effect, are used ranging from 1-5%(w/w) in deionized water. Their overall aspects, such as geometrical parameters (tip width, height, and width), piercing capabilities, and dissolution time, are measured and discussed. For breaking the skin barrier, two key parameters-a sharp tip and overall mechanical strength-are highlighted. Each material fails the piercing test at a concentration of 1%(w/w). Concentrations of 3%(w/w) and of 5%(w/w) are giving strong arrays able to pierce the skin. For the purpose of this study, HA at a concentration of 3%(w/w) results in arrays composed of microneedles with a tip width of 48 ± 8 μm and pierced through the foil with a dissolution time of less than 2 min.

We have established a novel concept of hybrid lithographic technology for non-planar surface patterning. Softlithography and photolithography are properly combined to transfer micro-patterns onto a curved area in an easy, low-cost way. As a first step, a film type of a photomask with micro-metal features is fabricated by the direct pattern transfer technique that has been presented in our preliminary work. Then, a flexible polymer photomask is wrapped on a curved surface to make conformal contact, and a variety of micro-features are formed on the surface via the photolithographic process. We have confirmed the validity of the technique for application in the industrial process by comparing the results transferred via the conventional photolithography with a rigid flat photomask. Subsequently, 3D polymer structures with a high aspect ratio (A/R) are fabricated on curved surfaces using this technique, followed by a discussion on several drawbacks due to the shape of the substrate. Overall, this paper has demonstrated a new method of micro-patterning, which would promise an emerging field of micro-fabrication on non-planar substrates.