Fabrication of Functional Film Provided with Microneedle Array By Thermal Imprint Method
Authors: Sato Ayumi , Kurose Takashi , Ito Hiroshi 
Published: February 2020

Introduction
In recent years, researches on the formation of surface microstructure by precision processing technology are progressing. Microneedle arrays which are one of the microstructures have attracted attention in the fields of beauty and medical care. Currently, fabrication of microneedle arrays with various materials is being studied. However, in a microneedle array made of a metal or an inorganic material, when a needle is broken in the skin, there is a risk that the needle may remain in the body. On the other hand, microneedle arrays with polymeric materials typified by plastics, especially biocompatible materials, are relatively safe due to biocompatibility or biodegradability, even if needles remain in the skin. Therefore, in this research, we applied a microneedle array on the surface of biocompatible plastic film using the thermal imprinting process and investigated the influence of molding conditions on transferability of molded products.
Method
As a sample, polyvinyl alcohol (PVA) (Mowiflex C17 CP-1000, manufactured by Kuraray Co., Ltd.) was used. Films (thiukness 300 μm, 500 μm) were prepared with a vacuum heat press machine (manufactured by Imoto Seisakusho). A mold of a microneedle array structure (independent microprojection structure array, hollow structure, height 90 μm, internal diameter 20 μm) was used. The needle array structure was transferred onto the PVA film with a vacuum small hot press machine (manufactured by Izumi Tec). A pressed set temperature was 90, 100, 110, 120 ° C, pressurization time was 1, 2, 3 minutes, and melt retention time was 1 or 2 minutes to prepare a molded article. Five needles having the highest transfer rate were selected and an average value of the form transfer rate of the molded article was obtained with a 3D laser microscope (LEXT OLS 4000, manufactured by OLYMPUS). In addition, the appearance of the molded needles was also observed using a scanning electron microscope (SEM) (JSM-6510, manufactured by JEOL).
Results and discussion
When the holding time was 2 minutes and the pressing temperature was 120 ℃, the transfer rate improved (Fig.1(a)) . It is thought that the temperature of the whole film increased with time and as a result the elastic modulus of the film decreased. However, when observing the whole structure by SEM roughly half of the structures caused defective transfer. There are three reasons for this. One is that they are not evenly heated by simultaneous heating both the film and the mold, which has large heat capacity, from the room temperature state. The second is because the film is thick and the resin flows easily. Thirdly the temperature at the bottom of the film is high and the elastic modulus (yield stress) is low, and the resin flows easily. In order to improve the above points, thin film, preheating the mold, lowering the set temperature were performed. As a result, the transfer rate was improved by thinning the film and preheating the mold. When the press temperature was lowered together with preheating the mold, although the form transfer rate was slightly lowered, the overall transfer rate improved to 100% (Fig.1(b)) .

Figure 1