###### Abstract  
The aim of the study was to investigate the effect of ethanol on the properties of acrylic-urethane resin products obtained by vat photopolymerization using the masked stereolithography method. The effect of alcohol at concentrations of 15, 25, and 35% in the resin on the chemical structure, weight, thickness of the samples, and mechanical properties in static haltensile tests performed immediately after printing and one month later were studied. The results obtained were evaluated in terms of the use of ethanol as a cosolvent to help load the resin with agomelatine for the potential of obtaining microneedle transdermal systems. It was shown that in terms of stability of properties, the most favorable system was resin with the addition of alcohol at a concentration of 15%. The greatest changes induced by the presence of the solvent in the resin were observed in the case of tensile properties, where the alcohol caused a decrease in the plasticity of the material, reducing the relative elongation at break from 14% for the pure resin to 4% when the alcohol concentration was 35%. [PERSON]'s modulus and tensile strength also decreased with the addition of alcohol by 18% and 31%, respectively, for testable samples with the maximum amount of alcohol in the resin. The deterioration in properties is most likely related to the effect of the solvent on the radical polymerization process of the resin, particularly the phenomenon of chain transfer to the solvent, which is important in view of the intended application of the developed material. macked stereolithography; vat photopolymerization; acrylic-urethane resin; radical polymerization; solvent +
Footnote †: journal: Applied Sciences  
0000-0002-0002-3300]M. [PERSON]  
0000-0002-3300]W. [PERSON]  
0000-0002-3300]M. Tome  
complex geometries and very high dimensional accuracy, especially when compared to the capabilities of the most popular polymer processing additive manufacturing method--fused filament fabrication [5]. One technique that stands out from the aforementioned vat photopolymerization is DLP, which, thanks to the use of a digital mirror device (DMD), allows exposure and simultaneous curing of the entire layer of the printed part, making the process faster but still very accurate. Masked stereolithography (MSLA) is another method that also allows full-layer exposure, but is easier to implement and, therefore, more accessible to a wider user base. To obtain parts by MSLA or DLP, photocurable resins are used, most often characterized by UV-activated radical polymerization [6]. The components of the resin are mainly acrylic oligomers and mers, a photoinitiator and additives in the form of a co-initiator, inert dyes, and photosensitizers [7]. During exposure to radiation, free radicals are formed from the photoinitiator, which, thanks to the functional groups present in the mers and oligomers, lead to cross-linking of the polymer structure. The design of a suitable package of images given to the DMD, corresponding to individual layers of the manufactured samples, makes it possible to obtain samples with dimensions on the order of micrometers [8]. A promising application of the MSLA technique is the manufacture of microneedle transdermal systems (MTSs). These are devices designed to create microchannels in the human skin that allow drug delivery and increase bioavailability. There are five main types of microneedles: solid, hollow, dissolving, coated, and hydrogel [9; 10]. An essential aspect of each type of microneedle is its geometry and dimensional accuracy, which must be maintained over time. Conventional injections can cause pain, but MTSs, with lengths that penetrate the stratum corneum without irritating nociceptors, offer a less painful alternative while maintaining mechanical strength. MSLA and DLP can produce various microneedles, particularly hydrogel drug-loaded systems that swell in the presence of a medium like water from human skin, allowing drug diffusion [11].