Science, technology, engineering, and mathematics (STEM) project refers to experiments conducted to address problems, make improvements, or discover new things in those fields that can be tested using the scientific method. This paper describes a long-term STEM project in which pre-service science teachers (PSSTs) have a chance to discover new application fields of microalgae and use them for bio-efficacy. One of the PSSTs' projects is described in detail. This STEM project-based activity lasted for 8 weeks. In the results, a product developed by PSSTs is a cling film which has an antioxidant effect owing to microalgae and sodium alginate. Besides, this environmentally friendly product is equivalent to plastic foils used nowadays. The production processes of this cling film will be presented in detail. According to these processes, some implications for improving the product and facilitating the process will be presented at the end of the paper.