## 1 Introduction  
As the outer shell of the human body, skin forms the human's largest organ [1]. When skin is injured, it is able to repair itself while passing through five stages of wound healing: haemostasis, inflammation, proliferation, migration, and maturation [2; 3]. As a part of this process, cells migrate into the wounded area [4]. Wound healing assays can be set up in order to enable further investigation of the skin wound-healing process in vitro [5; 6; 7]. The popularity of these assays can be explained by the simplicity of their procedures. In such procedures, a layer of adherent cells is wounded under specified conditions and the subsequent migration of the contiguous cells into the fresh and cell-free area is observed and analyzed [8; 9; 10]. Such a defined cell layer can be wounded by scratching, stamping using PDMS or neoprene [11; 12; 13], thermal wounding [14], electrical wounding [15], or optical wounding [16; 17]. Scratching is most commonly performed in well plates using pipette tips [18; 19; 20]. However, it can also be performed by using a variety of sharp-edged tools like toothpicks [21], metallic micro-indenters [22], or cell scrapers [23]. Furthermore, it is reported that scratches are performed on coverslips, which can be placed into a 6-well plate [24]. In other techniques, inlets within dishes or wells to create a cell free area have been used [25]. The general aim of a scratch assay is to provide straight borders in a reproducible way without damaging cells [26]. However, the reproducibility of manually performed scratch assays strongly depends on the performer and their skills [27]. An attachment for forceps-like scrapers to create scratches of defined direction, length, width, and depth in 6-well plates has been developed to investigate keratinocyte migration [28]. Scratches obtained with this scratch stencil are reported to be inflicted with high precision and in a reproducible fashion [29]. The material chosen by this group, polyoxymethylene, is generally a suitable construction material in concerns of laboratory apparatus construction [30]. In all aforementioned strategies, it is only possible to investigate the migration behavior of cells under different conditions. Subsequent molecular biological investigations using RNA or protein samples from migrated cells were impossible due to the fixation of the cells. Therefore, migration assays were performed separately by using transwell or Boyden chambers. After migration, the migrated cells were collected for further investigations [31; 32; 33]. Depending on the needs of experimental design and availability, the used cell types for the mentioned investigation methods of migration and wound healing may vary. For instance, to study epithelial repair, epithelial cells from the lung [34] or the retina [35] are commonly used. Another interesting source is represented by mesenchymal stem cells, which can be differentiated towards multiple cell types [36; 37; 38; 39; 40; 41; 42]. [PERSON] and colleagues even differentiated human adipose-derived stem cells towards epithelial cells for usage in scratch assays [43]. Fibroblasts from foreskin (HFF), skin (HDF), or nasal polyps (NPDF) have recently been used as well as microglia cells and keratinocytes [44; 45; 25; 46]. In addition to the mentioned cell types, endothelial cells are a favorable source to study migration. Many working groups used either human or bovine endothelial cells in scratch assays and other migration assays [47; 48; 49; 50; 51; 52; 53; 54; 55; 56]. This study covers the development and evaluation of a prototype scratch apparatus for wound assays, focusing on the comparison to conventional procedures. The presented device allows for the collection of migrated cells after scratching for further investigations, without the need of a second processing step. Moreover, the apparatus is adjustable in angle, normal force, shape, and material to adapt to a specific question and demanding substrates.