## 1 Introduction  
Tissue regeneration is a physiological process that is important to maintain organ homeostasis and to restore organs after an injury. This process is intrinsic of several human tissues, however some organs such as the heart, liver, and central nervous system present a very limited regenerative capacity [1]. Studies have demonstrated that mesenchymal stem/stromal cells (MSCs) have an important role in tissue regeneration, not only because they have the ability to differentiate into mesodermal lineage cells but also due to their rich secretome [1]. Today, MSCs isolated from different tissues have been tested in more than 300 clinical trials (www.clinicaltrials.gov, accessed on 1 February 2022) with promising results, and several MSC-based products have already received regulatory approval [2]; however, the advanced manufacturing of these cells still needs to overcome several issues, such as the use of animal-derived components such as fetal bovine serum (FBS) to supplement cell-culture media [34]. The use of FBS raises several concerns; not only ethical concerns related to the animal welfare used to produce it, but also due to immune reactions and the transmission of bovine pathogens to humans [5].  
An alternative to FBS that has been shown to be efficient in MSC expansion is the group of human platelet derivatives [6; 7; 8].  
Platelets--when activated--release a wide range of bioactive factors, including growth factors (GF) and cytokines, which are known to have an important role in cell expansion and survival [9]. Platelet concentrates (PCs) are regularly produced in blood banks, and due to their short-shelf life (5-7 days), around 5% of them are discarded before clinical use [10; 11].  
Giving new use to these PCs with no therapeutic value, using them as a source of cell culture media supplements has become a very attractive tool; however, the methodology used to induce GF release from platelets affects platelet derivatives' features [6; 12]. The most common methods used are hypo-osmotic shock (freeze and thaw cycles) and bovine thrombin [13; 14]. In the first methodology, the ideal number of cycles remains to be established, since a high number of cycles might lead to GF damage and a final lysate containing other platelet debris [14; 15]. Using bovine thrombin to induce GF release from platelets presents the same problems associated with animal-derived components [16]. Thus, more efficient strategies to activate platelet pools and consequently generate releases with an improved biological activity for MSC expansion are needed.  
Pulse electric field (PEF) is a technology characterized by inducing electroporation on cell membranes, with the application of ns-us range pulses and high electric fields kV/cm, allowing proteins to flow into and out of the cells, with a low thermal load [17; 18]. In a previous work, the potential for the application of PEF in the valorization of expired PCs into a biomedical product with therapeutic value was demonstrated [19]. In particular, it was shown that PCs subjected to a single PEF treatment can release PDGF into the supernatant, and a proof-of-concept study was performed to confirm the biological activity of this release to support MSC culture in vitro. In the current study, the impact of key PEF parameters, namely the pulse width, electric field, and pulse number, on platelet activation from expired PCs was further investigated in order to prove the concept, aiming at modulating the release of specific GFs to improve their bioactivity for MSC expansion while maintaining cells' quality attributes.