## 4 Discussion  
Due to their advantages in terms of their different degrees of biodegradability, resorbable membranes, especially collagen-type, are the most popular in the field of dentistry, maxilla-facial surgery, or regeneration medicine [25]. It is very difficult to compare collagen membranes in terms of their resorbability and biodegradability based exclusively on the information reported in their datasheets, as it refers to different processing conditions and provenience. Crosslinking techniques and decellularization approaches may have different influences on the mechanical, biological, and chemical properties of treated tissues [26]. Previous studies demonstrated that different types of natural polymeric membranes can serve not only as scaffolds but also as good candidates for localized delivery of morphogenetic proteins [27]. On the other hand, the concept of PRGF and its related mechanism is that by increasing the concentration of growth factors and cytokines that are released by platelets, the healing process is accelerated [28]. The actual definition of protein rich plasma (PRP) (and related PRGF) is accepted as a minimum platelet concentration three times that of whole blood [18; 19; 29]. In our study, as presented in Table 1, the counted platelets are in agreement with this definition. Moreover, the main growth factor levels and cytokines were quantitatively assessed (Table 2). The values presented in Table 2 are in agreement with previously reported data [19; 29; 30; 31; 32; 33]. These previous results demonstrated that different plasma formulations enhance proliferation and migration of fibroblast populations, protect and inhibit TGF-1-induced myofiblast differentiation, and regulate the metabolic activity of mesenchymal stem cells [33]. However, different PRGF formulations reported in the literature, prepared by various methodologies, consist of completely different compositions in terms of growth factors and cytokine concentrations, which may result in different therapeutic effects. Centrifugation conditions are fundamental to obtain a high PRP quality, together with quantification and identification of platelets and lymphocytes. For example, [PERSON] et al. [18] consider that inclusion of leukocytes may induce pro-inflammatory effects of the proteases and acid hydrolases contained in white blood cells, while other authors claim that these details are not important. The FTIR spectra recorded on both PRGF-modified and unmodified membranes demonstrated that there is no specific interaction between collagen and PRGF components, as the fingerprints of collagen molecules are very well preserved upon PRGF-loading in all the three cases. No denaturation  
Figure 6: Results of enzymatic degradation test of native (unmodified) and PRGF-modified collagen membranes. Legend: \(\text{Cova}^{\text{TM}}\text{Max}\); \(\text{Jason}^{\oplus}\); \(\text{Biocellagen}^{\oplus}\). or chemical modification were detected, according to Figure 2, so the collagen matrix may act as a natural reservoir for growth factor delivery. The cross-sectional images obtained by SEM revealed different ultrastructural details with respect to the three types of membrane selected for this study. This ultrastructure was preserved after PRGF-loading, except for Biocollagen(r), in which the porosity seemed to decrease, resulting in a more compact structure. According to the literature, natural biopolymers with porous structures (pore size >25 \(\upmu\)m) are well-known for their excellent biological functions, demonstrating better bone formation during the initial healing period than non-porous membranes [25; 34]. On the other hand, a more effective cell attachment and differentiation is achieved with nano-sized pores, while micro-sized pores are able to improve angiogenesis and nutrients transport, favoring the mineral deposits [25; 35]. By AFM investigation, we were able to emphasize the details of collagen fibers on the surface, especially their orientation and thickness.