On the other hand, in PRF prepared from fresh whole-blood without anticoagulants, it is thought that the centrifugal force increases the contact of factor XII with a glass surface, thereby primarily forming fibrin clots. Although platelets can also be activated by a glass surface, the centrifugal force accumulates platelets at and just below the interface between the red blood cell fraction and plasma fraction as well as in the surface area of PRF preparations [11]. Even though only a few red blood cells are embedded in the clot, in terms of the formation mechanism, this clot could be classified as a “red thrombus.” Therefore, it is plausible that growth factors that are stored in the platelets trapped in fibrin mesh may be released at early phases of degradation.

As described previously [24], glass tube production is no longer practiced by major manufacturers of medical equipment in Japan and in major Western countries. One of the possible solutions (when glassware is needed) is to coat the inner wall of plastic tubes with micronized silica particles, as in Greiner Bio-One Serum Clot Activator Tubes (Greiner Bio-One North America Inc., Monroe, NC, USA). Likewise, various types of plastic vacuum blood collection tubes containing a coagulation-activating film are produced by several manufacturers. Nevertheless, these products are intended for routine clinical examination, and their quality, especially safety, has never been ensured for preparation and implantation of platelet concentrates.

If platelet activity can be controlled in well-qualified plastic tubes, it is expected that safer PRF will be provided to patients without activation of the intrinsic coagulation pathway. In this study, we demonstrated that a collagen sponge can rapidly facilitate fibrin clot formation (without the aid of glassware) probably via activation of platelets. On the basis of this concept, we recently developed a PRF kit composed of plastic tubes containing a synthetic, RGD motif-enriched collagen-like peptide (RCP; FUJIFILM, Tokyo, Japan) and validated its utility in PRF preparation and its efficacy in bone regeneration [Tsukioka et al., manuscript submitted]. This fibrin clot formation is triggered by activated platelets.