## 2 Materials and Methods  
### PRP Systems and Protocols  
We used three commercially available PRP systems to prepare PRP. The first PRP system was the Harvest SmartPrep2 System (Harvest Technologies, Plymouth, MA, USA), using either one or two 60 mL process disposables. Briefly, 8 mL of anticoagulant citrate dextrose A (ACD-A) was drawn into the blood draw syringe, after which 2 mL was transferred to the sterile closed SmartPreP2 process disposable through the white resealable injection port into the plasma chamber of the disposable. We next added 54 mL of the subject's blood to the remaining 6 mL of ACD-A in the blood draw syringe. The blood was then transferred to the SmartPreP2 process disposable through the red resealable injection port into the SmartPreP2 process disposable, and the cartridge was placed in the automated Harvest centrifuge. During spinning, a floating shelf in the cartridge compartment with the blood rises to just below the buffy coat/red blood cell interface, allowing automated decanting of platelets, plasma and white blood cells into the plasma compartment of the cartridge for subsequent recovery. Approximately 10 mL of PRP was withdrawn from each process disposable (i.e., the yellow spacer was not removed), and the resultant PRP was tested for platelet concentration and pooled prior to further treatment when more than one disposable was used.  
The second PRP system was the Eclipse PRP (Eclipse Medical, The Colony, TX, USA), which used up to four of the 22 mL platelet separation tubes per donor, while following the manufacturer's protocol (including removing \(\sim\)75% of the platelet-poor plasma). Briefly, 20 mL of the subject's blood was drawn directly into a 22 mL platelet separation tube containing 2 mL of MNC7 (Eclipse Medical, The Colony, TX, USA), a proprietary anticoagulant, and a thixotropic separator gel. The tube was then spun in a centrifuge with a relative centrifugal force (RCF) of 1500 for 10 min and a braking time of 30-45 s. Upon completing centrifugation, we removed the platelet poor plasma (PPP) fraction by withdrawing \(\sim\)75% of the plasma fraction. The platelets were then resuspended by gently inverting/shaking the tube between one and seven times. Approximately 3-6 mL of PRP was withdrawn from each tube, tested for platelet concentration, and pooled prior to further treatment.  
The third PRP system was the EmCyte PurePRP II (EmCyte Corporation, Fort Myers, FL, USA), using the 60 mL concentrating device and following GS60 Pure II Protocol A. Briefly, 10 mL of sodium citrate anticoagulant was first drawn into a 60 mL syringe, followed by 50 mL of subject blood. The anticoagulated whole blood was then transferred into the concentrating device and placed into the centrifuge for an initial spin of 1.5 min at 3800 RPM. Using the 60 mL syringe, the platelet plasma suspension was withdrawn from the concentrating device and transferred to the concentrating accessory (a second device). The platelet plasma suspension was then spun in the centrifuge a second time for 5 min at 3800 RPM. After centrifugation, the platelet poor plasma was withdrawn from the concentrating accessory, leaving approximately 7 mL of PRP, which we then withdrew from the concentrating accessory, tested for platelet concentration, and pooled prior to further treatment.