PRF stands for Platelet-rich fibrin, and differs from Platelet-rich Plasma (PRP), in the fact that the platelets are activated in PRF. Fibrin matrix is a kind of gel which acts to support cellular repair and rebuilding process. It's very sticky and difficult to inject through a needle, but there is a small window of time when it can still be injected, up to 15 minutes after the activation process starts. Activation could be through an external agent, such as a tiny amount of calcium chloride, or sometimes the natural clotting process if blood is collected without anticoagulant in the tube.
Both PRP and PRF can be good treatments for under eye problems, such as infraorbital dark circles, volume loss in the tear trough area, crepey skin, fine lines, etc. For more information including links to published research on this topic, please see the link included with this answer.
The amount injected would depend on a few factors: age of the patient, amount of volume loss, whether or not the upper cheek is being included (can be helpful as this area supports the under eye area). A typical amount (per side), if the cheek was not included would be 1-2 mL, and up to 5 mL if the upper cheek is included.
On average, patients can expect 2-3 days of swelling, but this can vary depending on all of the above factors, as well as the concentration of platelets and the total number of platelets injected. It's great if the treating clinic has access to a hematology analyzer to be able to give you these numbers, but this is not available everywhere.
Another thing to remember is that a series of treatments will usually give best results, for example 3 treatments spaced about 2-4 weeks apart.
First, it should be stated that using Acell for hair growth is considered off-label. Even PRP for hair growth is still considered by many as experimental, although a quick search on PubMed will show the evidence for PRP is growing quickly.
Having said that, ssing Acell + PRP every 6 months should not be a problem. I suspect it may even be a good frequency for many patients with mild to moderate androgenetic hair loss. On the other hand, if your hair loss is mild, you may be fine with once a year treatments or even less frequently.
Acell, which is an extracellular matrix, seems to support the PRP and improve the results. Acell is used off-label with PRP for hair restoration and to improve the results of hair transplants. Although there are not a lot of studies on this, one small case series reported in 2012 by Dr Hitzig showed that when 150 terminal hairs were soaked in Acell prior to transplantation to another area of the scalp, after 6 months, there were approximately 550 terminal hairs growing in the transplanted area. This was a dramatic improvement if you consider that usually, with transplantation, after 6 months, there are less hairs growing than were transplanted, as they do not all survive.
One of the first studies showing the effect of PRP on hair growth was published in 2006 by Li et al. The researchers soaked 150 transplanted hairs in either saline or PRP prior to placing them into small marked areas on either side of patients' scalps. Six months later, out of 150 hairs, only 95 were growing in the square where the hair had been soaked in saline, but 20% more survived, (105 out of 150) in the PRP group. This was one of the first studies which led to the widespread use of PRP to promote hair growth.
In summary, from 150 hairs, about 70% survived with PRP, but when ACell was used, there seemed to be a multiplication effect as 150 hairs turned into 550. The researchers suggested that this could be a result of the ACell stimulating stem cells present in the bulge region and base of the hair follicles. These dramatic results led to ACell being used off-label by clinicians to treat different types of hair loss (mainly androgenetic alopecia).
With PRP, the most common protocol seems to be an initial treatment series of once a month for 3 treatments, followed by a booster after 3 months (see study by Amelia Hausauer). Maintenance PRP treatments for alopecia might be done once every 6-12 months.
With ACell, there are different approaches, and not enough research to be able to say which is best. Clinics take a range of approaches, from offering PRP + Acell every month, to every 3 or 6 months, or less often. Some doctors will even go as far as saying a single treatment with PRP Acell will be enough to stimulate ongoing hair growth for several years. Personally I think that's overly optimistic. At our clinic, we offer ACell as an add-on to PRP for patients who request it. We have done it as often as every month, or as just a single treatment. Most common is every 3-6 months.
The optimal frequency of PRP and Acell, either separately or in combination, probably depends on several factors, including the degree of hair loss, age, other medical conditions, whether other supportive treatments are being used, etc. I hope this helps.