In Vivo and In Vitro
The way that the academics performed the study is important for how we interpret its results.
A substance being applied topically (via skin) is the best as that is closest to real-world conditions.
A substance being tested in vivo (within a living thing) means the study is performed on a whole living organism, whether person, lab animal, or plant.
A substance being tested in vitro (in a glass) (think a test tube or petri dish) means the study is furthest from real-world conditions and its results are least reliable as to replicate in normal conditions.
Studied In Vivo (within a living thing) and In Vitro (in a glass)
Caffeine
Below 2017 study used a caffeine-based topical 0.2% liquid in vivo (within a living thing). Caffeine increases cyclic adenosine monophosphate levels in cells [2017].
Below 2017 study used caffeine at concentrations of 0.001% and 0.005% in vitro (in a glass)
Testosterone-induced hair follicle growth suppression was reversed with addition of ; moreover, caffeine alone led to significant stimulation of hair follicle growth [2017].
Below 2 studies used caffeine in vitro (in a glass).
(Study 1) Caffeine was found to enhance hair shaft elongation, prolong anagen (growth) phase duration and stimulate hair matrix keratinocyte proliferation. Caffeine counteracted testosterone-induced transforming growth factor-β2 expression, a major antagonistic hair growth regulatory factor [source].
(Study 2) Caffeine also resulted in increased expression of insulin-like growth factor-1, a promoter of hair growth, in both male and female hair follicles [source].
Stemoxydine
The below Stemoxydine study doesn’t say what percentage concentration was used. In vitro (in a glass) findings on Stemoxydine, led to in vivo (within a living thing) testing.
Stemoxydine mimics hypoxic signaling, which may sustain hair growth and cycling. Hypoxia may be an important regulator of stem/progenitor cells function in the human hair follicle. This converging data suggests that, in Androgenetic Alopecia (AGA), Stemoxydine could act as a hair kenogen (death) phase shortener, leading to an increase in visible scalp hair density [2014].
Studied In Vitro (in a glass)
L-carnitine L-tartrate (LCLT)
Below 2007 study was hair follicles cultured in the presence of 0.5-5 microm in vitro (in a glass).
Findings suggest that l-carnitine stimulates human scalp hair growth by up regulation of proliferation and down regulation of apoptosis in follicular keratinocytes [2007]. Carnitine also acts on the sensory organs suggesting a direct effect of palmitoyl-DL-carnitine on cochlear hair cells [1993].
Procyanidin B2
The below studies don’t say what percentage concentration, was in vitro (in a glass).
2002 and 2001 clinical trials have extensively demonstrated the positive hair-growing effects of procyanidin B2, a major dimeric procyanidin occurring in certain apples. Actually, these studies have exclusively investigated the potential benefits of topical applications of Procyanidin B2 under the form of alcoholic solutions in mixture with other bioactive components. Procyanidins B2 and B3 show evidence of protective actions versus apoptosis in hair epithelial cell cultures, thereby restricting catagen (transition phase) induction in the hair cycle [2018].
We can get Procyanidin B2 from different foods, mainly apples but also chocolate. Here's a list.
Studied by Eating
Creatine
Below 2009 study had athletes eat 25 grams a day creatine with 25 grams a day glucose.
Creatine supplementation may, in part, act through an increased rate of conversion of T to DHT [2009].
Studied on Hair
L-Arginine
The below L-Arginine studies don’t say what percentage concentration was used, but the 2007 study mentions it was applied to the hair via conditioner.
Nitric oxide (NO) initiates and maintains hair growth. Use of arginine (L-Arginine) can enhance NO and promote angiogenesis (formation of new blood vessels) for new hair growth [2010].
Uptake of pyrrolidone carboxylic acid (PCA), a derivative of glutamic acid, is enhanced by combining with arginine, an amino with strong affinity towards hair [2007].
Arginine prevents the undesirable attack by hydrogen peroxide on hair proteins and hair surface lipids. Furthermore, it is also suggested from amino acid analysis that a considerable amount of arginine is deposited on, or in hair fibers from coloring agents [2004].