Why Most Hair Serums Do Not Work: The Formulation Science of Getting Actives Into the Follicle

2026年8月18日

By Dr. Susan Lin, MD | MD HAIR | La Cañada Ventures, Inc.

You have a drawer. I know you do — almost everyone researching hair loss eventually accumulates one. Three or four half-used bottles of serum, each bought after a promising evening of research, each abandoned somewhere around week six. Rosemary oil. A peptide spray. Something with caffeine. Something with a percentage on the front in large type.

And the conclusion you have drawn, reasonably, is that hair serums do not work.

I would like to offer a different diagnosis. In many cases the molecules in those bottles were fine. The problem is that they never arrived. Between the pump and the follicle sits one of the most effective barriers in mammalian biology, and the overwhelming majority of hair products in the market are formulated as though that barrier does not exist.

I am Dr. Susan F. Lin, M.D. I trained at Boston University School of Medicine, I am board-certified in Obstetrics & Gynecology and in Anti-Aging Medicine (A4M), and I have been formulating topical products since 2008 as the physician behind the MD® line at La Cañada Ventures, Inc. This article is about the least glamorous and most decisive part of my job: not choosing an active, but getting it where it has to go. If you understand this one subject, you will never read an ingredient label the same way again.

The Barrier: Why Your Skin Is Supposed to Keep Things Out

Your skin’s outermost layer, the stratum corneum, is roughly 10 to 20 micrometers thick — a fraction of the width of a sheet of paper — and astonishingly good at its job.

Peter Elias’s foundational model describes it as a brick-and-mortar wall. The “bricks” are corneocytes: flattened, dead, keratin-packed cells with cross-linked protein envelopes. The “mortar” is a highly organized extracellular lipid matrix — ceramides, cholesterol, and free fatty acids in stacked lamellar sheets (Elias, 1983). That lipid mortar, not the cells, is what makes the barrier work.

The wall solves an evolutionary problem: keep water in, keep foreign molecules out. It does not know or care that you paid $89 for what you are applying; it treats a peptide the way it treats a pathogen. So the first thing to accept is that penetration is the exception, not the default. Every molecule that gets through does so because its physical chemistry and its vehicle allowed it to, and for no other reason.

Three Routes In — and Only One That Really Matters for Hair

A molecule applied to the scalp has three possible paths.

1. The Intercellular Route

The molecule threads through the lipid mortar between corneocytes. This is the dominant pathway for most topically applied substances — a tortuous, slow journey through alternating lipid and aqueous domains that strongly favors small, moderately lipophilic molecules.

2. The Transcellular Route

Straight through the corneocytes themselves — repeatedly crossing from lipid environment to protein-and-water environment and back. A chemical gauntlet few molecules navigate efficiently; it contributes comparatively little in practice.

3. The Follicular (Transappendageal) Route

The molecule travels down the hair follicle opening itself, bypassing the stratum corneum’s flat expanse by using a shaft already open to the surface.

For decades this route was dismissed as trivial, on the reasonable-sounding grounds that follicular orifices occupy only a small percentage of total skin surface area. That dismissal turned out to be wrong — and correcting it is one of the more consequential developments in modern delivery science.

The Follicular Reservoir: What Otberg’s Group Demonstrated

Two findings changed the field.

First, follicle density and geometry vary enormously by body site. Otberg and colleagues systematically quantified hair follicle size and distribution across human body regions and found dramatic differences — with the head and forehead showing far greater follicular density and considerably larger follicular infundibula than sites such as the forearm, where the earlier “negligible surface area” assumption had been formed (Otberg et al., 2004). The scalp is, in delivery terms, the most follicle-rich real estate on the body.

Second, the follicle is not just a hole — it is a reservoir. The follicular infundibulum functions as a storage depot: material deposited there remains in place far longer than material on flat skin, which is cleared within hours by desquamation, sebum flow, and washing. Blume-Peytavi and Vogt reviewed the follicle’s reservoir function and its implications for selective targeting, noting that this compartment sits in close anatomical proximity to structures of therapeutic interest — the sebaceous gland, the bulge region, the perifollicular vasculature and immune cells (Blume-Peytavi & Vogt, 2011).

Third, and most persuasively, the kinetics. Otberg’s group ran an elegant human experiment: they applied caffeine to skin in two conditions — one with follicular openings left open, one with the openings selectively blocked — and compared systemic uptake. With follicles open, caffeine appeared in the bloodstream markedly faster. Blocking the follicular route substantially delayed absorption (Otberg et al., 2008).

That is direct human evidence that the follicular pathway is a fast lane, not a footnote. Subsequent work confirmed that particles and vehicles of appropriate size are transported deep into the follicular duct, and that mechanical action — movement of the hair shaft in its canal, essentially a pumping motion — actively drives material downward (Lademann et al., 2007; Knorr et al., 2009).

Assemble the implication. If the enzymatic and cellular events of hair thinning happen at the follicle, and the follicle offers a preferential, reservoir-forming route into the skin, then a scalp product should be engineered specifically for follicular delivery. Most are not. Most are engineered for texture, fragrance, spreadability, and shelf appeal, with the active added to justify the label.

The Physical Chemistry: Not Every Molecule Can Get In

Even with a favorable route, molecules must satisfy hard physicochemical constraints.

Molecular Weight: The 500 Dalton Rule

Bos and Meinardi’s widely cited analysis proposed that compounds above roughly 500 Daltons do not effectively cross intact human stratum corneum — an observation supported by the fact that essentially all common topical drugs and contact allergens fall below that threshold (Bos & Meinardi, 2000).

That single number invalidates a great deal of marketing. Intact collagen, elastin, and most “protein” actives are orders of magnitude above 500 Da and cannot pass an intact barrier by the intercellular route. They may sit on the surface and condition the hair shaft — a legitimate cosmetic function — but they are not entering the follicle. It is also why peptides, not proteins, are the biologically interesting class in topical hair formulation: a short, deliberately engineered peptide can be built small enough to have a genuine delivery story.

Lipophilicity: The Goldilocks Problem

A molecule must be lipid-loving enough to enter the lipid mortar — but not so lipid-loving that it lodges there and never partitions out into the viable tissue beneath. Delivery science expresses this as an optimal partition-coefficient window: too hydrophilic and it never enters, too lipophilic and it enters and stays. Formulators spend real effort placing actives inside that window, usually by adjusting the vehicle rather than the molecule.

Solubility, Charge, and the Vehicle’s Grip

An active must also be genuinely dissolved to be available for transfer — and, critically, willing to leave the vehicle. This is the point almost every consumer misses: a vehicle that dissolves an active too well will not release it. Thermodynamic activity, not concentration, drives partitioning into skin. Charge matters as well; ionized molecules cross lipid membranes poorly, so formulation pH is not a stability footnote but a determinant of how much of your active exists in a penetrating form at the moment of application.

Why the Same Active at the Same Percentage Behaves Differently

Here is the sentence I most want you to take away from this article: the same molecule, at the same concentration, in two different vehicles, is effectively two different products.

The vehicle governs almost everything that matters after the pump.

Release. How readily the active partitions out of the formula and into the skin.
Follicular targeting. Particle size, viscosity, and surfactant content determine whether material enters the follicular duct or sits on top of it. Nanoparticle and vesicular carrier research repeatedly shows that appropriately sized systems concentrate in follicles rather than on flat skin (Lademann et al., 2007; Sinico & Fadda, 2009).
Barrier interaction. Some vehicle components transiently increase permeability; others form an occlusive film that hydrates the stratum corneum — and hydration alone increases permeability by swelling corneocytes and loosening lipid packing.
Residence time. A formula that runs off the scalp, absorbs into the hair fiber, or evaporates in ninety seconds has surrendered its contact window.
Stability. Antioxidant polyphenols in particular degrade in the bottle. A label percentage describes what was added at manufacture, not what remains at month nine.

This is why you cannot compare two products by reading their percentages, and why mixing an active into a carrier oil at home so often disappoints. The percentage is an input; delivery is the output, and the two are related only through formulation work that never appears on the label.

Contact Time and Application Method

Two more variables sit in your hands rather than mine.

Contact time is how long the active spends against the scalp at an effective concentration. Wash-off products — shampoos, conditioners — have contact times measured in seconds. Whatever their actives, the delivery window is tiny, which is why a “DHT-blocking shampoo” is a fundamentally harder proposition than a leave-on serum with the same molecule.

Application method determines whether the product reaches the scalp at all. Hair is extremely good at intercepting liquid before it reaches skin, and a serum distributed across the fiber does nothing for the follicle. Applying to scalp skin along exposed part lines, with a format that parts the hair and deposits at the surface, is not a comfort feature — it is a delivery variable of the same class as molecular weight. Gentle massage afterward matters too: movement of the hair shaft within its canal helps pump material into the follicular duct (Patzelt & Lademann, 2013).

The Bottom Line

The stratum corneum is a brick-and-mortar barrier evolved to exclude foreign molecules, and it makes no exceptions for expensive ones (Elias, 1983). Of the three routes across it, the follicular pathway matters most for hair — because the scalp is the most follicle-dense site on the body, because the infundibulum forms a long-lived reservoir adjacent to the structures we care about, and because human kinetic data show follicular transport is measurably faster than the transepidermal route (Otberg et al., 2004; Otberg et al., 2008; Blume-Peytavi & Vogt, 2011). Molecules must be small enough — roughly under 500 Daltons — and lipophilic enough, but not too lipophilic, to make the crossing (Bos & Meinardi, 2000). And the vehicle determines release, follicular targeting, hydration, residence time, and stability. Change the vehicle and you have changed the product, whatever the percentage on the front says.

Which brings me to the uncomfortable commercial truth underneath this whole article: formulation expertise is the product. The active is a commodity — anyone can buy the same raw material. What separates a serum that engages the follicle from one that decorates the hair shaft is delivery engineering, and delivery engineering is invisible on a label. That asymmetry is exactly why the drawer fills up.

Dr. Susan Lin’s Clinical Perspective

“The question patients ask me is ‘which active is best?’ The question I ask in the lab is ‘will this active arrive, and in what state?’ Those are not the same question, and the second one is harder. I have watched excellent molecules fail because they were suspended in a vehicle that would not release them, applied through hair that intercepted them, and given a contact window of ninety seconds. I have also watched a modest concentration outperform a headline one because it was engineered for the follicular route and applied directly to scalp skin. Percentages are the easiest thing to print and the least informative thing to read. If a brand can explain what its vehicle is doing and why its applicator exists, you are talking to a formulator. If it can only tell you how much is in the bottle, you are talking to a marketer.”

— Dr. Susan F. Lin, M.D., Physician Formulator, MD HAIR

Mechanism Spotlight: The Follicular Infundibulum as a Depot

Picture the top segment of a hair follicle — the infundibulum — as a funnel open to the surface, lined with a stratum corneum that becomes progressively thinner and less organized as it descends. Two features make it the most interesting real estate on the scalp. First, its barrier is incomplete: deeper in the duct, the tightly stacked lipid lamellae of surface skin give way to a more permeable epithelium, so a molecule that reaches this depth faces a much weaker wall than it would on flat skin. Second, it retains. Material on the skin surface is cleared within hours by desquamation, sebum flow, and washing; material in the infundibulum persists substantially longer, forming a genuine depot from which the active elutes over time (Blume-Peytavi & Vogt, 2011). Otberg’s caffeine work supplied the human kinetics: with follicular openings available, absorption was distinctly faster than when they were blocked, confirming the duct as an active transport route rather than an anatomical curiosity (Otberg et al., 2008). And the duct is not passive — the hair shaft moving within its canal acts as a pump, driving material downward with mechanical stimulation (Lademann et al., 2007). This is the anatomy that turns “apply to the scalp and massage gently” from a pleasant ritual into a delivery instruction.

MD HAIR Product Recommendation

MD® Follicle Energizer — Hair Density Scalp Serum

This is the product in our line where everything above became a design constraint rather than a talking point.

The molecule was chosen for its delivery profile. MD® Follicle Energizer is built on Biotinoyl Tripeptide-1 — a tripeptide, not a protein. Three amino acids, in the size class the 500-Dalton literature says can actually cross an intact barrier, which is precisely why I chose a peptide over the collagen and keratin “actives” that cannot.

The vehicle was engineered to release and to target, because a serum that holds its active too tightly delivers nothing — and the applicator is a delivery device, not packaging. The precision brush parts the hair and deposits serum on scalp skin at the hairline, crown, and part line, where thinning shows first, instead of scattering it across fiber that will be washed away. Apply to the scalp, then massage gently; as the research above shows, that movement helps carry material into the follicular duct.

It is drug-free and hormone-free, so it works with or without minoxidil, with no rebound shedding on discontinuation. Physician-formulated by Dr. Susan F. Lin, M.D., and manufactured in FDA-registered, GMP-compliant facilities in the USA. In a 119-day (17-week) open-label study conducted by Spincontrol North America on the two-step topical serum system (n=24, no placebo arm, outcomes self-reported by questionnaire), 71% of participants agreed their hair growth had improved; the report itself cautions that the overall satisfaction rate was “not significantly validated.” Individual results vary; study on file. MD® Follicle Energizer is a cosmetic product and is not intended to diagnose, treat, cure, or prevent any disease.

And the barrier problem explains one more design choice: some inputs are better delivered from inside than through the skin at all. That is the role of MD Nutri Hair™, one capsule daily — the internal arm of the routine, carrying lilac stem-cell extract standardized for verbascoside, the plant phenol which in laboratory studies on human dermal papilla cells induced cell proliferation, prevented testosterone-induced cell death, and reduced pro-inflammatory signals (Wisuitiprot et al., 2022; cell studies, not human trials).

Genuine MD HAIR and MD Nutri Hair™ products are sold only through mdhair.com, md-factor.com, and the official La Cañada Ventures, Inc. stores on Amazon and Walmart. Formulation integrity depends on how a product is stored and handled — which is why the channel matters as much as the label.

References

Elias PM. (1983). Epidermal lipids, barrier function, and desquamation. Journal of Investigative Dermatology, 80(Suppl), 44s–49s. PubMed
Bos JD, Meinardi MMHM. (2000). The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Experimental Dermatology, 9(3), 165–169. PubMed
Otberg N, Richter H, Schaefer H, Blume-Peytavi U, Sterry W, Lademann J. (2004). Variations of hair follicle size and distribution in different body sites. Journal of Investigative Dermatology, 122(1), 14–19. PubMed
Otberg N, Patzelt A, Rasulev U, et al. (2008). The role of hair follicles in the percutaneous absorption of caffeine. British Journal of Clinical Pharmacology, 65(4), 488–492. PubMed
Blume-Peytavi U, Vogt A. (2011). Human hair follicle: reservoir function and selective targeting. British Journal of Dermatology, 165(Suppl 2), 13–17. PubMed
Lademann J, Richter H, Teichmann A, et al. (2007). Nanoparticles — an efficient carrier for drug delivery into the hair follicles. European Journal of Pharmaceutics and Biopharmaceutics, 66(2), 159–164. PubMed
Knorr F, Lademann J, Patzelt A, Sterry W, Blume-Peytavi U, Vogt A. (2009). Follicular transport route — research progress and future perspectives. European Journal of Pharmaceutics and Biopharmaceutics, 71(2), 173–180. PubMed
Patzelt A, Lademann J. (2013). Drug delivery to hair follicles. Expert Opinion on Drug Delivery, 10(6), 787–797. PubMed
Sinico C, Fadda AM. (2009). Vesicular carriers for dermal drug delivery. Expert Opinion on Drug Delivery, 6(8), 813–825. PubMed
Wisuitiprot V, et al. (2022). Effects of Acanthus ebracteatus Vahl. extract and verbascoside on human dermal papilla and murine macrophage. Scientific Reports, 12, 1491. PubMed

Dr. Susan F. Lin, M.D. is the physician formulator behind MD HAIR and MD Nutri Hair™, product lines of La Cañada Ventures, Inc. — physician-formulated since 2008 under the MD® mark. She trained at Boston University School of Medicine, is board-certified in Obstetrics & Gynecology and in Anti-Aging Medicine (A4M), and is the named inventor on U.S. Patent No. 8,206,695 (the MD® lash formulation).

MD HAIR topical products are cosmetics manufactured in FDA-registered, GMP-compliant facilities; facility registration is not product approval by the FDA. MD Nutri Hair™ is a dietary supplement. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. The penetration and delivery research described here characterizes skin and follicle biology and formulation science generally; it is not a clinical outcome claim for any product. Individual results vary. This article is for educational purposes and does not constitute medical advice. Consult your own physician for personalized guidance. Because there are no clinical data in pregnant or breastfeeding women, we do not advocate using MD HAIR products during pregnancy or lactation.