The Skin Barrier and Delivery Routes
This document is technical information about delivery routes. It does not recommend or evaluate any procedure, product or device.
Question
Under what conditions does a topical formulation reach below the stratum corneum?
The stratum corneum blocks most molecules. The literature reports that penetration of topically applied substances increases when physical channels are created, and on that axis methods divide into those that breach the barrier and those that preserve it.
Delivery routes — one axis: is a channel created?
| Route | Stratum corneum | Reported depth | What the literature reports |
|---|---|---|---|
| Needle-based — microneedling and needle radiofrequency | Reported to vary with needle length and device settings | Needles pierce the stratum corneum and create reversible microchannels, through which permeation of macromolecules that would not otherwise cross is reported to increase. 36678906 | |
| Ablative fractional laser | Channel depth is reported to vary with treatment parameters | Tissue is vaporised, leaving vertical channels. Channel depth and density are reported to influence the tissue distribution of topically applied substances. 24664987 | |
| Focused ultrasound | Focal zones are reported from the dermis to deeper planes | In ex vivo human skin, confined thermal coagulation was reported at the focal zone while the overlying epidermis was spared. Leaving the epidermis intact is a design objective of this modality. 18429926 | |
| Non-invasive radiofrequency | Heating of the dermis and deeper planes is reported | Surface cooling protects the epidermis while deeper layers are selectively heated. Preserved epidermal integrity is reported as a factor that reduces downtime and complication risk. 17544932 | |
| Topical application without additional intervention | — | A rule of thumb has been proposed that compounds above roughly 500 Daltons do not readily cross the stratum corneum. It is presented as an observed tendency rather than an absolute cut-off. 10839713 |
This axis is not a ranking. For the methods marked as preserving the barrier, leaving the epidermis intact is the design objective.
Evidence
| PMID | Study type | Summary |
|---|---|---|
| 10839713 | review | Proposes 500 Daltons as a practical upper limit for percutaneous absorption |
| 36678906 | review | Review of biopharmaceutical delivery through microneedle-created channels |
| 24664987 | review | Review of laser-assisted delivery technology |
| 25417565 | review | Review of reports that ablative fractional lasers enhance topical delivery |
| 26266688 | in vivo | Optical coherence tomography imaging of laser-ablated channels closing over time |
| 18429926 | in vitro | Reports confined dermal thermal coagulation with epidermal sparing in ex vivo human skin |
| 17544932 | review | Review of surface cooling and selective deep heating in non-invasive radiofrequency |
Limitations & open questions
This page concerns physical delivery routes, not outcomes or efficacy. A report that channels form does not indicate the result of any particular formulation. Most cited studies examined specific drugs or molecules and do not transfer directly to every topical formulation. Channels are reported to close over time (PMID 26266688), so timing of application may affect the outcome. Methods that preserve the stratum corneum do so because leaving the epidermis intact is itself the design objective, so this table describes a difference in purpose rather than a ranking. Behaviour varies by device type and settings, and questions about individual products should be directed to that manufacturer's regulatory documentation.
Material references usually stop at describing what a material is. The question that decides matters in practice comes after that: does it get into the skin, and if so, under what conditions.
The stratum corneum is where that question starts. The literature reports that crossing it becomes harder as molecular size increases, and a widely cited review proposes roughly 500 Daltons as a practical upper limit. It is presented as an observed tendency rather than a hard boundary, but it serves as the reference line when the limits of topical formulations are discussed.
The table above examines a single axis: whether a physical channel is created through the stratum corneum. Methods reported to create channels separate from methods reported to preserve the barrier.
One point is easy to misread. Methods that preserve the stratum corneum do so not through any shortcoming but because leaving the epidermis intact is the design objective. The literature describes preserved epidermal integrity as an advantage that reduces downtime and complication risk. This table does not rank the methods; it compares technologies built for different purposes along one axis only.
Depth of action is a separate matter from this axis. A method reported to act at dermal depth still creates no route for a topically applied substance if it does not open the stratum corneum. Overlapping in depth and creating a channel are two different facts.