Science — Conceptual Framework
The Functional Depth Model in Chemical Peeling
Revisiting peel depth beyond concentration-based classification. A practical framework integrating penetration dynamics, tissue response, and metabolic tolerance.
1) Limits of Concentration-Based Models
Traditional peel depth classifications (superficial, medium, deep) are often presented as if acid concentration were the primary determinant of depth. In real clinical settings, however, concentration alone does not reliably predict biological depth or outcome.
Why “concentration = depth” can be misleading
- Skin permeability varies widely between patients and anatomical zones.
- Barrier status, hydration, and inflammation modify penetration and tissue response.
- Protocol sequencing and preparation can change outcomes at identical concentrations.
- “Depth” is not only chemical: it is also biological (cellular tolerance and reaction kinetics).
The goal of the Functional Depth Model is not to reject historical classification, but to extend it with clinically relevant variables that influence functional depth and safety.
2) Biological Variables Influencing Depth
Depth is better understood as a function of penetration dynamics and tissue response. The following variables frequently influence how “deep” a protocol behaves clinically.
Penetration & Tissue Access
- Barrier integrity and stratum corneum compactness
- Superficial vs deep dehydration (often underestimated)
- Vehicle, layering strategy, and protocol sequencing
- Anatomical zone variability (face, peri-oral, neck, body)
Tissue Biology & Reaction Kinetics
- Inflammation baseline and sensitivity/reactivity
- Redox balance and oxidative stress status
- Melanin distribution and “deep” hyperchromia patterns
- Cellular tolerance influencing predictability and safety
Practical takeaway: a protocol may behave “deeper” not because concentration is higher, but because penetration and tissue response are optimized (or destabilized).
3) The Functional Depth Equation
The Functional Depth Model proposes that depth should be interpreted through three interacting domains: penetration, tissue response, and metabolic tolerance.
Proposed Conceptual Equation
Functional Depth = Penetration × Tissue Response × Metabolic Tolerance
Penetration
The effective access of actives to targeted layers, shaped by barrier status, vehicle, and sequencing.
Tissue Response
The biological reaction pattern (inflammation, melanin response, repair kinetics) that shapes clinical outcome.
Metabolic Tolerance
The skin’s capacity to tolerate controlled stress while maintaining predictability and limiting adverse reactions.
This model reframes “depth” as a functional output—rather than a simple proxy of concentration.
4) Clinical Implications
Clinically, the Functional Depth Model supports a strategy where depth is achieved through protocol engineering and biological stabilization—rather than concentration escalation.
Deep Dehydration Is Often Overlooked
Dehydration may be superficial or deeper. Addressing only surface hydration can lead to unstable responses and reduced tolerance in corrective protocols.
Hyperchromia Can Be Deeper Than Expected
Pigment patterns may be more than epidermal. Depth targeting can require a protocol capable of controlled penetration and stable tissue response.
Low-Concentration Acid Strategies
- In selected protocols, lower-concentration TCA may achieve meaningful functional depth when penetration is optimized.
- A similar logic may apply to salicylic acid when the objective is controlled depth without escalation.
- The clinical target becomes functional efficacy with improved tolerance—rather than higher chemical load.
Practical takeaway: protocol design can shift depth strategy from “more concentration” to “better biology + better penetration control.”
5) Risk Reduction Perspective
Traditional “deep peel” strategies are often associated with increased complication probability, partly due to higher concentrations and more aggressive tissue stress. The Functional Depth Model supports an alternative: depth through controlled penetration and stabilized tissue biology.
Predictability
Stabilizing tissue response can reduce variability between patients and anatomical zones.
Tolerance
Better tolerance can reduce the need for concentration escalation and limit adverse reaction patterns.
Safety Logic
Depth obtained via protocol engineering may reduce reliance on high-concentration “deep peel” exposure.
Note: this framework describes a conceptual approach. Clinical outcomes remain patient-dependent and require professional judgment.
6) Conceptual Shift & Conclusions
The Functional Depth Model proposes a conceptual shift: peel depth should be interpreted as a functional output rather than a chemical label.
Key Conclusions
- Concentration alone does not define functional depth.
- Penetration dynamics and tissue biology strongly influence clinical “depth behaviour.”
- Metabolic tolerance is a central variable for predictability and safety.
- Protocol engineering may allow deeper functional outcomes without concentration escalation.
Functional Depth = Penetration × Tissue Response × Metabolic Tolerance
A framework designed to complement traditional classification with clinically actionable variables.