Chapter 38: Bioengineering of the Skin

INTRODUCTION

Bioengineering of the skin is a scientific discipline devoted to developing standardized methodologies to measure skin findings in a scientific manner. The International Society for Bioengineering of the Skin has its own journal and there are multiple books published on these methods. It is vital to understand these methodologies in order to effectively evaluate the scientific merit of studies in the dermatology literature. These measurements allow the cosmetic dermatologist to evaluate the efficacy of particular treatments and to objectively measure treatment outcomes. These tests are a necessary addition to cosmetic dermatology clinical trials and they aid cosmetic dermatologists in their pursuit of practicing evidence-based medicine in a cosmetic setting.

SUBJECTIVE MEASUREMENTS

The Lactic Acid Stinging Test

Even when this method lacks objective criteria, this test is widely accepted as a marker of sensitivity and as a standard method for evaluating individuals who report invisible and subjective cutaneous irritation. Stinging is considered to be a variant of pain that develops rapidly and fades quickly any time the appropriate sensory nerves are stimulated¹ (see Chapter 17). In one study, Frosch and Kligman applied 5% lactic acid to the nasolabial fold (a site highly innervated with sensory fibers), when the subject was sweating profusely.² Under these conditions, approximately 20% of the research group reported experiencing an unpleasant sensation. In a different study, Seidenari et al. applied 10% lactic acid solution to the nasolabial fold and observed that individuals with “sensitive skin” experienced a much stronger stinging sensation than those in the healthy control group.³ However, not all studies agree that patients with sensitive skin are more likely to be “stingers.” In order to achieve a more reliable response, applying an inert control substance such as saline solution to the contralateral test site is recommended. In addition to lactic acid, other water-soluble substances such as capsaicin can be used.

The Chloroform Methanol and Sorbic Acid Tests

Like the test mentioned above, these may help the practitioner identify patients with sensitive skin. In these procedures, the practitioner applies the substance to the face of the patient to determine if stinging or burning occurs.⁴

Challenge Patch Test

This test uses sodium lauryl sulfate and other detergents that are known to be irritants. The irritants are placed on the skin to determine a patient’s sensitivity. In cases of an impaired epidermal water barrier, demonstrated by increased transepidermal water loss (TEWL), the detergents will be more likely to cause irritation. This test is used to examine epidermal barrier function. In one study, the purpose was to discover whether “stingers” (patients with sensitive skin) might represent an easily and rapidly identifiable subpopulation with a more generally increased tendency to manifest skin responses. The response to a 0.3% sodium dodecyl sulfate patch test was assessed in a ...