3.4 Skin · ThinkerQAQ

This is a historical learning note and may contain outdated or incomplete understanding.

1. Skin

1.1. What Is Skin?

An organ that covers the surface of the body.

1.2. Why Do We Need Skin?

Protection, excretion, regulation of body temperature, and sensing external stimuli.

1.3. Skin Structure

1.3.1. Epidermis

Stratum corneum: it can reflect light. If this layer is smooth, it can reflect light evenly. Granular layer Spinous layer Basal layer Basement membrane: contains parent cells that can generate other cells and push them upward.

The epidermis contains NMF. In a dry environment, more NMF is produced.

1.3.2. Dermis

There are many kinds of cells. The most numerous are fibroblasts, which secrete collagen, elastic fibers, hyaluronic acid, and other substances, forming an environment that binds strongly with water and has high elasticity. It also contains abundant blood vessels and sensory nerve receptors.

1.3.3. Subcutaneous Fat Layer

1.3.4. Skin Appendages

1.3.4.1. Hair Follicles

Skin appendages in which hair grows. They open onto the skin surface (pores) and extend deep into the dermis.

1.3.4.1.1. Pores

Openings from which hair grows and through which sebaceous-gland secretions are discharged.

1.3.4.2. Sebaceous Glands

Made of sebaceous-gland cells, which can synthesize large amounts of oil.

1.3.4.2.1. Sebum

Sebaceous glands secrete oil, which is transported to the skin surface and broken down by esterases to form a film covering the skin surface. Skin barrier, moisturization.

1.3.4.3. Sweat Glands

Glands that secrete sweat. Their openings are called sweat pores.

2. Skin Classification

Oily vs. dry
Resistant vs. sensitive
Pigmented vs. non-pigmented
Tight vs. wrinkled

3. Skin Problems

3.1. Very Oily Skin

3.1.1. What Is Very Oily Skin?

Excess sebum means very oily skin.

The skin becomes visibly shiny with oil two hours after washing the face with cleanser.

3.1.2. How to Handle It

Hormones: reduce oil and sugar in the diet. Temperature: cool down. Psychological factors: avoid excessive stress. Reactive oil secretion: cleansing and hydration. Skincare products:

A Video to Make Oily Skin Less Greasy - Bilibili

3.2. Enlarged Pores

Dirt blockage: cleansing and hydration. Aging: sun protection, IPL photorejuvenation. Inflammation: anti-inflammatory medication. Structural cause: dermal atrophy; fractional laser.

3.3. Acne

Three major factors: increased sebum secretion, clogged pores, and Propionibacterium acnes. Increased sebum secretion makes dead skin cells stick together and block pores, forming blackheads or whiteheads. Then Propionibacterium acnes enters the pores and causes inflammation, eventually producing redness, swelling, and pustules.

3.3.1. Comedones

3.3.1.1. What Are Comedones?

Whiteheads: closed comedones; neither red nor painful, about the size of a grain of rice. Blackheads: open comedones.

3.3.1.2. Causes of Comedones

Whiteheads: after pores are blocked, sebum cannot drain properly. A mixture of keratinocytes, skin flakes, and other material accumulates and forms whiteheads. Blackheads: after pores are blocked, oil cannot drain properly. A mixture of keratinocytes, skin flakes, and other material accumulates and is oxidized by the air, forming blackheads.

3.3.1.3. How to Handle Comedones

Physical removal: peel-off masks, comedone extractors, etc. Cleansing and hydration. Medication: fusidic acid + tretinoin/adapalene + benzoyl peroxide.

3.3.2. Inflammatory Acne

3.3.2.1. What Is Inflammatory Acne?

Red acne lesions.

3.3.2.2. Causes of Inflammatory Acne

Comedones + inflammation (excess oil secretion promotes anaerobic bacterial growth, which triggers an immune response and inflammation) -> inflammatory acne, such as papules, pustules, and nodules.

3.3.2.3. How to Handle Inflammatory Acne

Cleansing and hydration. Medication: tretinoin/adapalene + benzoyl peroxide.

Do not squeeze acne in the facial “danger triangle”: Why You Shouldn’t Squeeze Pimples in the “Danger Triangle,” and What to Do Instead - The Paper

3.3.3. Acne Marks

3.3.3.1. What Are They?

Red or darkened areas of skin.

3.3.3.2. Causes of Acne Marks

Red acne marks: inflammatory acne causes capillary dilation. After the acne subsides, the blood vessels do not immediately contract, leaving flat red patches. Dark acne marks: red acne marks + pigmentation.

3.3.3.3. How to Handle Acne Marks

Reduce inflammation. Sun protection. Medication: Hirudoid, Contractubex, and related scar products.

3.3.4. Acne Scars

3.3.4.1. What Are They?

Depressed areas of skin.

3.3.4.2. Causes of Acne Scars

If inflammatory acne is accompanied by dermal damage, depressed scars can form after the acne resolves.

3.3.4.3. How to Handle Acne Scars

Fractional laser. Fractional Laser — Guangzhou Shenzhen Institute of Dermatology / Shenzhen Center for Chronic Disease Control — Do People Regret Fractional Laser or Dermabrasion for Acne Scars? How Effective Is It? Shenzhen Institute of Dermatology / Shenzhen Center for Chronic Disease Control — Is Ablative Fractional Laser Available? Which Machine Is Used and How Much Does It Cost? Explainer: Why Is Fractional Laser Skin Resurfacing So Popular? What Is It, What Can It Treat, and What Are the Risks? - Zhihu

3.4. Dark Circles

3.4.1. What Are They?

Darkening around the eyes.

3.4.2. Causes of Dark Circles

Bluish dark circles: there are many capillaries around the eyes. If the oxygen content of hemoglobin in red blood cells decreases, the area can look bluish. Brown dark circles: post-inflammatory pigmentation.

3.4.3. How to Handle Dark Circles

Bluish dark circles: sleep early + exercise + warm compress. Brown dark circles: sun protection.

3.5. Eye Bags

3.5.1. What Are They?

Sagging and swelling of the lower-eyelid skin, producing a bag-like appearance.

3.5.2. Causes of Eye Bags

Excess orbital fat accumulation and weakening of eyelid support structures.

3.5.3. How to Handle Eye Bags

Surgery: remove excess fat and tighten the skin and muscles around the eyes.

3.6. Skin Aging

3.6.1. What Is It?

Intrinsic aging: controlled by a person’s genes and cannot be intervened in. Extrinsic aging: affected by environmental factors such as smoking and sunlight and can be intervened in.

3.6.2. How to Handle It

Prevent loss of collagen, elastic fibers, and hyaluronic acid. For example, vitamin C and retinoic acid can help prevent collagen loss.
Sun protection.

3.7. Damaged Skin Barrier

3.7.1. What Is It?

Redness, itching, and peeling.

3.7.2. How to Handle It

Moisturize.

4. Skincare Pyramid

4.1. What Is It?

It originally came from a 2014 paper by Dr. Julie Kenner and Dr. Zoe Draelos in JDD (Journal of Drugs in Dermatology). It is a tool intended to help doctors scientifically recommend functional skincare products.

4.2. First Version

Three major levels and eight smaller levels. The most important is sun protection, followed by vitamin A. The higher the level, the more vague the claims become.

4.3. Misreading of the First Version

Three major levels and six smaller sections. The bottom level is basic skincare, the middle level is advanced skincare, and the top level is the highest level of skincare. This can encourage people to pursue “advanced” skincare.

4.3.1. First Level

4.3.1.1. Cleansing

Remove dirt from the skin.

How to wash your face:

Wash with clean water.
Wash with facial cleanser.
Facial cleanser is not always necessary. For example, oily skin should use cleanser no more than twice per day.
Water temperature: 30–35°C.
Do not exceed 15 seconds.
Spread in one direction instead of rubbing back and forth. Gently press the T-zone.
Press with a tissue to absorb water; do not rub.
Use moisturizing lotion/cream after washing depending on the situation. For example, oily skin may not need moisturizer after washing with water, but it is better to use some moisturizing lotion after using cleanser. You Don’t Actually Know How to Wash Your Face! | Essential Skincare Steps | The First Step to Good Skin | Four Face-Washing Principles - Bilibili How to Wash Your Face Properly? Seven Incorrect Face-Washing Methods - Bilibili

4.3.1.2. Moisturizing

Prevent evaporation of water from the skin.

Lotion / cream
Face mask (not necessary)

For oily skin, it is not necessary unless the skin barrier is damaged (redness and itching), for example after using retinol or exfoliating acids. La Roche-Posay Hyaluronic Acid vs. CeraVe Moisturizing Cream vs. La Mer Cream — Moisturizing Effect Comparison - Bilibili

4.3.1.3. Sun Protection

Prevent ultraviolet light from damaging the skin, such as sunburn, tanning, and photoaging.

Physical sun protection: parasols, sun hats, face coverings, etc.
Apply sunscreen.

Notes:

Sun protection is not needed on rainy days. On sunny days use hard physical barriers; if looking at the sky feels dazzling, apply sunscreen.
Areas in shade are safe and direct sunlight is unsafe. Do not expose skin to direct sunlight.
Opaque barriers are reliable. Semi-transparent barriers, including sunscreen, are often not strong enough.
Sunscreen is important. Even a thin layer can block about half the radiation, but no amount can withstand intense midday sun.
UV radiation from lamps, computers, and phones is extremely low. Sunscreen is not needed for computer or phone use.
On clear winter days, UV can still be strong, so use sun protection as needed.
During active acne breakouts, try to avoid sunscreen and instead use a parasol, wide-brimmed hat, and other physical coverings, because many sunscreens are occlusive and may accelerate growth of acne-associated fungi.
Chemical sunscreen can be removed with facial cleanser, while physical sunscreen cannot.

SPF: protects against short-wave UVB, meaning protection against redness and sunburn. PA: protects against long-wave UVA, meaning protection against tanning and photoaging.

For ordinary skin tones, SPF 8–12 is suitable. For fair skin, SPF 30 sunscreen is recommended. For people with photosensitivity, SPF 12–20 is suitable. For ordinary office work, SPF 15 and PA+ can be used. For outdoor exercise, SPF 25–35 and PA++ are suitable. For swimming at the beach, SPF 35–50 and PA+++ can be used. It is best to apply sunscreen to exposed areas half an hour before going outside and reapply every 3–4 hours.

4.3.2. Second Level

4.3.2.1. Neutralization

Use antioxidant ingredients to neutralize oxidative stress caused by environmental factors such as ultraviolet light and pollution. Neutralize excessive free radicals and inflammatory factors.

Use products with antioxidant effects (such as vitamin C, vitamin E, and green-tea polyphenols) and anti-inflammatory effects (such as olive extract and Centella asiatica extract).

4.3.2.2. Repair

Use extracellular-matrix-like ingredients to strengthen the skin barrier and improve skin quality.

Use vitamin B5 to repair damaged skin, retinol to repair ultraviolet damage, and ceramides to repair the skin barrier. Use products containing skin-repair ingredients such as ceramides, glycerin, and sodium hyaluronate.

4.3.3. Third Level

4.3.3.1. Regeneration

Enable intercellular communication, renew and differentiate cells at the cellular level, and promote regeneration of cells and extracellular matrix.

Use products containing ingredients that promote cell and extracellular-matrix regeneration, such as growth factors, peptides, and skin-tissue components. Use acids and vitamin A to activate mechanisms of skin-cell regeneration.

4.4. Second Version

Three levels.

Bottom level: protection and repair
- Addresses skin damage from ultraviolet light, air pollutants, smoking, near-infrared light, blue light (phone and computer screens), etc.
  - Sun protection should be the main approach, combined with antioxidants. These include protective ingredients such as retinol, pure vitamin C, and niacinamide. Oral antioxidants have not been sufficiently supported by evidence.
- Maintenance of the skin microbiome
  - Some prebiotic and probiotic ingredients may protect the skin barrier, but more evidence is still needed.
Middle level: renewal (moisturizing, exfoliation, cell renewal)
- Use moisturizers to maintain normal skin-barrier function. For example, glycolic acid and salicylic acid can be used for exfoliation and should be combined with moisturization; vitamin A promotes cell renewal.
Top level: activation and regeneration
- Mainly at the dermal level. New technologies include peptides, growth factors, stem cells, and circadian-rhythm regulators. The claims are relatively vague.

4.5. How to Apply It

Use good sun protection and protect the skin barrier.

Known sources of damage such as ultraviolet light, air pollution, near-infrared light, and blue light can receive some protection from sunscreen, moisturizing cream, and even foundation. If microbiome-oriented skincare ingredients are useful for you, they can be used, usually together with antioxidants such as pure vitamin C, niacinamide, and green-tea polyphenols.

Moisturize well and regularly use glycolic acid, salicylic acid, or vitamin-A/retinol ingredients.

This is currently one of the most effective approaches for repairing photodamage and making the skin stronger, smoother, and firmer.

Maintain regular exercise, a healthy diet, and sleep.

Respecting the circadian rhythm is currently one of the best ways to support activation and regeneration.

5. Functional Ingredients in Skincare Products

5.1. Cleansing

Soap base: essentially no different from soap. Suitable for oily, resistant skin.
Surfactants:
- SLS/SLES: gradually being phased out because of strong irritation and safety concerns. Suitable for oily, resistant skin and combination skin.
- Anionic or amphoteric surfactants: suitable for most skin types except very oily skin, such as amino-acid, betaine, and sulfonate types.
- Nonionic surfactants: suitable for dry and sensitive skin, such as glucosides.

5.2. Moisturizing

Glycerin: a natural humectant that attracts water from the air and hydrates the skin.
Petrolatum: chemically stable and can form a protective film on the skin, keeping it in a soothing state. It does not add water to the skin but locks water in.
Aloe vera: can penetrate into the skin and retain moisture, preventing evaporation of surface water.
Provitamin B5: also called panthenol. It is a penetrating moisturizer. After being absorbed by the skin, it can penetrate directly into the stratum corneum and increase hyaluronic-acid content in the skin, providing moisturization.
Natural moisturizing factor: abbreviated NMF. Its main components include amino acids, sodium lactate, urea, etc. It is an adsorptive, water-soluble substance that binds with water in the stratum corneum and regulates and stores water to maintain the water content of corneocytes, keeping the skin naturally hydrated.
Hyaluronic acid: has very strong water absorption and is an important mucopolysaccharide in the dermis. It can absorb hundreds of times its own weight in water and quickly fill the stratum corneum with moisture, so it is called a “high-efficiency moisturizing ingredient.”
Hydrolyzed collagen: can effectively retain moisture in the skin and prevent excessive water loss, keeping the skin moist and supple.
Squalene: can provide oxygen and nutrients to cells, promote cell metabolism, and form a sebum film outside the skin to prevent water loss.
Seaweed extract: contains multiple active compounds. Combined with body proteins, it can form a gel-like collagen structure, enhancing the skin’s ability to retain water and improving moisturization.
Ceramides: contain many hydrophilic groups and have a strong affinity for water. They are highly efficient moisturizers that can strengthen the skin barrier and lock in moisture.
Shea butter: can form an emulsion-like protective layer on the skin surface, preventing water loss and providing strong moisture retention.
β-glucan: a natural polysaccharide that can form a transparent, elastic, breathable film on the skin. Its macromolecular structure can lock in water and prevent water loss, providing efficient moisturization.

5.3. Vitamin C

Antioxidant; counters free radicals. Take a Look at the Pioneer of Vitamin C Serums - Zhihu

5.4. Vitamin A and Its Derivatives

Adapalene.

5.5. 377

Skin darkens because melanin increases in deeper layers, is transported to the skin surface, and accumulates in the stratum corneum, changing skin color. How is melanin produced? Melanocytes produce it through the following pathway: tyrosine + oxygen –tyrosinase–> melanin. Whitening either inhibits this process or accelerates metabolism.

377 takes the inhibition route: it inhibits tyrosinase activity.

Skincare Explainer: What Is the Whitening Ingredient “377”? (Including an Analysis of the Whitening Mechanism) - Zhihu

5.6. Zinc

Reduces sebum secretion.

5.7. Hydroquinone and Arbutin

Whitening.

5.8. Niacinamide

Whitening: intercepts melanin transport in the middle of the pathway. Improves the skin barrier. Regulates sebum secretion.

When Using Niacinamide for Whitening, Pay Attention to These Points to Avoid Ineffectiveness or Skin Damage

5.8.1. Niacinamide + Vitamin C

Vitamin C needs an acidic environment (pH < 3) to work, while niacinamide needs a neutral environment (6–7.5) to remain stable; otherwise at high temperatures (90–100°C) it may produce nicotinic acid. Vitamin C derivatives do not require acidic conditions, so vitamin C derivatives can be used.

5.8.2. High-Concentration Niacinamide

Why High-Concentration Niacinamide Does Not Whiten Skin, Based on the Literature - Bilibili

The Ordinary 10% niacinamide mainly targets oil control and acne rather than whitening.

5.9. Acids

Salicylic acid: oil-soluble (strong pore penetration). Exfoliates keratin and has some oil-control, anti-inflammatory, and antibacterial effects. Effective concentration is 1–2%.
Alpha hydroxy acids: water-soluble (weaker pore penetration). Exfoliate keratin; effective concentration is around 8–10%.
- Mandelic acid: an oil-soluble AHA. Suitable for acne-prone skin that is relatively sensitive and thin.
Retinoic acids: exfoliate keratin, treat acne, and help prevent photoaging.
- Retinoic acid: tretinoin cream, adapalene gel.
- Retinoic-acid derivatives: need to be converted into retinoic acid; conversion efficiency is retinal > retinol > retinyl ester.
Azelaic acid: antibacterial, oil-controlling, helps prevent keratinization, and helps prevent dark acne marks. More suitable for pustular and inflammatory acne. Not recommended for use over the entire face.

6. Types of Skincare Products

6.1. Facial Cleanser

6.1.1. Recommendations

Dove amino-acid cleansing foam. ~~Dove anti-acne cleanser: contains IPMP.~~ ~~CeraVe salicylic-acid cleanser.~~

6.2. Facial Mist

Not necessary.

Cools the skin and reduces oil secretion.

6.2.1. Recommendations

Kong Feng Chun purslane moisturizing spray
Facial-Mist Recommendations: Avène, La Roche-Posay, Uriage, Uniskin, RELLET, Collgene, and Kong Feng Chun — Which One Suits You? - Bilibili

6.3. Toner

Not necessary.

Astringent toner

Astringent: contains alcohol.
Exfoliating: contains AHAs.

Moisturizing toner

Moisturizing; suitable for dry skin.

Cleansing water

Makeup remover.

6.4. Eye Cream

Not necessary.

Although the skin around the eyes is especially thin, its difference from ordinary skin is not that large. Eye cream does not cause milia.

6.5. Serum

Different serums have different functions and generally focus on one function. They belong to the second and third levels of the skincare pyramid.

6.6. Lotion / Cream

6.6.1. Formula

Water
Humectants: draw water up from deeper layers of the skin.
Occlusives: form a film on the skin surface to prevent water evaporation. The higher the content, the stickier and oilier the skin feel.
Emollients: quickly fill gaps between corneocytes.