1. Skin Tissue Mechanics BME 615 “ Carving (for sculpture) is easy, you just go down to the skin and stop.” Michelangelo

2. Function – Insulation – Protection of underlying organs – Production of vitamins D and B – Sensation – Excretion – Heat regulation – Storage of lipids and H2O – Absorption of N2, O2, and CO2

3. Largest organ in body, in an adult – Approximately 15% of body weight –1.5-2.0 square meters (~20 square feet) and 1-2 mm thick – Average sq. in. contains: 650 sweat glands, 20 blood vessels, 60,000 melanocytes, >1,000 nerve endings –Thick (hands & feet) and thin (elsewhere, can grow hair)

4. Anatomy

5. Epidermis  Outer layer of skin  Cells in deeper layer divide and move towards surface (takes ~ 2 weeks)  Cells flatten and develop keratin fibers (cells toughen and become waterproof)  Cells (keratinocytes and melanocytes) die and flake off (every minute we loose ~30,000 to 40,000 cells or ~ 10 pounds or 4.5 kilograms of cells per year)  Pigment or melanin give skin its color and blocks suns UV rays  No blood vessels, nourished by diffusion

6. Dermis  Inner layer of skin containing capillaries, sensory nerve endings, lymphatic vessels, sweat glands, and hair follicles (with erector muscles for goose bumps)  Areolar connective tissue (ECM is collagen, elastin, ground substance, etc.)  Cells are fibroblasts, adipocytes, macrophages  Fibroblasts are constantly dividing because skin flakes off so fast

7. Subcutaneous Fat  Hypodermis  50% of body fat  Padding and insulation

8. Aging and Disease  Aging causes skin to become thinner and more easily damaged with sagging (elasticity loss), less blood flow, lower gland activity  Benign tumors  Skin cancer  Rashes, blisters, acne, fungal infections, microbial infections, ulcers, etc.

9. Ehlers-Danlos Syndrome (defect in collagen I)

10. Why skin mechanics? Function –Stretching appropriately over joints –Toughness for extreme activities Skin expansion for free flaps in plastic surgery Wound closure Cosmetic surgical procedures Tissue engineering Leather goods??? A tissue that exhibits hyperelastic behavior