Tissues, skin, and contact with an object

Why this chapter exists

• The most common serious harm from a maker's contact object
• Almost always caused by someone competent
• A well-machined splint with a crisp edge
• A moulded handle with its parting line across the point of highest load
• A strap sized in the morning on a limb that swells by afternoon

Epithelium — two facts with design consequences

• Named by cell shape and layers: simple squamous in alveoli, stratified in skin
• Avascular: fed by diffusion from the connective tissue beneath
• Damage to the layer beneath is what causes real trouble
• A continuous epithelium is the primary barrier
• Breach it — even a 3 mm pressure break — and it becomes a wound project

The epidermis and its barrier

• Keratinocytes form the barrier; Langerhans cells keep watch
• Melanocyte number is similar across populations; melanin amount differs
• That matters for optical sensors — see the pulse oximeter
• Strip the stratum corneum and the barrier fails before any wound is visible
• Medical adhesive-related skin injury is a design-caused harm

What the pressure calculation cannot establish

• Distribution — a ridge, seam, or fastener head carries the peak
• Depth — damage often starts near bone, before the surface shows it
• Time — load and duration trade off non-linearly
• The person — perfusion, age, moisture, sensation, mobility
• Movement — sliding adds shear and friction

Inflammation and repair

• Haemostasis — minutes: clot and platelet plug
• Inflammation — hours to days: redness, heat, swelling, pain
• Proliferation — days to weeks: new vessels and collagen
• Remodelling — weeks to months, or longer
• Swelling changes dimensions within hours
• Scar is not replacement

Preventing a 5 mm skin break matters more than being good at responding to one.

Training technology — and the maker's entry point

• Suture and skin pads: layered silicone, consumed by the hundred
• Moulage and wound-care trainers
• Instrumented interface rigs: force sensors under cast silicone
• Sewn demonstration straps that vary one thing each

The skill is mould making — and making the hundredth pad match the first.

Workshop exercise and exit criteria

• Build a layered skin model — label it schematic, not to scale
• Compare a narrow and a broad contact; mark where seams concentrate force
• Demonstrate shear with foam sheets — and list how it is unlike skin
• Draw a contact map for the elbow model

Do not run pressure or prolonged-contact experiments on yourself or anyone else.

What this chapter does not establish

• You cannot set a pressure limit
• You cannot assess anyone's skin
• A bench interface-pressure measurement is not a safety assessment
• "Skin-safe" needs evidence about a finished object in a specific use
• No part of this authorises prolonged-contact or wound-contact work

Next — Bones, joints, and movement.