⚠ ARCHITECTURE UPDATE (May 2026):
This page now describes the May 2026 spec — Nylon 30–50 strand tendon, capstan/screw/washer/backup ferrule termination at the bone.
The earlier "TPU elastic cord + Kevlar X-wrap" tendon has been superseded.
TPU is optional and used only for stretchy side ligaments, never as the main load path. Kevlar remains in fascicle cores (DWG-03) and ligament panels only.
TENDONS — NYLON 30–50 STRAND (May 2026)
Biological tendons are force transmitters that happen to be slightly elastic. In the May 2026 architecture we replicate that: 30–50 strand Nylon monofilament (P-24) bundles act as near-inextensible cables that carry muscle force directly to bone. TPU and Kevlar-X-wrap constructions were tried and rejected — see rationale below.
MATERIAL RATIONALE — WHY NYLON WON
Human tendon operates at 0.7–1.2 GPa Young's modulus. Among practical fibers:
TPU at 10–50 MPa — 15–65× too soft → mushy control, no positional authority.
Nylon monofilament at 2,000–4,000 MPa (2–4 GPa) — 2–6× stiffer than human. Workable.
Kevlar/Aramid at 70,000–112,000 MPa — 60–175× too stiff. No energy storage, brittle failure.
Conclusion: Nylon replaces both Kevlar and TPU as the primary tendon material. Kevlar remains in fascicle core (DWG-03) and ligament panels. TPU is optional for stretchy side joints only.
PRIMARY TENDON (P-24)
Force transmission from muscle to bone. The main load path.
Stretchy side-joint element only. Never the main tendon.
MATERIALTPU ∅0.15mm cord (P-09)
STRETCH~20–30%
WRAPNone, or sparse Kevlar wrap
FUNCTIONJoint limit / soft return
// NYLON TENDON — CROSS-SECTION (30 STRAND)
TENDON BUILD + TERMINATION
Three steps: bundle the nylon, crimp one end with a ferrule, then secure the other end to bone with a capstan wrap + washer + backup ferrule. No adhesive. Fully serviceable.
NYLON TENDON — BUILD STEPS
1
Bundle 30× Nylon ∅0.104mm
Parallel, no twist. 1 + 6 + 12 + 11 in three concentric rings. Each strand slides independently → very flexible.
2
Crimp M1.0 ferrule at muscle end
Crimp with pliers under firm even pressure. The ferrule is the muscle-side termination. Repeat for 40 or 50 strand builds if you need higher working load.
3
Capstan wrap + washer + backup ferrule at bone
3 turns around M3 screw head → rubber washer → M1.0 backup ferrule. Self-energizing under load. Two failure catches. No adhesive. Fully serviceable.
LIGAMENTS — JOINT CONSTRAINT (P-06 KEVLAR PANELS)
Ligaments prevent dislocation, limit range of motion, maintain joint geometry — exactly like biological ligaments. They never actuate. Pure Kevlar/Aramid (P-06), woven into flat panels anchored bone-to-bone.
ELBOW LIGAMENT — CONSTRUCTION GUIDE
A
Measure the joint gap first
Build the joint, then cut ligament to fit. Rough guide for elbow: ~15mm wide × 20–25mm long, 2–3 layers of weave.
B
Weave rectangular Kevlar panel
Internal X-cross pattern resists valgus (sideways) and rotational stress simultaneously. ~0% stretch by design.
C
Anchor bone-to-bone
Both ends fixed. Medial (inner) and lateral (outer) ligament panels = two separate pieces, one per side of joint. ∅1.5mm anchor holes at 4 corners.
LIGAMENT PATTERNS
Rectangular panel — basic constraint, both sides of joint
X-cross internal — resists valgus + rotation
Figure-8 wrap — limits rotation range at a joint axis
Loop + cinch — prevents pull-apart on compression joints
WHY NOT RUBBER ELASTIC
Rubber elastic cord creeps under sustained pre-tension — it permanently elongates over hours/days of load. The new architecture uses Nylon as the main tendon, so creep is no longer a concern. TPU is reserved for stretchy side ligaments (where the cycle count is low) — even there, TPU's superior fatigue resistance over rubber is preferred. TPU is stiffer than rubber but durability wins.