💪 Muscle Fascicle — Build Guide

A fascicle is the base actuator unit — a self-contained artificial muscle fiber pair wound into a protective sleeve, ready to contract when heated.

1. Fiber Construction

Each fiber is a co-inserted nylon + nichrome pair bound with sewing thread, then wound into a helix.

Materials Per Fiber

• Nylon monofilament — PA6/PA66, either 0.3mm (coarse) or 0.1mm (fine)
• Nichrome wire — Ni90, either AWG34/0.15mm (coarse) or AWG40/0.08mm (fine)
• Sewing thread — binds the pair permanently (not removed)

Construction Steps

1. Cut materials — Cut nylon and nichrome to 20cm each. Strip any coating from nichrome ends.
2. Co-insert — Lay nichrome alongside nylon (side-by-side, NOT wrapped). The nichrome sits loosely alongside, not wound around.
3. Bind with thread — Wrap sewing thread tightly every ~5mm along the length. This holds the pair together permanently. Do not remove these bindings.
4. Wind into helix — Secure one end in a drill or hand winder. Twist 50–100 full turns for a 20cm fiber until it "springs" — the coil springs open on its own when you let go.
5. Heat-set — Hang the fiber with a light weight (enough to keep tension, not stretch). Bake at 150°C for 10–15 minutes. The nylon will anneal into the helical shape.
6. Cool — Let cool naturally. Coarse fibers: 3–8 seconds. Fine fibers: ~1 second.

Fiber Specifications

2. 2-Ply Fiber Pairs

The base actuator unit is a fiber pair, not a single fiber. Two identical finished fibers are twisted together.

How to Make a Pair

1. Take two identical finished fibers (same nylon size, same nichrome, same coil pitch)
2. Twist together in the OPPOSITE direction to their coils — if each fiber coils clockwise, twist counter-clockwise. A few turns is enough — just until they interlock.
3. Fix both ends (clip or clamp)
4. The pair now contracts in balance with no return spring needed — the opposing twist provides passive return

The two fibers mechanically cancel each other's twist tendency, creating a self-stabilizing actuator that returns passively when cooled.

3. Fascicle Assembly

A fascicle wraps a fiber pair (or multiple pairs) in protective sleeving with strain limiters.

Materials Per Fascicle

• 2-ply fiber pair — two identical fibers twisted together
• 4mm PET braided sleeving — protective outer sleeve (NOT 6mm!)
• Kevlar strain limiters — 2–4 parallel strands running alongside the fiber (NEVER twisted in)
• Mineral oil — fill for thermal management
• Heat shrink — termination at both ends
• Ferrule — crimp connection to tendon

Assembly Steps

1. Thread PET sleeve — Cut 4mm PET braided sleeving to length (slightly shorter than the fascicle target length)
2. Insert fiber pair — Thread the 2-ply fiber pair through the sleeve
3. Add Kevlar strain limiters — Run 2–4 strands of Kevlar alongside the fiber inside the sleeve. These prevent over-stretch but don't restrict normal contraction. "Leash not fighter" — they limit travel, don't fight motion.
4. Fill with mineral oil — Draw mineral oil through the sleeve to fill the air gaps. Oil acts as thermal mass and conducts heat away from the nichrome faster than air alone.
5. Terminations — Slide heat shrink over both sleeve ends, apply heat to secure. At the load end, crimp a ferrule over the fiber pair + Kevlar for tendon connection.

Two Fascicle Tiers

Cross-Section Diagram

Side View Diagram

Electrical Connection

ESP32 GPIO → MOSFET Gate → MOSFET Drain → Nichrome (both fibers) → Ground

Each fascicle requires:

• 1 MOSFET (IRLZ44N recommended) — switches current on/off
• 1 flyback diode (1N4007) — reverse voltage protection across nichrome leads

Wiring

Positive lead: nichrome ends (both fibers) → MOSFET drain
Negative lead: nichrome ends → ground
Gate: ESP32 GPIO pin (via 100Ω resistor recommended)
Diode: placed across nichrome leads (cathode to +, anode to -)

Operating Principle

1. ESP32 sends GPIO HIGH → MOSFET gate closes
2. Current flows through nichrome → resistive heating
3. Heat transfers to nylon fibers → they contract
4. 2-ply pair contracts symmetrically → pulls on ferrule → mechanical load movement
5. ESP32 sends GPIO LOW → MOSFET opens → nichrome cools → opposing twist extends fibers (passive return, no spring needed)

Notes

• The 2-ply construction means no return spring — the fiber pair returns passively via the opposing twist geometry
• Heat shrink at both ends is critical — the PET sleeve ends are the main failure points under flexing
• Ferrules are only needed at the ACTUATOR connection end (the end that pulls on the load)
• Always slightly active fine fascicles maintain pre-tension on the tendon system
• Coarse fascicles fire on demand into the pre-tensioned system for faster movement

📄 Source: 💪 Muscle Fascicle — Build Guide