Find out about.......Plastics, Polymer Engineering and Leadership: Plastics and Polymers Selection for Humanoid Robot Applications: From Structural Skeletons to Artificial Muscles

Hello and welcome to a new material selection post in which we discuss suitable plastics for humanoid robotic applications. It is a longer post and I have structed it into five chapters:

Chapter 1: The Humanoid Market – From Industrial Tools to Bio-mimicry
Chapter 2: The Structural Skeleton – PEEK and Reinforced Composites
Chapter 3: The “Artificial Muscle” – Electroactive Polymers (EAPs)
Chapter 4: Actuation and Tribology – Gears, Wear, and Sealing
Chapter 5: The PFAS Challenge – Engineering Without “Forever Chemicals”

Introduction

Humanoid robots represent one of the most demanding convergence points of mechanical engineering, electronics, materials science, and biology-inspired design. Unlike traditional industrial robots, humanoids must be lightweight, energy-efficient, safe for human interaction, and capable of complex, biomimetic motion.

In this context, optimal polymer selection is a key engineering decision. The correct choice of plastic materials is essential to:

Ensure reliable function of all robot subsystems
Balance stiffness, weight, wear resistance, and durability
Prevent premature part failure due to fatigue, creep, wear, or environmental exposure

Poor material selection can lead to unexpected breakdowns, excessive wear, thermal deformation, or regulatory non-compliance, often only discovered late in development or during field operation.

Polymers and polymer-based composites are no longer auxiliary materials in this space — they are enablers. This article explores how advanced plastics underpin modern humanoid robot design, from structural frames to artificial muscles, while addressing emerging regulatory and sustainability challenges.

Chapter 1: The Humanoid Market – From Industrial Tools to Bio-mimicry

Market Context: From Cobots to Autonomous Humanoids

Early collaborative robots (“cobots”) were essentially industrial manipulators made safer through sensors and control algorithms. Their material choices reflected this heritage: steel, aluminum, and classical engineering plastics.

Today’s humanoid robots represent a paradigm shift:

Designed for unstructured environments
Expected to interact safely with humans
Required to move with human-like kinematics
Increasingly autonomous, powered by onboard batteries

This shift has forced engineers to rethink mass distribution, inertia, and energy efficiency — areas where polymers outperform metals.

Design Step: Lightweighting as a System-Level Strategy

In humanoid robots, weight is not neutral:

Every gram saved in the frame can be reassigned to:
- Battery capacity (longer runtime)
- Sensors and AI hardware
- Payload capability

Polymers enable functional integration (snap-fits, ribs, channels, cable guides) that would require multiple machined metal parts, thereby reducing both part count and mass.

Chapter 2: The Structural Skeleton – PEEK and Reinforced Composites

Material Focus: High-Performance Structural Polymers

The “skeleton” of a humanoid robot must provide high stiffness, fatigue resistance, and dimensional stability under cyclic loads.

Key materials include:

PEEK (Polyetheretherketone)
- Carbon- or glass-fiber reinforced
- Exceptional stiffness-to-weight ratio (interesting for metal replacement)
- High thermal stability and chemical resistance
- Structural frames, gears, joints, bushings, insulation
PA 6.6 (Polyamide 6.6)
- Tough, cost-effective
- Suitable for secondary load-bearing structures, housings, covers
- 3D printing possible (SLS)
- PolyArylAmide (PARA, PA-MXD6): for appliactions in robotics where low moisture, high dimensional stability (enabling complex parts), excellent surface appearance (“best-in-class” among the Polyamides), and outstanding stiffened and strength is needed.
Polycarbonate (PC)

Impact-resistant, transparent, stable
Sensor/camera covers, shields, panels

PPS (Polyphenylene Sulfide)
- Glass-fiber reinforced
- Excellent dimensional stability
- High thermal and creep resistance
- Connectors, housings, parts near heat sources
PEI (Polyetherimide)

High strength, flame retardant, heat resistant, good electrical properties
Connector housings, structural frames
Strong, safe, reliable in harsh environments

LFT (Long Fiber Thermoplastics)
- Continuous or long glass/carbon fibers
- Ideal for large, injection-molded structural parts
- High load capacity, reduces robot weight

Design Example: Replacing Aluminum with CNC-machined or 3D-printed PEEK

CNC-machined or 3D-printed carbon-filled PEEK can achieve:

Elastic modulus approaching aluminum
Up to 50% weight reduction
Significantly lower rotational inertia

Lower inertia directly translates into:

Faster acceleration and deceleration
Reduced motor size
Lower energy consumption

Additive manufacturing further enables topology-optimized structures, closely mimicking biological bones with hollow cores and load-aligned fiber orientations.

Chapter 3: The “Artificial Muscle” – Electroactive Polymers (EAPs)

Defining EAPs

Electroactive Polymers (EAPs) are materials that change shape, size, or mechanical properties when subjected to an electrical stimulus. They are often described as “muscle-like” materials because their actuation principles resemble biological muscle contraction.

Electronic vs. Ionic EAPs

Electronic EAPs

Examples:

Dielectric Elastomers (DEAs)
Electrostrictive Graft Elastomers
Ferroelectric Polymers

Characteristics:

Fast response times
High energy density
Require high operating voltages
Suitable for dry environments

Ionic EAPs

Examples:

Ionic Polymer-Metal Composites (IPMCs)
Conducting Polymers
Carbon Nanotube (CNT) networks

Characteristics:

Operate at low voltages
Slower response
Often require moisture or electrolytes
Ideal for fine, low-force movements

Design Example: Eliminating Gears and Motors

EAPs enable soft robotics, where motion is achieved without:

Gearboxes
Bearings
Lubricants

Applications include:

Facial expression systems
Dexterous fingers
Artificial skin and haptics

The result is silent, compliant, and lifelike motion, impossible to achieve with rigid electromechanical systems alone.

Chapter 4: Actuation and Tribology – Gears, Wear, and Sealing

Motion Control: Tribological Plastics

Despite advances in soft actuation, many humanoid joints still rely on conventional rotary actuation. Here, tribological performance is critical.

Key materials:

POM (Acetal) – low friction, dimensional stability
PA 4.6 – high melting point, excellent fatigue resistance
PA 6.10 – outstanding wear resistance
Polyketone – good wear resistance
Self-lubricating materials using fillers such as Graphite, Molybdenum Disulfide (MoS2), Carbon fiber, and PTFE/UHMWPE – maintenance-free bearings and joints

These materials allow:

Dry-running systems
Reduced maintenance
Long service life under oscillating motion

Sealing: Protecting Sensitive Electronics

Humanoid robots operate in dusty, humid, and unpredictable environments. Advanced sealing systems are essential.

IPSR (Ingress Protection Seals) / PSS (Precision sealing systems)
Advanced EPDM, NBR and FKM elastomers
High-temperature thermoplastics such as Quantix® ULTRA (1.200 °C)

These seals protect:

Motors
Sensors
Control electronics
…without adding excessive friction or bulk.

Chapter 5: The PFAS Challenge – Engineering Without “Forever Chemicals”

The Regulatory Hurdle

Historically, many high-performance plastics relied on PTFE additives to reduce friction and wear. However:

PTFE belongs to the PFAS ( Per- and polyfluoroalkyl substances) containing family
Increasingly restricted under REACH / ECHA and EU 2019/1021
Long-term environmental persistence (“forever chemicals”)

This has forced a fundamental rethink of tribological design.

The Innovation Step: Molecular Engineering

Instead of relying on fluorinated additives, modern polymers achieve performance through intrinsic molecular structure.

A prime example:

PA4.6
- Higher melting point than PA 6 or PA 6.6
- Superior crystallinity (80%)
- Excellent fatigue and wear resistance
- Performs under high speed and load without PTFE

This enables PFAS-free gears and bearings suitable for humanoid robot actuators.

Apart from PA 4.6, there are polymers which are inherently wear-resistant too:

-Polyketone (PK)

-Polyoxymethylene (POM): crystallinity level above 90% possible

-Ultra-high molecular weight polyethylene (UHMWPE)

-Polyamide-Imide (PAI)

-Polybenzimidazole (PBI)

-Polyetheretherketone (PEEK)

As alternative, Hexagonal boron nitride (hBN) which can offer a fluorine- and micro plastic-free replacement. The very good lubricating properties of hBN come from its crystal structure. We discussed this in detail with Michaela Schopp - Product Manager at Henze BNP AG in this guest interview here.

Conclusion

The selection of high-performance plastics in humanoid robotics is driven by the need for lightweight, durable, and reliable components that can withstand mechanical stress, environmental exposure, and regulatory requirements. Materials like PEEK, PA, PC, POM, PPS, PEI, PU, and LFT each offer unique advantages for specific robot parts, from structural frames to gears and sensor housings (Figure 1). As the robotics industry evolves, the role of advanced, sustainable polymers will only increase, enabling the next generation of agile, efficient, and compliant humanoid robots.