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 structured 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:
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Ensure reliable function of all robot subsystems
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Balance stiffness, weight, wear resistance, and durability
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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:
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Designed for unstructured environments
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Expected to interact safely with humans
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Required to move with human-like kinematics
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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:
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:
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PEEK (Polyetheretherketone)
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Carbon- or glass-fiber reinforced
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Exceptional stiffness-to-weight ratio (interesting for metal replacement)
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High thermal stability and chemical resistance
Structural frames, gears, joints, bushings, insulation
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PA 6.6 (Polyamide 6.6)
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Tough, cost-effective
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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
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PPS (Polyphenylene Sulfide)
Glass-fiber reinforced
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Excellent dimensional stability
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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
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LFT (Long Fiber Thermoplastics)
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Continuous or long glass/carbon fibers
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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:
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Elastic modulus approaching aluminum
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Up to 50% weight reduction
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Significantly lower rotational inertia
Lower inertia directly translates into:
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:
Characteristics:
Ionic EAPs
Examples:
Characteristics:
Design Example: Eliminating Gears and Motors
EAPs enable soft robotics, where motion is achieved without:
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Gearboxes
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Bearings
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Lubricants
Applications include:
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
Despite advances in soft actuation, many humanoid joints still rely on conventional rotary actuation. Here, tribological performance is critical.
Key materials:
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POM (Acetal) – low friction, dimensional stability
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PA 4.6 – high melting point, excellent fatigue resistance
PA 6.10 – outstanding wear resistance
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Polyketone & PEEK – good wear resistance
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Self-lubricating materials using fillers such as Graphite, Molybdenum Disulfide (MoS2), Carbon fiber, and PTFE/UHMWPE – maintenance-free bearings and joints
These materials allow:
Sealing: Protecting Sensitive Electronics
Humanoid robots operate in dusty, humid, and unpredictable environments. Advanced sealing systems are essential.
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IPSR (Ingress Protection Seals) / PSS (Precision sealing systems)
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Advanced EPDM, NBR and FKM elastomers
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High-temperature thermoplastics such as Quantix® ULTRA (1.200 °C)
These seals protect:
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:
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PTFE belongs to the PFAS ( Per- and polyfluoroalkyl substances) containing family
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Increasingly restricted under REACH / ECHA and EU 2019/1021
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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:
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PA4.6
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Higher melting point than PA 6 or PA 6.6
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Superior crystallinity (80%)
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Excellent fatigue and wear resistance
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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.
Figure 1: Summary of materials used for Humanoid Robotic applications.
If you need selection support, or a deeper dive into a specific material or application, please let me know!
Thanks for reading & #findoutaboutplastics
Greetings,
Herwig Juster
Literature:
[1] https://www.fst.com/de/news-stories/pressemitteilungen/2024/thermoplaste-fuer-bis-zu-1200-grad-celsius/
[2] https://www.fst.com/markets/robotics/6-axis-robot/
[12] https://schunk.com/de/de/news/schunk-gruendet-tech-spin-off-fuer-humanoide-roboterhaende/36224
[13] https://www.linkedin.com/pulse/syensqos-ketaspire-peek-enables-durability-flexible-joints-baleno-5oj9c/?trackingId=DBuQvo0eWn86WUTpTpAECQ%3D%3D
