The review, led by Hajar Naemah Sohif and colleagues, takes aim at a persistent gap in the literature. While previous surveys have catalogued TPU printing parameters, modification strategies and end-use applications, the authors contend that the connections between what goes into a filament formulation, how the material’s microstructure develops during printing, and what properties ultimately emerge from the nozzle have remained insufficiently integrated. Their proposed solution is a processing-microstructure-property framework, a conceptual map that links material formulation and processing conditions to phase morphology, crystallinity, interlayer adhesion and filler dispersion, and ultimately to the mechanical and functional performance of the finished part.

Understanding why this framework matters requires a brief tour of TPU’s inner architecture. Thermoplastic polyurethanes are block copolymers composed of alternating soft segments, which confer elasticity, and hard segments, which act as physical crosslinks and provide strength. The relative proportions and degree of ordering of these segments, a phenomenon known as microphase separation, governs much of the material’s mechanical behaviour. Studies cited in the review have visualised and quantified how varying hard segment content changes microphase separation and, with it, tensile performance. This internal structure, however, is only part of the story; in fused deposition modeling and related extrusion-based processes, the way molten layers bond to one another and how fillers distribute themselves within the melt can make or break the final part.

The most heavily explored strategy for upgrading TPU is polymer blending, and the review devotes considerable attention to three partner polymers in particular: polylactic acid, polycaprolactone and acrylonitrile butadiene styrene. Blending TPU with PLA, a rigid and bio-based polymer, can dramatically increase stiffness and has been shown to improve fracture toughness in ways that depend strongly on the blend ratio. PLA/TPU blends have also demonstrated shape memory behaviour, opening the door to eco-friendly filaments that can be programmed into temporary shapes and recover their original form on heating. Researchers have even prepared PLA-TPU copolymers via in situ ring-opening polymerization to improve compatibility between the two otherwise immiscible phases, a reminder that in polymer blending, chemistry at the interface is often as important as the bulk composition.

Polycaprolactone, by contrast, brings a different set of talents to the table. With its low melting temperature and biocompatibility, PCL has been blended with TPU to create shape memory materials aimed at biomedical use, including self-knotting sutures that tighten when warmed to body temperature. Composites of TPU/PCL blends with hydroxyapatite have been explored for bone-related applications, combining the elastomer’s compliance with the ceramic’s bioactivity. ABS, the workhorse of consumer 3D printing, offers yet another route: ABS/TPU blends balance the stiffness and dimensional stability of ABS with the toughness and flexibility of TPU, and studies have shown that blend composition directly affects printability, weld line quality and mechanical performance. Carbon nanotube-modified ABS/TPU nanocomposites have further demonstrated improvements in fluidity and fracture behaviour.

Blending, however, is not a free lunch. The review is candid about the pitfalls. Most polymer pairs, including TPU with PLA or ABS, are thermodynamically immiscible, meaning they tend to separate into distinct phases rather than mixing at the molecular level. The resulting phase morphology, whether droplets, fibers or co-continuous networks, depends on composition, viscosity ratio and processing history, and it exerts decisive control over properties. Poor interfacial adhesion between phases can create weak points that crack propagation exploits. This is where compatibilizers enter the picture: molecules designed to sit at the interface, reduce interfacial tension and anchor the two phases together. The review highlights compatibilization as a critical lever, noting that well-chosen compatibilizers can transform a brittle, phase-separated blend into a material with genuinely useful combinations of stiffness and toughness.

Additives beyond compatibilizers round out the formulation toolbox. Fillers of various dimensions, from nanoparticles such as silica-coated titanium dioxide and zinc oxide to carbon nanotubes, graphene, boron nitride and short carbon fibers, can impart electrical conductivity, thermal conductivity, flame retardancy or reinforcement. Conductive TPU composites loaded with multiwalled carbon nanotubes have been 3D printed into stretchable strain sensors, while graphene-filled TPU filaments produced by melt compounding show enhanced thermal conductivity for thermal management applications. The catch, the authors stress, is dispersion: filler agglomeration is a recurring failure mode that undermines both mechanical performance and functionality, and filler size itself matters, as demonstrated by studies of boron nitride particles in flexible thermally conductive TPU composites. Plasticizers, meanwhile, soften the material and modify the microphase-separated morphology, with documented effects on the electromechanical properties of TPU films.

The practical payoff of all this formulation science is already visible across four application sectors the review identifies. In medicine, TPU-based filaments are used for tissue engineering scaffolds, tracheal constructs, surgical planning models and shape memory devices, where biocompatibility and tunable mechanics are paramount. In electronics and sensors, stretchable conductive TPU composites enable wearable and flexible devices that survive repeated deformation. Non-pneumatic tires, which replace air with engineered lattice structures, exploit TPU’s energy absorption and rebound behaviour, with 3D-printed lattice architectures offering designers unprecedented control over cushioning and durability. Textiles and sports equipment round out the list, with flexible printed TPU finding roles in functional garments, protective gear and auxetic structures designed for energy absorption.

Looking ahead, the review identifies sustainability as the defining frontier. Bio-based TPUs derived from renewable feedstocks, including polylactic acid-derived polyols, are advancing rapidly, and fully bio-based composites with enhanced flame retardancy have been demonstrated using additives such as phytic acid. Recyclability is another theme, with researchers developing polyurethane elastomers that can be reprocessed without catastrophic loss of mechanical properties, and studies of polymer blend recycling offering guidance for circular material flows. Perhaps most intriguingly, the authors point toward multi-material and 4D printing, in which printed objects change shape over time in response to environmental stimuli. Shape memory TPU formulations, hybrid composites combining shape memory polymers with thermoplastic elastomers, and programmable multi-material structures all suggest a future in which printed TPU components are not merely flexible, but actively responsive.

What emerges from the Malaysian team’s synthesis is a discipline coming of age. The era of treating TPU as a single, fixed filament grade is giving way to a design-space approach in which chemists, processors and printers jointly tune soft and hard segment chemistry, blend ratios, compatibilizer architecture, filler type and loading, and printing parameters as one integrated system. The review’s framework, connecting formulation and processing to microstructure and final properties, offers researchers a common language for that effort. If the authors are right, the humble spool of flexible filament on a workbench today is merely the starting point for a generation of smart, high-performance and sustainable elastomeric parts, printed layer by layer and engineered from the molecule up.

Subject of Research: Tailoring the mechanical and functional properties of thermoplastic polyurethane filaments for additive manufacturing through blends and additives.

Article Title: Tailoring mechanical and functional properties of Thermoplastic Polyurethane (TPU) filaments for additive manufacturing: A review of blends and additives

Article References: Sohif, H. N., Manshor, H., Hatta, M. A. M., & Sajab, M. S. (2026). Tailoring mechanical and functional properties of Thermoplastic Polyurethane (TPU) filaments for additive manufacturing: A review of blends and additives. Polymer Bulletin, 83(12), Article 681. https://doi.org/10.1007/s00289-026-06739-1

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06739-1

Keywords: thermoplastic polyurethane, TPU, additive manufacturing, 3D printing, polymer blends, PLA, PCL, ABS, fillers, compatibilizers, shape memory polymers, sustainability