Nature produces materials with a fascinating combination of stiffness, strength and functionality through energy-efficient, environmentally friendly processes. These excellent properties arise from a complex interplay between structural control across multiple scales and variations in local composition. Achieving similar structural and compositional precision in synthetic soft materials remains a challenge, largely due to differences in processing methods compared to natural systems. Nature often employs compartmentalization and extrusion-based techniques to produce polymer-based materials. Adapting these strategies to synthetic polymers is possible by formulating them as deformable microparticles. However, the resulting granular materials are typically very soft.
In this presentation, I will showcase how we 3D print a wide range of polymers, including hydrogels, elastomers and thermosets through direct ink writing (DIW). This is achieved by formulating polymers as microparticles and optionally add some microgels to enable the jamming of the microparticles. The resulting inks display rheological properties that are ideal for DIW. Yet, poor inter-particle interactions result in viscoelastic, very week granular systems. We transform these soft granular structures into elastic load-bearing materials by polymerizing reagents contained within the soft particles to create double-network granular polymers. I will illustrate how stiffness and toughness of these granular materials can be independently tuned and how interfaces can be leveraged to introduce functionalities, such as electrical conductivity, to the stretchable materials. These 3D-printable functional materials have the potential to be used as multi-responsive sensors and soft actuators capable to undergo pre-defined deformations.

Polyethylene terephthalate (PET) is widely used across various industries (Suhaimi et al., 2022), yet its foaming process typically requires chemical additives such as branching agents and chain extenders to improve melt strength and foam stability (Dolatshah et al., 2022; Kruse & Wagner, 2017). This study introduces an additive-free 3D foam printing method that exploits pre-induced crystallization via CO₂ impregnation to enhance bubble nucleation and structural integrity without chemical modifications. By adjusting extrusion speed and hot-end geometry, we regulate the melting of pre-induced crystals, directly influencing foam morphology and mechanical properties.
To characterize the thermal behavior within the 3D printer hot-end, we employ the Graetz number (Gz) as a dimensionless parameter to quantify the interplay between convective and conductive heat transfer. Results indicate that when Gz < 10, uniform thermal distribution leads to complete crystal melting, limiting foam expansion. Conversely, at Gz > 10, strong thermal gradients allow partial crystal retention, significantly improving bubble nucleation, foam density, and mechanical strength. A modified scaling law incorporating crystallinity effects is introduced to predict the elastic modulus across a range of foam densities, demonstrating that crystallinity reinforces the polymer matrix and maintains specific mechanical properties despite increasing porosity.
This approach is scalable, cost-effective, and compatible with commercial PET and standard 3D printing technologies, enabling the production of lightweight, high-performance foams for automotive, aerospace, and packaging applications. By eliminating chemical additives, it also supports sustainable and eco-friendly manufacturing practices.

3D Printing offers high flexibility and precise control over the final construct characteristics and composition (1). Granular hydrogels, typically made of jammed microgels have emerged as promising inks, due to their extrudability, modularity, and porosity (2). Dynamic crosslinking allows the development of stimuli-responsive granular hydrogels with intrinsic reversibility triggered by tunable conditions. Our study focuses on designing a granular hydrogel whose yield-stress and ultimate printability can be tuned by in-situ modulating individual microgel's swelling degree through selective cleavage of a disulfide-based crosslinker.

The poly(N-isopropylacrylamide), pNiPAM, based microgels were synthesized by surfactant-free radical polymerization and the cleavable dynamic disulfide crosslinks were introduced by co-polymerization with, N,N′-bis(acryloyl)cystamine, BAC. The effect of chemical reduction of the disulfide crosslinks by tris-(2-carboxyethyl)phosphine hydrochloride, TCEP, on individual microgels and the granular hydrogels was characterized by the combination of DLS, wet-AFM, cryo-electron microscopy, and rheology. The addition of TCEP effectively led to microgel swelling accompanied by partial chain release and significantly increased the yield-stress of the resulting granular hydrogels. The granular hydrogels were then 3D printed into square-mesh scaffolds. Inks that initially exhibited poor shape fidelity were turned into stable, multi-layered 3D structures capable of supporting their weight, up to 32 layers, after reduction-induced jamming.

Finally, after printing, the scaffolds were annealed in a sodium periodate, NaIO4, bath. This post-printing process enabled to reforming intra- and inter-particles disulfide bonds that endowed the scaffolds with long-term dimensional stability in physiological conditions. The printed scaffolds showed full cytocompatibility permitting their use for tissue engineering.

Three-dimensional (3D) printing has revolutionized the fabrication of complex, patient-specific structures for biomedical applications. Light-based 3D-printing techniques—such as stereolithography (SLA), digital light processing (DLP), and two-photon polymerization (2PP)— utilize spatially controlled illumination to cure photo-crosslinkable resins with unparalleled precision, surpassing conventional extrusion-based methods. Despite their high resolution, these techniques often rely on voxel-by-voxel or layer-by-layer fabrication, which can be time-intensive.
A transformative advancement in the field is volumetric additive manufacturing (VAM), which enables the rapid production of micrometer-precise, patient-specific constructs in less than one minute. VAM operates by projecting sequential two-dimensional images into a rotating vial of photo-crosslinkable resin, forming a complete 3D structure in one single step. This technique offers unmatched speed and precision, making it highly suitable for fabricating customized medical implants.
We previously optimized thiol-ene photo-crosslinkable poly-ɛ-caprolactone (E-PCL) resins for VAM by fine-tuning parameters such as light dose, photo-initiator concentration, radical scavenger levels, and solvent composition.¹ The resulting E-PCL constructs demonstrated excellent computer-aided design/computer-aided manufacturing (CAD/CAM) fidelity and suitability for biomedical applications.
Despite these significant advancements, a major challenge remains: the in vivo monitoring of biodegradable implants, which is crucial for their clinical translation. To address this limitation, we incorporated a radiopaque monomer 5-acrylamido-2,4,6-triiodoisophthalic acid (AATIPA)² into a tetra-functional E-PCL network³ and performed in vivo studies in mice. The capability to fabricate intricate, porous 3D-structures and monitor their performance in vivo marks a substantial leap forward, enabling the early detection of implant malfunctions and advancing the development of next-generation biodegradable medical devices.
(Research Foundation Flanders (FWO), 1SHDP24N)

The development of intelligent 3D printing materials with photo- and thermally responsive properties remains a challenge, particularly for sustainable applications requiring reprocessability, multifunctionality, and precise control over dynamic behavior. Herein, we report a solvent-free photoprintable ink featuring dual dynamic covalent bonds (DCBs) designed for photo- and thermally responsive 3D printing. The resin combines dynamic disulfides and β-hydroxy esters, which are further enhanced with photoswitchable spiropyran additives, enabling tunable photochromic and thermochromic properties. This material also exhibits shape memory functionality, allowing simultaneous shape and color transitions under heat. Printed prototypes demonstrate exceptional thermal stability, controllable fading times, and multifunctionality, supporting applications in anticounterfeiting, imaging, and sensing. These results provide a sustainable and innovative solution for intelligent 3D printing materials, bridging the gap between multifunctionality and reprocessability. By advancing the capabilities of responsive materials, this work paves the way for transformative progress in adaptive manufacturing and practical applications in next-generation functional devices.

Unmet freedom in the design of complex and application tailored parts gears the industrialization of Additive Manufacturing (AM). In the case of thermoplastics, the techniques offer unique control in spatially directed function once the printing and molecular time-scales are aligned. However, insufficient macromolecular diffusion and re-entangling across weld interfaces causes inferior weld mechanics, i.e. stress transfer in the transverse direction.1
Interfacial co-crystallization is opportune as solution and specific to the use of biobased building blocks. Using polylactides (PLA), low crystallization rates of the individual PLAs provide time for interfacial diffusion, while high stereocomplex (SC) crystallization rates after molecular mixing of enantiomers facilitate thermodynamic stabilization and increased weld strength, are reported during 3D printing experimentally.2,3 Since SC induces intrinsic mechanical embrittlement, the last hurdle is the realization of ductility. ε-caprolactone has been copolymerized in 3, 5, 7 and 15 mol% with enantiomerically pure L and D lactides to unsure ductility. Prior to melt SC crystallization, melt or solution mixed PLLA and PDLA are reported to form left- and right-handed helical pre-cursors that de-mix and as such a limited volume fraction of SC crystals form.
Upon cooling from the melt, poly(lactide-co-caprolactone) random copolymers in combination with enantiomerically opposite homopolymers, yield phase separated morphologies with opportunities in designing ductile SC reinforced interfaces. In this presentation, we will disclose a structural and functional study of SC blends and interfaces using advanced spectroscopic techniques such as nanometer resolved X-ray Scattering imaging using a 70 nm synchrotron X-ray beam and Scattering Near-field Optical Microscopy.