Fotis Christakopoulos

All Publications

• Tuning Viscoelasticity of Dynamic Covalent Hydrogels for Human Tissue Modeling ADVANCED FUNCTIONAL MATERIALS de Paiva Narciso, N., Christakopoulos, F., Huang, M. S., Baugh, N. J., Huerta-Lopez, C., Matos, E. X., Pashin, K. P., Spakowitz, A. J., Heilshorn, S. C. 2026

  View details for DOI 10.1002/adfm.76427

  View details for Web of Science ID 001849611600001

• In situ rheological monitoring of diffusion-controlled hydrogel crosslinking for embedded 3D bioprinting. Biofabrication Christakopoulos, F., Shih, A., Chung, S. J., Huerta-López, C., Brunel, L. G., De Paiva Narciso, N., Messin-Roizard, C., Tao, J., Myung, D., Heilshorn, S., Fuller, G. G. 2026

  Abstract

  In embedded 3D bioprinting, biomaterial inks are extruded into sacrificial support baths to facilitate the fabrication of complex shapes, even from soft, liquid-like materials. Post-printing, the diffusion of small molecules into or out of the support bath can facilitate ink crosslinking to stabilize the printed structure. In these coupled reaction-diffusion systems, the rheological properties of the ink will change over time. Despite the importance of tuning the mechanical properties of these inks for biological applications, there are currently no methods to accurately predict ink stiffness over time throughout the crosslinking process. Here, we use a custom-developed magnetic stress rheometer to continuously monitor diffusion-driven crosslinking in situ. Our approach reveals how gelation kinetics depend on the thickness of the ink layer, and enables predictive estimation of mechanical evolution in these reaction-and diffusion-driven systems. With these insights, we fabricate specimens with predetermined mechanical properties and observe changes in cell phenotype as a response. These insights help inform the design of inks and timing of bioprinting protocols to achieve prints with desirable mechanical properties and further allow the fabrication of prints with patterned mechanical properties.

  View details for DOI 10.1088/1758-5090/ae7ed3

  View details for PubMedID 42309146

• Dual-orientation of collagen fibers to guide cell alignment in 3D-printed constructs. Acta biomaterialia Singhal, D., Christakopoulos, F., Brunel, L. G., Borkar, S., Doulames, V. M., Mozipo, E. A., Myung, D., Fuller, G. G., Heilshorn, S. C. 2025

  Abstract

  Natural tissue comprises fibrous proteins with complex fiber alignment patterns. Here, we develop a reproducible method to fabricate biomimetic scaffolds with patterned fiber alignment along two independent orientations. While extrusion-based approaches are commonly used to align fibrous polymers in a single orientation parallel to the direction of flow, we hypothesized that extrusion-based 3D printing could be utilized to achieve more complex patterns of fiber alignment. Specifically, we show that control of lateral spreading of a printed filament can induce fiber alignment that is either parallel or perpendicular to the flow direction. Theoretical prediction of the printing parameters that control fiber orientation was experimentally validated using a collagen biomaterial ink. The velocity ratio of the printhead movement relative to the ink extrusion rate was found to dictate collagen fiber alignment, allowing for the informed fabrication of collagen scaffolds with prescribed patterns of fiber alignment. For example, controlled variation of the ink extrusion rate during a single print resulted in scaffolds with specified regions of both parallel and perpendicular collagen fiber alignment. Human corneal mesenchymal stromal cells seeded onto the printed scaffolds adopted a spread morphology that aligned with the underlying collagen fiber patterns. This technique worked well for filaments either printed onto a printbed in air or extruded within a support bath using embedded 3D printing, enabling the fabrication of 3D structures with aligned collagen fibers. Taken together, this work demonstrates a theoretical and experimental framework to achieve the reproducible fabrication of 3D printed structures with controlled collagen fiber patterns that guide cellular alignment. Statement of Significance Natural tissues contain collagen fibers aligned in multiple directions, which are essential for guiding cell behavior; however, most existing fabrication methods can achieve only unidirectional fiber alignment. Here, we introduce an extrusion-based 3D printing strategy that enables precise control over collagen fiber orientation in both parallel and perpendicular directions. This allows multidirectional collagen fiber alignment patterned spatially within a single construct, thereby guiding corneal mesenchymal stromal cells to align multidirectionally. This approach works when printing in air or with embedded printing in a support bath. Thus, this strategy can enable the fabrication of complex 3D scaffolds that mimic the anisotropic architecture of native tissues.

  View details for DOI 10.1016/j.actbio.2025.11.013

  View details for PubMedID 41232895

• Reinforcement of Fibrillar Collagen Hydrogels with Bioorthogonal Covalent Crosslinks. Biomacromolecules Brunel, L. G., Long, C. M., Christakopoulos, F., Cai, B., de Paiva Narciso, N., Johansson, P. K., Singhal, D., Baugh, N. J., Zhang, D., Enejder, A., Myung, D., Heilshorn, S. C. 2025

  Abstract

  Bioorthogonal covalent crosslinking stabilizes collagen type I hydrogels, improving their structural integrity for tissue engineering applications with encapsulated living cells. The chemical modification required for crosslinking, however, interferes with the fibrillar nature of the collagen, leading instead to an amorphous network without fibers. We demonstrate an approach to perform bioconjugation chemistry on collagen with controlled localization such that the modified collagen retains its ability to self-assemble into a fibrillar network while also displaying functional groups for covalent crosslinking with bioorthogonal click chemistry. The collagen matrix is formed through a sequential crosslinking process, in which the modified collagen first physically assembles into fibers and then is covalently crosslinked. This approach preserves the fibrous architecture of the collagen, guiding the behavior of encapsulated human corneal mesenchymal stromal cells while also reinforcing fibers through covalent crosslinks, strengthening the stability of the cell-laden collagen hydrogel against cell-induced contraction and enzymatic degradation.

  View details for DOI 10.1021/acs.biomac.5c00398

  View details for PubMedID 40554673

• Interpenetrating networks of fibrillar and amorphous collagen promote cell spreading and hydrogel stability. Acta biomaterialia Brunel, L. G., Long, C. M., Christakopoulos, F., Cai, B., Johansson, P. K., Singhal, D., Enejder, A., Myung, D., Heilshorn, S. C. 2025

  Abstract

  Hydrogels composed of collagen, the most abundant protein in the human body, are widely used as scaffolds for tissue engineering due to their ability to support cellular activity. However, collagen hydrogels with encapsulated cells often experience bulk contraction due to cell-generated forces, and conventional strategies to mitigate this undesired deformation often compromise either the fibrillar microstructure or cytocompatibility of the collagen. To support the spreading of encapsulated cells while preserving the structural integrity of the gels, we present an interpenetrating network (IPN) of two distinct collagen networks with different crosslinking mechanisms and microstructures. First, a physically self-assembled collagen network preserves the fibrillar microstructure and enables the spreading of encapsulated human corneal mesenchymal stromal cells. Second, an amorphous collagen network covalently crosslinked with bioorthogonal chemistry fills the voids between fibrils and stabilizes the gel against cell-induced contraction. This collagen IPN balances the biofunctionality of natural collagen with the stability of covalently crosslinked, engineered polymers. Taken together, these data represent a new avenue for maintaining both the fiber-induced spreading of cells and the structural integrity of collagen hydrogels by leveraging an IPN of fibrillar and amorphous collagen networks. STATEMENT OF SIGNIFICANCE: Collagen hydrogels are widely used as scaffolds for tissue engineering due to their support of cellular activity. However, collagen hydrogels often undergo undesired changes in size and shape due to cell-generated forces, and conventional strategies to mitigate this deformation typically compromise either the fibrillar microstructure or cytocompatibility of the collagen. In this study, we introduce an innovative interpenetrating network (IPN) that combines physically self-assembled, fibrillar collagen-ideal for promoting cell adhesion and spreading-with covalently crosslinked, amorphous collagen-ideal for enhancing bulk hydrogel stability. Our IPN design maintains the native fibrillar structure of collagen while significantly improving resistance against cell-induced contraction, providing a promising solution to enhance the performance and reliability of collagen hydrogels for tissue engineering applications.

  View details for DOI 10.1016/j.actbio.2025.01.009

  View details for PubMedID 39798635

• Organoid bioprinting: from cells to functional tissues NATURE REVIEWS BIOENGINEERING Huang, M. S., Christakopoulos, F., Roth, J. G., Heilshorn, S. C. 2024

  View details for DOI 10.1038/s44222-024-00268-0

  View details for Web of Science ID 001388933500001

• Structure Formation and Unexpected Ultrafast Re-entanglement Dynamics of Disentangled Ultrahigh Molecular Weight Polyethylene MACROMOLECULES Wang, Z., Li, B., Christakopoulos, F., Xie, K., Zhu, C., Xu, J., Muller, A. J. 2024

  View details for DOI 10.1021/acs.macromol.4c01733

  View details for Web of Science ID 001340168900001

• Solid-state extrusion of nascent disentangled ultra-high molecular weight polyethylene POLYMER ENGINEERING AND SCIENCE Christakopoulos, F., Busato, S. P., Kong, X., Troisi, E., Friederichs, N., Tervoort, T. A. 2024

  View details for DOI 10.1002/pen.26787

  View details for Web of Science ID 001224721200001

• Disentangled Melt of Ultrahigh-Molecular-Weight Polyethylene: Fictitious or Real? MACROMOLECULES Litvinov, V., Christakopoulos, F., Lemstra, P. 2024

  View details for DOI 10.1021/acs.macromol.4c00271

  View details for Web of Science ID 001200689900001

• A microrheological examination of insulin-secreting β-cells in healthy and diabetic-like conditions SOFT MATTER Woolley, L., Burbidge, A., Vermant, J., Christakopoulos, F. 2024

  Abstract

  Pancreatic β-cells regulate glucose homeostasis through glucose-stimulated insulin secretion, which is hindered in type-2 diabetes. Transport of the insulin vesicles is expected to be affected by changes in the viscoelastic and transport properties of the cytoplasm. These are evaluated in situ through particle-tracking measurements using a rat insulinoma β-cell line. The use of inert probes assists in decoupling the material properties of the cytoplasm from the active transport through cellular processes. The effect of glucose-stimulated insulin secretion is examined, and the subsequent remodeling of the cytoskeleton, at constant effects of cell activity, is shown to result in reduced mobility of the tracer particles. Induction of diabetic-like conditions is identified to alter the mean-squared displacement of the passive particles in the cytoplasm and diminish its reaction to glucose stimulation.

  View details for DOI 10.1039/d3sm01141k

  View details for Web of Science ID 001196634300001

  View details for PubMedID 38573072

• Embedded 3d Bioprinting of Collagen Inks into Microgel Baths to control hydrogel Microstructure and Cell Spreading. Advanced healthcare materials Brunel, L. G., Christakopoulos, F., Kilian, D., Cai, B., Hull, S. M., Myung, D., Heilshorn, S. C. 2023: e2303325

  Abstract

  Microextrusion-based 3D bioprinting into support baths has emerged as a promising technique to pattern soft biomaterials into complex, macroscopic structures. We hypothesized that interactions between inks and support baths, which are often composed of granular microgels, could be modulated to control the microscopic structure within these macroscopic-printed constructs. Using printed collagen bioinks crosslinked either through physical self-assembly or bioorthogonal covalent chemistry, we demonstrate that microscopic porosity is introduced into collagen inks printed into microgel support baths but not bulk gel support baths. The overall porosity is governed by the ratio between the ink's shear viscosity and the microgel support bath's zero-shear viscosity. By adjusting the flow rate during extrusion, the ink's shear viscosity was modulated, thus controlling the extent of microscopic porosity independent of the ink composition. For covalently crosslinked collagen, printing into support baths comprised of gelatin microgels (15-50 µm) resulted in large pores (∼40 µm) that allowed human corneal mesenchymal stromal cells to readily spread, while control samples of cast collagen or collagen printed in non-granular support baths did not allow cell spreading. Taken together, these data demonstrate a new method to impart controlled microscale porosity into 3D printed hydrogels using granular microgel support baths. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adhm.202303325

  View details for PubMedID 38134346

• Gas-phase polymerization of ultra-high molecular weight polyethylene with decreased entanglement density JOURNAL OF POLYMER SCIENCE do Rosario, R., Christakopoulos, F., Tervoort, T. A. A., Brunel, F., McKenna, T. F. L. 2023; 61 (12): 1183-1195

  View details for DOI 10.1002/pol.20230038

  View details for Web of Science ID 000950544700001

• Additive Manufacturing of Polyolefins POLYMERS Christakopoulos, F., van Heugten, P. M. H., Tervoort, T. A. A. 2022; 14 (23)

  Abstract

  Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. However, the semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. First, the crystallization process results in severe shrinkage upon cooling, while the processing temperature and cooling rate affect the mechanical properties and mesoscopic structure of the fabricated parts. In addition, for ultra-high-molecular weight polyolefins, limited chain diffusion is a major obstacle to achieving proper adhesion between adjunct layers. Finally, polyolefins are typically apolar polymers, which reduces the adhesion of the 3D-printed part to the substrate. Notwithstanding these difficulties, it is clear that the successful processing of polyolefins via additive manufacturing techniques would enable the fabrication of high-end engineering products with enormous design flexibility. In addition, additive manufacturing could be utilized for the increased recycling of plastics. This manuscript reviews the work that has been conducted in developing experimental protocols for the additive manufacturing of polyolefins, presenting a comparison between the different approaches with a focus on the use of polyethylene and polypropylene grades. This review is concluded with an outlook for future research to overcome the current challenges that impede the addition of polyolefins to the standard palette of materials processed through additive manufacturing.

  View details for DOI 10.3390/polym14235147

  View details for Web of Science ID 000896227000001

  View details for PubMedID 36501543

  View details for PubMedCentralID PMC9740552

• "Tying the Knot": Enhanced Recycling through Ultrafast Entangling across Ultrahigh Molecular Weight Polyethylene Interfaces MACROMOLECULES Christakopoulos, F., Troisi, E., Friederichs, N., Vermant, J., Tervoort, T. A. 2021; 54 (20): 9452-9460

  View details for DOI 10.1021/acs.macromol.1c01427

  View details for Web of Science ID 000711772900021

• Melting-Induced Evolution of Morphology, Entanglement Density, and Ultradrawability of Solution-Crystallized Ultrahigh-Molecular-Weight Polyethylene MACROMOLECULES Christakopoulos, F., Bersenev, E., Grigorian, S., Brem, A., Ivanov, D. A., Tervoort, T. A., Litvinov, V. 2021; 54 (12): 5683-5693

  View details for DOI 10.1021/acs.macromol.1c00667

  View details for Web of Science ID 000665741600037

• Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder POLYMER Christakopoulos, F., Troisi, E. M., Sologubenko, A. S., Friederichs, N., Stricker, L., Tervoort, T. A. 2021; 222

  View details for DOI 10.1016/j.polymer.2021.123633

  View details for Web of Science ID 000641147600006

• Melting Kinetics of Nascent Poly(tetrafluoroethylene) Powder POLYMERS Christakopoulos, F., Troisi, E., Tervoort, T. A. 2020; 12 (4)

  Abstract

  The melting behavior of nascent poly(tetrafluoroethylene) (PTFE) was investigated by way of differential scanning calorimetry (DSC). It is well known that the melting temperature of nascent PTFE is about 344 ∘ C, but reduces to 327 ∘ C for once molten material. In this study, the melting temperature of nascent PTFE crystals was found to strongly depend on heating rate, decreasing considerably for slow heating rates. In addition, during isothermal experiments in the temperature range of 327 ∘ C < T < 344 ∘ C, delayed melting of PTFE was observed, with complete melting only occurring after up to several hours. The melting kinetics of nascent PTFE were analyzed by means of the isoconversional methodology, and an apparent activation energy of melting, dependent on the conversion, was determined. The compensation effect was utilized in order to derive the pre-exponential factor of the kinetic model. The numerical reconstruction of the kinetic model was compared with literature models and an Avrami-Erofeev model was identified as best fit of the experimental data. The predictions of the kinetic model were in good agreement with the observed time-dependent melting of nascent PTFE during isothermal and constant heating-rate experiments.

  View details for DOI 10.3390/polym12040791

  View details for Web of Science ID 000535587700061

  View details for PubMedID 32252294

  View details for PubMedCentralID PMC7240677