Fotis Christakopoulos

All Publications

• 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