UCL laser printing creates sensors inside chicken embryos

16 Jul 2024

Two-photon process creates elastic spring structures responding to physical forces during growth.

A project at University College London (UCL) and the University of Padua has created mechanical force sensors directly in the developing brains and spinal cords of chicken embryos.

Published in Nature Materials, the breakthrough could lead to better understanding and prevention of birth malformations such as spina bifida.

The mechanical forces exerted by the embryo during its development are crucial to the formation of organs and anatomical structures, such as the creation of the neural tube that ultimately gives rise to the central nervous system.

Monitoring these "morphogenetic" forces in living embryos has been challenging, and although methods to quantify mechanical or tensile stresses within individual large cells have been developed, a method to quantify dynamic tissue-level forces has been missing, said the project team.

"The force sensor technology must be compatible with embryo development to provide mechanical read-outs over developmentally relevant time frames of several hours, not seconds to minutes," said the project in its paper. "Sensor size, spatial position and orientation should be precisely controllable through in situ microfabrication at cell-level and tissue-level length scales during live imaging."

The team's answer was to apply laser bioprinting and create elastic shapes within the embryos through two-photon polymerization of a suitable material, expanding on previous research at the University of Padua into intravital 3D (i3D) bioprinting operations.

In 2020 a Padua project demonstrated that an i3D technique could successfully crosslink gelatin constructs to support and organize cell development, and create millimeter-scale structures under the epidermis of the skin in mice.

Advanced microscopy and novel biomaterials

The new project has built on this work, redeveloping the technique to enable micrometer-scale photo-crosslinking of biocompatible photo-active polymers in a three-dimensional elastic hydrogel, to suit the demanding and fragile environment of a developing embryo and in particular the folds of the developing neural tube.

Two-photon bioprinting was identified as the route to crating the 3D shapes with high positional and structural accuracy directly in chick embryos, crosslinking a i3D polymer introduced into the area of interest. When exposed to a strong laser, the liquid transforms into a spring-like solid attached to the growing spinal cord of the embryos and deformed by the mechanical forces produced by the embryo's cells, said the project.

"We adapted i3D bioprinting to create elastic, compliant shapes anchored to the closing neural folds, such that their deformation serves as a read-out of forces generated by medial apposition of the neural folds. We refer to these structures as intravital mechano-sensory hydrogels (iMeSHs)."

A combination of live imaging and mechanical modelling was used to monitor the deformation of the new structure during embryo development. This let researchers observe the forces involved in normal growth of the neural tube, and also start to investigate whether pharmaceutical intervention might increase positive forces or decrease negative ones enough to help prevent malformations such as spina bifida.

"Thanks to the use of novel biomaterials and advanced microscopy, this study promises a step change in the field of embryonic mechanics and lays the foundation for a unified understanding of development," commented lead author Eirini Maniou. "Our work paves the way for identifying new preventative and therapeutic strategies for central nervous system malformations."