Nitrogen gas flow created microchannels in printed scaffolds and guided skin and bone repair in animals
Nitrogen gas flow created microchannels in printed scaffolds and guided skin and bone repair in animals
On August 7, Science Advances published a study on the cryoprinting of hydrogel scaffolds, which are temporary porous structures used to support tissue repair. A stream of nitrogen gas makes ice grow inside the freshly printed gel either vertically or from the edges toward the center. After thawing, the ice leaves behind microchannels. In experiments with mice and rats, the orientation of these channels changed how the tissue healed.
Bone, skin, tendons, and nerves have directional structures. Their cells and fibers extend along the lines of mechanical load, growth, or signal transmission. A scaffold must match the shape of the defect and provide a path for cells to enter the material.
In this study, the authors print a hydrogel, which is a soft, water-rich gel made from gelatin, and cool it with gas from liquid nitrogen. The gas flow removes heat along a selected path. Ice crystals grow along that path, and after thawing, they leave interconnected voids tens of micrometers wide. A vertical flow creates channels along the height of the scaffold, while a ring that supplies nitrogen at the edges directs the channels toward the center.
In a system for freezing cell suspensions, liquid nitrogen served the opposite purpose: it prevented ice crystals from damaging cell membranes. Here, the ice serves only as a temporary template for voids in the hydrogel. Cells are introduced into the scaffold after it has thawed.
In the animal experiments, the channel orientation was matched to the direction of tissue growth. In rat skull defects, radial channels followed the growth of new bone from the edges toward the center. In femoral defects, vertical channels aligned with the axis of the bone marrow cavity. After four and eight weeks, micro-CT showed the densest and most organized bone growth when the channel orientation matched the direction of tissue growth.
In full-thickness skin wounds in mice, the radial scaffold produced 99.15% wound closure by day 14, compared with 74.14% in untreated animals. In culture, human cells remained near the surface of a conventional gel but moved along the channels of the printed scaffold. This migration shows how the pattern of voids within a material can determine the direction in which cells populate the scaffold.
The scaffold's outer shape matches the defect, while its microchannels guide cells within it. The printer nozzle determines the external shape, while the direction of the heat-removing gas flow determines the arrangement of the channels. The authors therefore extended vertical cryoprinting with aligned channels, described in 2022, to include radial geometries and hollow structures up to two centimeters tall.