Cellular threads folded a printed hydrogel into tubes

Cellular threads folded a printed hydrogel into tubes

In a preprint posted on July 28, a team from the University of Illinois Chicago and the University of Aveiro described a 4D bioprinting method in which dense threads of living cells reshape a printed scaffold during culture. The authors produced bends, helices, tubes, and cartilage-like and bone-like constructs.

In tissue engineering, geometry contributes to function. Cartilaginous half-rings keep the airway open, a tendon transfers force from muscle to bone, and curved structures distribute loads. An engineer therefore needs not only to print cells, but also to arrange them so that the material assembles into the intended shape.

The printer establishes the initial geometry, and living cells alter it during culture. The printer first creates a soft base from hydrogel, a water-rich polymer material that can support living cells. It then deposits dense cellular threads within the base using a rapidly degrading carrier. Gelatin microspheres gradually dissolve, the base softens, and the cells within the threads proliferate, connect, and contract. This local pulling force bends the surrounding hydrogel.

The authors tested this causal sequence. Over seven days, empty scaffolds, threads made from dead cells, and constructs treated with cytochalasin D, which inhibits actin-based contraction, largely retained their original shape. Living fibroblasts bent the hydrogel strips, while myoblasts and human mesenchymal stromal cells produced greater curvature.

The pattern of the threads determines the scaffold's eventual geometry. A single short thread folded a strip into a V shape. Six angled threads formed either a right-handed or left-handed helix, with the deposition angle determining its handedness. Seven or nine parallel threads rolled a sheet into a closed tube.

The authors then changed the cells and culture medium. Over 21 days, human mesenchymal stromal cells formed curved cartilage-like tissue. A construct containing seven threads became a tube with seven C-shaped rings resembling tracheal rings. In chondrogenic medium, the compressive modulus reached 33 kPa, compared with 1.17 kPa in standard medium. Over 28 days, osteogenic medium produced curved bone-like strips and tubes with signs of mineralization. In a separate experiment, fibroblasts and myoblasts occupied different regions of the same bending scaffold, mimicking the interface between muscle and tendon.

In common forms of 4D bioprinting, materials change shape in response to heat, light, acidity, electricity, or a magnetic field. In this study, cellular contraction provides the driving force: the final shape depends on where the cells are placed and how quickly the hydrogel yields to their force.

The engineer controls the placement of the cellular threads and the properties of the base that allow their pulling force to alter its shape. Printing establishes the starting conditions, while the final shape develops during culture.