Bioprinting: Functional droplet networks.

To make artificial constructs that mimic the structural complexity of native tissues, scientists typically use photolithography, soft lithography, stamping or microfluidic approaches. Recent advances in such microand nanoscale technologies, emerged at the convergence of engineering, biology, chemistry and materials science, have also enabled great progress in the understanding of living systems1. Yet the microenvironment of native tissues — which consists of multiple cell types precisely organized in three dimensions — is still complex in comparison to what can be achieved today with fabrication techniques. In particular, the spatiotemporal manipulation of cells remains a challenge. Moreover, fabricated scaffolds are typically rigid and thus lack the potential to mimic contractile tissues, such as cardiac muscle or vascular structures2. Bioprinting — the rapid, layer-by-layer deposition of cells and extracellular matrix — has also been used to make tissue mimics at the microscale, but imparting them with functionality has remained elusive. Now, Hagan Bayley and colleagues report in Science the fabrication, by means of a bioprinting approach, of three-dimensional (3D) tissue-like materials that fold in ways similar to muscles and, like neural tissues, transmit electrical signals3 (Fig. 1). Bayley and co-workers used an automated 3D printer to eject aqueous droplets (each about 65 pl) into a lipidcontaining oil bath, and assembled the droplets into networks by programming BIOPRINTING