Fluid dynamics of the droplet impact processes in cell printing

Cell printing is an emerging technique for use in tissue engineering and bio-manufacturing. The essential idea is to deliver living cells to target positions using droplets generated via various mechanisms including thermoor piezo-jetting, laser-guided direct write technique, and others (Boland et al. 2006; Ringeisen et al. 2006; Calvert 2007). By taking advantage of the intrinsic properties of jet-based printing such as high spatial resolution (comparable to or even smaller than the size of single cells), high throughput, non-contact printing, and capable of delivering droplets with different compositions, cell printing can potentially address many critical challenges in tissue engineering and bio-manufacturing applications (Mironov et al. 2003; Varghese et al. 2005; Boland et al. 2006; Withers 2006; Villar et al. 2013). Despite the caveats inherent in the feasibility of cell printing (Jakab et al. 2004), specifically, whether enough cells can survive the harsh printing process, and whether those cells can form useful bio-structures after droplet deposition, advancements have been nonetheless forthcoming (Mironov et al. 2003; Roth et al. 2004; Xu et al. 2005, 2006). For example, cell damage is now a manageable process for some cells (Calvert 2007). Not only can printed cells self-organize into coherent structures, but also some printed structures have even been implanted into animals and grow into functional tissues (Xu et al. 2008). Research on cell printing experienced explosive growth and several comprehensive reviews of cell printing technique and its applications in various fields are now available (Nakamura et al. 2005; Abstract Cell printing, in which cell-laden droplets are delivered to target positions using inkjets or other devices, is an emerging technique in tissue engineering. Despite significant progress, the survival rate of cells delivered to these positions is often inadequate for targeted applications. Here, we developed a simple model for cell printing based on multiphase fluid–structure interactions. Using this model, we reconstructed the droplet and cell dynamics during the droplet impact process in cell printing. Based on extensive simulations, we developed a general picture of the droplet impact process by dividing it into four stages: the inertia stage, the interfacial flow stage, the elastic response stage, and the viscous flow stage. We provided a simple estimation of the duration of each stage and the magnitude of stress within the cell during each stage. From that estimation, we determined that surface tension is essential for controlling the deformation and stress inside cell under low-to-moderate droplet impact velocities relevant to inkjet-based cell printing. Based on extensive parametric studies, strategies for controlling the stress and deformation of cells during cell printing are examined and their practical implementations are discussed.