Microfluidic droplet-based bioprinting offers several advantages over conventional extrusion-based bioprinting methods such as (i) high-precision spatial patterning of the biologics (including cells, molecules, drugs and bioinks) and (ii) ease of their compartmentalization. These advantages, combined with high reproducibility of the generated microdroplets, facilitate high-throughput fabrication of well-defined 3D tissue constructs with complex inner architecture which could be used, e.g., in drug- or biomaterial-screening.
Most of the techniques currently availabe in the literature rely on the use of simple W/O emulsion droplets . In this context, double-emulsion W/O/W core-shell droplets consisting of an aqueous ‘core’ encapsulated by a (biocompatible) oil ‘shell’ in an external aqueous phase could offer additional benefits. The shell phase could serve as a selective permeable barrier allowing the transport of small molecules and oxygen from the external environment while ensuring that the cells and bioinks contained in different droplets remain compartmentalized and develop into separate microtissues.
Here, we establish the possibility of printing of single-file chains of double emulsion aqueous core-droplets onto a glass substrate under external aqueous media. This strategy allows generation of ordered arrays of droplets for future use as microfluidic biomaterial- or drug-testing assays. In particular, we also demonstrate printing of hydrogel (GelMA) droplets.
Double emulsion droplets are generated using an aqueous solution of GELMA 6% w/v + 0.2% w/v photoinitiator (LAP) as the inner phase, NOVEC 7500 + 3 % PFPE-PEG-PFPE surfactant as the shell/intermediate phase and distilled water as the external phase. The substrate is a glass slide treated with a fluorophilic coting NOVEC 1720.
GelMA droplets are encapsulated in NOVEC 7500 using a microfluidic T-junction micromilled in a polycarbonate chip. The generated droplets are then directed towards a substrate through a 25G needle immersed in an external aqueous media. The spacing between the tip of the needle and the substrate is precisely adjusted (<200 micrometers) to allow immediate deposition of the droplets at the substrate via wetting by the oil phase while leaving enough space for the droplets to remain stable upon extrusion.
We show that the GelMA droplets can be printed onto a substrate in the form of a line of liquid ‘cores’ encapsulated by a thin oil ‘shell’ under external aqueous media. The presence of the surfactant-rich oil phase not only prevents coalescence of the droplets but also leads to adhesive capillary forces between them which stabilizes the printed lines. We demonstrate printing of hundreds of droplets at various substrates and in particular find optimal printing conditions using rough or porous substrates which facilitate rapid droplet deposition (prevent droplet ‘sliding’ at the substrate).
We present a microfluididc-bioprinting platform that could, in the future, serve as a novel tool for high-throughput reproducible production and printing of thousands of compartmentalized microtissues. The technology could be developed towards, e.g., high-throughput biomaterial-screening via incorporating different hydrogel in each droplet followed by the deposition of the droplets into ordered arrays and their long-term culture.
1 Zhou, L. et al., Adv. Mat. 32, 2002183, 2020