Design considerations for digital light processing bioprinters

Abstract

With the rapid development and popularization of additive manufacturing, different technologies, including, but not limited to, extrusion-, droplet-, and vat-photopolymerization-based fabrication techniques, have emerged that have allowed tremendous progress in three-dimensional (3D) printing in the past decades. Bioprinting, typically using living cells and/or biomaterials conformed by different printing modalities, has produced functional tissues. As a subclass of vat-photopolymerization bioprinting, digital light processing (DLP) uses digitally controlled photomasks to selectively solidify liquid photocurable bioinks to construct complex physical objects in a layer-by-layer manner. DLP bioprinting presents unique advantages, including short printing times, relatively low manufacturing costs, and decently high resolutions, allowing users to achieve significant progress in the bioprinting of tissue-like complex structures. Nevertheless, the need to accommodate different materials while bioprinting and improve the printing performance has driven the rapid progress in DLP bioprinters, which requires multiple pieces of knowledge ranging from optics, electronics, software, and materials beyond the biological aspects. This raises the need for a comprehensive review to recapitulate the most important considerations in the design and assembly of DLP bioprinters. This review begins with analyzing unique considerations and specific examples in the hardware, including the resin vat, optical system, and electronics. In the software, the workflow is analyzed, including the parameters to be considered for the control of the bioprinter and the voxelizing/slicing algorithm. In addition, we briefly discuss the material requirements for DLP bioprinting. Then, we provide a section with best practices and maintenance of a do-it-yourself DLP bioprinter. Finally, we highlight the future outlooks of the DLP technology and their critical role in directing the future of bioprinting. The state-of-the-art progress in DLP bioprinter in this review will provide a set of knowledge for innovative DLP bioprinter designs.

FIG. 1.

Overview of DLP bioprinter design considerations.

FIG. 2.

Stage and vat designs according to configuration. (a) Bottom-up configuration: (i) adhesion issue that may occur; (ii) rough surface to promote surface adhesion; (iii) vacuum/suction forces on print and film; and (iv) having an oxygen-permeable film enables a dead zone that promotes print separation. (b) Top-down configuration: (i) inconsistent layer issue that may occur; and (ii) implementation of wiper to create a homogeneous layer.

FIG. 3.

Specific stage examples. (a) Generic stage design using glass surface for flatness. (b) iCLIP design of stage with a secondary bioink inlet. (c) Robotic arm as a stage to create complex structures without supports. (d) Delta-mounted stage for increased print area and resolution (stitching). (e) Multi-technique DLP bioprinting with extrusion for the fabrication of multi-material constructs.

FIG. 4.

Specific vat examples. (a) Generic vat design to control heat to regulate bioink viscosity and for cellular viability. (b) HARP. (c) Microfluidic vat. (d) Carousel-inspired top-down multi-vat. (e) Dynamic support bath incorporated into the vat.

FIG. 5.

Optical workflow. Starts with selecting a light source (see inset and Table I for more information); projectors are not considered in this setup. Next, if needed, collimate the light, and play with the magnification until impacting the DMD surface. Also, g-DLP and its functionality in respect to the DMD technology is illustrated; an example a 2-× 2-pixel array emitting four different tones of gray. Afterward, it is necessary to filter (utilizing lenses and apertures) unwanted diffraction orders. Throughout the setup, mirrors can be used to redirect the light in a particular direction (i.e., bottom-up vs top-down). Finally, more magnification methods can be utilized, such as a microscope objective, to obtain the projected image to the desired size.

FIG. 6.

Electronics components. (a) Diagram showing the five main electronics components of a DLP bioprinter (motor, motor-driver, position-sensor, controller, and power source); each category is then labeled with some pertinent models. (b) Overview of the connection of main electronic components is provided. (c) Main electronic layout example in a DLP bioprinter.

FIG. 7.

Software architecture. (a) Diagram flow chart of the functionality of a DLP bioprinter's software workflow. (b) Graphical representation of the process of voxelizing, slicing, and projecting an STL file.

FIG. 8.

Materials considerations. Left: Typical components that encompass uncrosslinked bioinks. Right: Main factors to control once the crosslinked hydrogel is obtained.