A Path Toward High-Resolution Bioprinting

The emerging field of embedded bioprinting could be a pivotal next step in tissue engineering. Although spatial resolution and structural integrity have remained consistent obstacles, new technology could provide the solution.

Since the advent of bioprinting, the technology has held the promise of enabling advanced biomimetic tissue fabrication. In embedded bioprinting, low-viscosity hydrogel-based bio-inks are deposited into a temporary support matrix that prevents the collapse of the printed structure. While this method expands the potential for tissue engineering by enabling the fabrication of complex, functional, three-dimensional (3D) tissues, limited spatial resolution is a major obstacle. The rheological and physical properties of the support matrix are critical parameters that significantly impact the resolution and fidelity of the printed construct.

Bioprinting — a pioneering technique in tissue engineering and regenerative medicine — employs the precise layer-by-layer deposition of bio-inks to recapitulate complex tissue structure and function. Applications for these tissues in healthcare include physiologically relevant 3D in vitro models and patient-specific medical devices for tissue regeneration and organ transplantation (1). A crucial success factor in fabricating these tissues is the structural fidelity of the printed construct, which is partly determined by the minimum feature size, also referred to as the bioprinting resolution.

Typically composed of cell-laden hydrogels, bio-inks must have a low viscosity to eliminate the need for high extrusion pressures during printing, which could threaten cell viability (2). Consequently, these bio-inks are non-self-supporting and collapse when deposited onto a hard surface as in traditional 3D printing methods. Embedded bioprinting techniques resolve this property-level dichotomy by depositing bio-inks into a temporary support matrix that suspends the bio-ink and prevents the structure from collapsing until it can be stabilized via crosslinking. Crosslinking induces the gelation of the bio-ink, during which the bio-ink transforms from exhibiting pseudoplastic behavior to a solid viscoelastic state. Crosslinking methods can be chemical, physical, or enzymatic, and the choice of crosslinking method depends on the bio-ink composition. The most common crosslinking methods involve treating the structure with ultraviolet (UV) light or an ionic solution such as calcium chloride.

Several embedded bioprinting techniques have been devised, which utilize a range of bioprinting technologies, support matrix materials, and formulations. Extrusion-based embedded bioprinting balances bio-ink versatility, cell viability, scalability, and cost, making it the most widely used technology for fabricating functional tissues and organs. Utilizing an agarose fluid gel as a support matrix, suspended layer additive manufacturing (SLAM) facilitates the extrusion bioprinting of soft hydrogel-based bio-inks into complex, functional, 3D tissue structures (2). Composed of micrometer-sized gel particles, fluid gels behave in bulk as viscoelastic fluids with shear-thinning and self-healing properties. This enables the support matrix to act as a liquid as the cartridge needle passes through it, yet it rapidly recovers solid-like properties to support the deposited bio-ink, preventing the spread and collapse of the printed structure.

Despite being one of the leading bioprinting techniques for tissue engineering, limited spatial resolution is a major hindrance to extrusion bioprinting. Previous work in bioprinting has focused on optimizing print resolution through printing parameters such as bio-ink formulation, print mechanisms, extrusion pressure, and nozzle geometry; however, the impact of the support matrix on resolution has received less attention. This article discusses the importance of print resolution in bioprinting and explores the influence of the support matrix on resolution and material deposition.

Importance of resolution in tissue functionality

High printing resolution is essential for bioprinting to accurately replicate the structure, function, and behavior of native tissues, improving...