From Simple to Architecturally Complex Hydrogel Scaffolds for Cell and Tissue Engineering Applications: Opportunities Presented by Two‐Photon Polymerization

Abstract

Direct laser writing via two-photon polymerization (2PP) is an emerging micro-and nano-fabrication technique to prepare predetermined and architecturally precise hydrogel scaffolds with high resolution and spatial complexity. As such, these scaffolds are increasingly being evaluated for cell and tissue engineering applications. This article first discusses the basic principles and photoresists employed in hydrogel fabrication in 2PP, followed by an in-depth introduction of various mechanical and biological characterization techniques used to assess the fabricated structures. The design requirements for cell and tissue related applications are then described to guide the engineering, physicochemical, and biological efforts. Three case studies in bone, cancer, and cardiac tissues are presented that illustrate the need for structured materials in the next generation of clinical applications. This paper concludes by summarizing the progress to date, identifying additional opportunities for 2PP hydrogel scaffolds, and discussing future directions for 2PP research.

Zwitterionic 3D‐Printed Non‐Immunogenic Stealth Microrobots

“…[35] 3D microprinting via two-photon polymerization (2PP) is an emerging nanofabrication technique that enables 3D complex polymeric structures with down to 100 nm resolution and has found broad applications in fabricating photonic crystals, metamaterials, cell scaffolds, microfluidic devices, and microrobots. [6,36,37] Briefly, a confined nanoscale voxel within a volume of photoresist is illuminated with focused femtosecond laser pulses following a complex computer-aided design (CAD) file, resulting in the photopolymerization of complex 3D structures (Figure 2a). [38] Current commercially available photoresists for 2PP lack chemical versatility, and significant research efforts are being made in the development of new functional photoresists.…”

“…[38] Current commercially available photoresists for 2PP lack chemical versatility, and significant research efforts are being made in the development of new functional photoresists. [36,37] Our zwitterionic photoresists have multiple advantages over the state-of-the-art natural and synthetic materials for 2PP: i) CB, SB, SBX, and CBX are highly soluble in water, and therefore, they do not require organic solvents and can be polymerized with water-soluble photoinitiators, which are less toxic. [37] Furthermore, water-only-compatible materials, such as biomolecules or cells, can be integrated into a single printing step.…”

“…[36,37] Our zwitterionic photoresists have multiple advantages over the state-of-the-art natural and synthetic materials for 2PP: i) CB, SB, SBX, and CBX are highly soluble in water, and therefore, they do not require organic solvents and can be polymerized with water-soluble photoinitiators, which are less toxic. [37] Furthermore, water-only-compatible materials, such as biomolecules or cells, can be integrated into a single printing step. ii) Natural polymers, including gelatin, chitosan, hyaluronic acid, alginate, etc., have limited metha crylation of functional groups (adjusted by reaction time), which restricts the crosslinking density available to such hydrogels.…”

Challenges on optimization of 3D-printed bone scaffolds

“…The 3D printing machines, based on the stimulation used for integrating matter, can be categorized into (1) laser-based 3D printing technologies which operate using laser stimulation to bond either material powders or fluid medium; (2) extrusion-based 3D printing technologies which extrude molten materials that either cool and physically bond or are further solidified by UV stimulation, and (3) ink-based 3D printing technologies which print liquid or aerosol chemical binders to chemically bond the material powders together. Laser-based technologies include stereolithography (SLA) [118][119][120][121][122][123][124][125][126][127][128][129][130], selective laser sintering (SLS) [131][132][133][134][135][136], electron beam melting (EBM) [137][138][139], LENS [140], SLM [39,[141][142][143][144][145][146][147][148][149], and two-photon polymerization (2PP) [150,151]. Extrusionbased technologies include fused deposition modeling (FDM) [152], and material jetting (MJ) [96].…”

Harnessing Multi-Photon Absorption to Produce Three-Dimensional Magnetic Structures at the Nanoscale

“…The technique, which made use of pulsed lasers within the picosecond or femtosecond regime, was capable of defining any 3D geometry within a polymer, at a resolution of order 100 nm. From the 2000s onwards it has been used to great effect in order to realise photonic crystals [22][23][24][25][26], microfluidic channels [27] and cell scaffolds [28][29][30][31]. It would be another 15 years before the potential of two-photon lithography was exploited to realise 3D magnetic nanostructures.…”