Three-dimensional printing of neuron-inspired structures by direct laser writing

Neuron-inspired structures are 2D or 3D artificial structures that emulate the structural features in biological neural networks (BNNs). Due to their inherent varied characteristics, such as geometrical characteristics, mechanical properties and biocompatibility, these neuron-inspired structures not only provide structural supports and direct neuron shapes and affect neuron differentiation, migration and proliferation in the field of neuron tissue culturing but have proved to reflect their involvement in different neuron functional tasks in neuroelectronic interfacing applications and neuromorphic computing purposes.
The design, fabrication and characterisation of neuron-inspired structures has received considerable attention over the past decades, with many fabrication techniques, including electron beam lithography (EBL) and three-dimensional (3D) additive printing, utilised in fabrication of two-dimensional (2D) or 3D neuron-inspired structures.

In addition, future applications of neuron-inspired structures require development in the fabrication of 3D neuron-inspired structures at the sub-micrometre scale with high biocompatibility as well as emulation of the structural features in BNNs.
However, owing to the fact that BNNs possess extraordinary complexity and connectivity at the sub-micrometre scale in 3D space, traditional fabrication of neuron-inspired structures cannot emulate the complexity in BNNs. One reason is the constraints rooted in the fabrication technologies used in the fabrication of neuron-inspired structures, such as EBL, mask lithography and other recent developments in 3D fabrication techniques. These techniques are generally limited to fabrication in either 2D substrate or 3D space lacking resolution. Another reason is the lack of emulation of the structural features in BNNs. Neuron-inspired structures that have been fabricated are generally very simple, such as microholes or micro-grooves. These structures cannot emulate geometrical features such as the branching structures in BNNs.

The solution lies in the combination of 3D direct laser writing (DLW), biomimetics and recently developed biocompatible hydrogel materials. 3D DLW based on two-photon absorption (TPA) is a cost-effective fabrication technique that can fabricate 3D arbitrary structures down to the 9 nm feature size. Biomimetics is a multidisciplinary field that has provided numerous solutions in the fields of physics, chemistry and engineering.
In this PhD project, we propose and demonstrate the use of 3D DLW to fabricate biomimetic neuron-inspired structures with a sub-micrometre feature size. In the main, we focus on four aspects of research:

1. discovery of unique structural features in BNNs networks and mathematical definition of the corresponding structures based on biomimetics

2. investigating the challenges in the fabrication of these structures and identifying solutions to tackle these challenges using 3D DLW

3. understanding the relationship between structure and properties (such as mechanical properties) based on experimental and theoretical study, revealing the physics principles and underlying mechanisms

4. developing a biocompatible photosensitive material suitable for 3D DLW.

3D biomimetic neuron-tracing structures can be directly fabricated using 3D DLW with a sub-micrometre feature size, tenfold smaller than biological counterparts. By introducing the mathematical model behind the elastic-capillary phenomenon, stable 3D biomimetic neuron-tracing structures can be fabricated by tuning fabrication conditions such as laser power and writing speed. This work solves the fabrication challenge faced in the fabrication of 3D neuron-tracing structures at the sub-micrometre scale.
Inspired by the mathematical formula for the `shortest connection distance' in BNNs, biomimetic 3D Steiner tree microstructures were introduced and fabricated using galvo-dithering DLW. The mechanical properties of the fabricated 3D Steiner tree microstructures are theoretically and experimentally studied, and the power-law scaling relationships between relative density and Young's modulus / yield strength confirmed. 3D Steiner tree microstructures have the smallest relative density compared with traditional low-density structures, rendering them potential candidates in many fields, including biomedical engineering and mechanical metamaterials.

A novel biocompatible hydrogel suitable for 3D DLW for future applications in biomedical science was developed. Demonstration of the properties of our selected components were performed using z-scan methods. The range of the Young's modulus of the hydrogel material was studied by fabricating hydrogel microcubic structures with different laser powers. Further experimental fabrication and theoretical study showed the reversibility of the hydrogel microstructures resulted from the swelling and shrinking effect of hydrogels. A series of neuron-inspired fractal tree structures were fabricated using optimised conditions in the hydrogel. This chapter demonstrated the properties of the biocompatible hydrogel for 3D DLW.