Team demonstrates fabrication method to build 3D structures that mimic bone microstructure

Scientists combined laser 3D printing technology and an alternative dipping process to build complex 3D structures mimicking bone microstructure. This is the first demonstration of this manufacturing method, and will lead to the development of 3D cell culture systems that can support bone grafts or create artificial bone marrow.

Their research is published in the journal ACS Biomaterials Science and Engineeringand appears on the cover of the issue published on February 12, 2024.

Bone is a hybrid material composed of organic and inorganic substances, primarily collagen fibers and an inorganic mineral called hydroxyapatite (HAp). Mineralized collagen fibers assemble to form a hierarchical structure that provides excellent mechanical strength and toughness to cortical bone. Cortical bone is the strong outer layer of long bones.

Bone marrow microstructures, called marrow niches, act as regulators of hematopoietic stem cells. These are primitive cells that transform into all types of blood cells. However, the mechanism by which the bone marrow niche maintains hematopoietic stem cells remains unclear.

Hematopoietic stem cell transplantation offers a possible strategy to treat leukemia, lymphoma and immune diseases. But it is difficult for hematopoietic stem cells to spread outside the body. The creation of a transplantation model mimicking the bone marrow environment could therefore be a solution to these challenges, allowing hematopoietic stem cells to multiply in vitro and then be transplanted. Furthermore, a model mimicking the bone marrow environment could help clarify the mechanism of maintaining hematopoietic stem cells in the bone marrow in vivo.

In previous research, scientists developed HAp-based biomaterials that mimic bone microstructure. They used microfabrication techniques to create 3D models with HAp, with the aim of constructing a bone microstructure that mimics a biological environment. HAp-coated materials have been used as in vivo bone substitutes to bond defective bones by implantation. Previous research has shown that HAp-coated materials can provide an environment that is favorable for cellular function and has high affinity for bone.

However, this previous research had limitations. “It has been difficult to fabricate 3D organic and inorganic composite materials with precise structure by laser 3D printing,” said Kazutoshi Iijima, associate professor at the Faculty of Engineering at Yokohama National University.

Laser scanning stereolithography, a 3D printing technology, can produce high-definition bone models. The team chose a manufacturing method combining laser scanning stereolithography with an alternative dipping process. With this manufacturing method, the team constructed micro-sized hydrogel models of polymerized gelatin methacrylate, a biocompatible cross-linkable polymer used in bioprinting. They modified the models with HAp using the alternative soaking process with a solution of calcium and phosphate ions. This study is the first demonstration of HAp modification on 3D printed models with a more complex structure, using the alternative dipping process.

They designed and manufactured simple line-shaped models and a pyramid model with a complex structure. These allowed them to modify the fabricated models of different sizes with HAp, using the alternative dipping process method, without altering the microstructure created by stereolithography.

They tested their models under various conditions by changing the immersion time and the number of alternating cycles of the soaking process. The team was able to control the thickness of the HAp layer by changing the conditions of the alternative dipping process. They analyzed the composite line patterns and studied the formation mechanism of HAp by an alternative soaking process in the hydrogels.

“By combining laser 3D printing technology and reciprocating dipping process, it became possible to construct precise 3D composite materials of gelatin methacrylate and hydroxyapatite with precise structure,” said Hiroki Miyajima, assistant professor specially appointed to the Faculty of Engineering at Yokohama National University. .

Looking ahead, the team hopes to develop bone and bone marrow models mimicking bone microstructure that contribute to regenerative medicine, such as bone tissue regeneration and hematopoietic stem cell expansion.

The research team includes Kaori Kojima, Hiroki Touji, Kodai Onodera, Masaru Mukai, Shoji Maruo and Kazutoshi Iijima from Yokohama National University, Japan.