Cryogenics and 3D Printing Used to Create Super-Soft Replicas of Biological Structures for Tissue Regeneration

Researchers with Imperial College London (ICL) have developed a new 3D printing technique for creating replicas of biological structures, which could potentially be used for tissue regeneration, and even replica organs. Scaffold use is becoming more common in the medical field, and we are more frequently hearing about 3D printing technology in combination with tissue scaffolds and tissue regeneration.

If doctors were able to regenerate damaged human tissue by ‘seeding’ porous scaffolds with cells and encouraging their growth, the body would be able to heal from transplant procedures that replace tissue without having to deal with the many issues, such as rejection by the body, that normally plague that type of operation.

The method, created in collaboration with researchers from Kings College London, uses 3D printing and cryogenics to create super-soft structures which could eventually replicate complex organs.

“Cryogenics is the novel aspect of this technology – it uses the phase change between liquid and solid to trigger polymerisation and create super soft objects that can hold their shape,” explained Dr. Antonio Elia Forte, one of the researchers from ICL’s Department of Bioengineering. “This means that the technology has a wide variety of possible uses.”

The team’s work builds on previous research, but took it a step further, and developed the first method that can create structures soft enough to mimic the mechanical properties of organs such as the brain and lungs. The researchers published the new technique in a paper, titled “Cryogenic 3D Printing of Super Soft Hydrogels,” in the journal Scientific Reports; co-authors include Zhengchu Tan, Cristian Parisi, Lucy Di Silvio, Daniele Dini, and Dr. Forte.

Schematic of the cryogenic 3D printing procedure and set up (not to scale).

The abstract reads, “In this contribution we present a cryogenic 3D printing method able to produce stable 3D structures by utilising the liquid to solid phase change of a composite hydrogel (CH) ink. This is achieved by rapidly cooling the ink solution below its freezing point using solid carbon dioxide (CO₂) in an isopropanol bath. The setup was able to successfully create 3D complex geometrical structures, with an average compressive stiffness of O(1) kPa (0.49 ± 0.04 kPa stress at 30% compressive strain) and therefore mimics the mechanical properties of the softest tissues found in the human body (e.g. brain and lung). The method was further validated by showing that the 3D printed material was well matched to the cast-moulded equivalent in terms of mechanical properties and microstructure. A preliminary biological evaluation on the 3D printed material, coated with collagen type I, poly-L-lysine and gelatine, was performed by seeding human dermal fibroblasts. Cells showed good attachment and viability on the collagen-coated 3D printed CH. This greatly widens the range of applications for the cryogenically 3D printed CH structures, from soft tissue phantoms for surgical training and simulations to mechanobiology and tissue engineering.”

The research team’s technique uses solid carbon dioxide, also known as dry ice, to quickly cool down a hydrogel ink as it’s extruded from a 3D printer; for the purposes of the experiment, the researchers used a modified Ultimaker 3D printer. Once the ink thaws out, it forms a gel that’s as soft as human tissue.

“At the moment we have created structures a few centimetres in size, but ideally we’d like to create a replica of a whole organ using this technique,” said Tan, a postgraduate researcher from ICL’s Department of Mechanical Engineering.

While similar techniques have resulted in structures that collapse under their own weight, that’s not been the case with ICL’s work. The method is extremely unique in that can be used to create super-soft scaffolds, similar to the softest tissues found in the human body. Because the team was able to match the softness and structure of human body tissues, the resulting structures could be used to form scaffolds in medical procedures that could be a template and promote tissue generation.

(a) Cylindrical pore microstructure and (b) 8 unit cells printed; thawed printed 8 cell structure in (c) isometric view and (d) side view. Scale bars, (c) 10 mm and (d) 5 mm.

The team seeded their 3D printed structures with dermal fibroblast cells to test them – the cells generate connective tissue in the skin – and reported successful attachment and survival. This research could lead to many opportunities and applications in the medical field, such as growing stem cells and seeding neuronal cells involved in the spinal cord and brain. In the future, scientists may even be able to use the new 3D printing technique to create replica body parts and organs, which could improve medical training, and allow for experiments to be conducted that are not possible to complete on live human subjects.

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