Three dimensional (3D) bioprinting is the utilization of 3D printing–like techniques to combine cells, growth factors, and biomaterials to fabricate biomedical parts that maximally imitate natural tissue characteristics. Generally, 3D bioprinting utilizes the layer-by-layer method to deposit materials known as bioinks to create tissue-like structures that are later used in medical and tissue engineering fields. Bioprinting covers a broad range of biomaterials.
Currently, bioprinting can be used to print tissues and organs to help research drugs and pills. However, emerging innovations span from bioprinting of cells or extracellular matrix deposited into a 3D gel layer by layer to produce the desired tissue or organ. In addition, 3D bioprinting has begun to incorporate the printing of scaffolds. These scaffolds can be used to regenerate joints and ligaments.
Pre-bioprinting is the process of creating a model that the printer will later create and choosing the materials that will be used. One of the first steps is to obtain a biopsy of the organ. Common technologies used for bioprinting are computed tomography (CT) and magnetic resonance imaging (MRI). To print with a layer-by-layer approach, tomographic reconstruction is done on the images. The now-2D images are then sent to the printer to be made. Once the image is created, certain cells are isolated and multiplied. These cells are then mixed with a special liquefied material that provides oxygen and other nutrients to keep them alive. In some processes, the cells are encapsulated in cellular spheroids 500μm in diameter. This aggregation of cells does not require a scaffold, and are required for placing in the tubular-like tissue fusion for processes such as extrusion.:165
In the second step, the liquid mixture of cells, matrix, and nutrients known as bioinks are placed in a printer cartridge and deposited using the patients' medical scans. When a bioprinted pre-tissue is transferred to an incubator, this cell-based pre-tissue matures into a tissue.
3D bioprinting for fabricating biological constructs typically involves dispensing cells onto a biocompatible scaffold using a successive layer-by-layer approach to generate tissue-like three-dimensional structures. Artificial organs such as livers and kidneys made by 3D bioprinting have been shown to lack crucial elements that affect the body such as working blood vessels, tubules for collecting urine, and the growth of billions of cells required for these organs. Without these components the body has no way to get the essential nutrients and oxygen deep within their interiors. Given that every tissue in the body is naturally composed of different cell types, many technologies for printing these cells vary in their ability to ensure stability and viability of the cells during the manufacturing process. Some of the methods that are used for 3D bioprinting of cells are photolithography, magnetic bioprinting, stereolithography, and direct cell extrusion.:196
The post-bioprinting process is necessary to create a stable structure from the biological material. If this process is not well-maintained, the mechanical integrity and function of the 3D printed object is at risk. To maintain the object, both mechanical and chemical stimulations are needed. These stimulations send signals to the cells to control the remodeling and growth of tissues. In addition, in recent development, bioreactor technologies have allowed the rapid maturation of tissues, vascularization of tissues and the ability to survive transplants.
Bioreactors work in either providing convective nutrient transport, creating microgravity environments, changing the pressure causing solution to flow through the cells, or add compression for dynamic or static loading. Each type of bioreactor is ideal for different types of tissue, for example compression bioreactors are ideal for cartilage tissue.:198
Researchers in the field have developed approaches to produce living organs that are constructed with the appropriate biological and mechanical properties. 3D bioprinting is based on three main approaches: Biomimicry, autonomous self-assembly and mini-tissue building blocks.
The first approach of bioprinting is called biomimicry. The main goal of this approach is to create fabricated structures that are identical to the natural structure that are found in the tissues and organs in the human body. Biomimicry requires duplication of the shape, framework, and the microenvironment of the organs and tissues. The application of biomimicry in bioprinting involves creating both identical cellular and extracellular parts of organs. For this approach to be successful, the tissues must be replicated on a micro scale. Therefore, it is necessary to understand the microenvironment, the nature of the biological forces in this microenvironment, the precise organization of functional and supporting cell types, solubility factors, and the composition of extracellular matrix.
The second approach of bioprinting is autonomous self-assembly. This approach relies on the physical process of embryonic organ development as a model to replicate the tissues of interest. When cells are in their early development, they create their own extracellular matrix building block, the proper cell signaling, and independent arrangement and patterning to provide the required biological functions and micro-architecture. Autonomous self-assembly demands specific information about the developmental techniques of the tissues and organs of the embryo. There is a "scaffold-free" model that uses self-assembling spheroids that subjects to fusion and cell arrangement to resemble evolving tissues. Autonomous self-assembly depends on the cell as the fundamental driver of histogenesis, guiding the building blocks, structural and functional properties of these tissues. It demands a deeper understanding of how embryonic tissues mechanisms develop as well as the microenvironment surrounded to create the bioprinted tissues.
The third approach of bioprinting is a combination of both the biomimicry and self-assembly approaches, which is called mini tissues. Organs and tissues are built from very small functional components. Mini-tissue approach takes these small pieces and manufacture and arrange them into larger framework.
Akin to ordinary ink printers, bioprinters have three major components to them. These are the hardware used, the type of bio-ink, and the material it is printed on (biomaterials). "Bio-ink is a material made from living cells that behaves much like a liquid, allowing people to "print" it in order to create a desired shape. To make bio-ink, scientists create a slurry of cells that can be loaded into a cartridge and inserted into a specially designed printer, along with another cartridge containing a gel known as bio-paper."
In bioprinting, there are three major types of printers that have been used. These are inkjet, laser-assisted, and extrusion printers. Inkjet printers are mainly used in bioprinting for fast and large-scale products. One type of inkjet printer, called drop-on-demand inkjet printer, prints materials in exact amounts, minimizing cost and waste. Printers that utilize lasers provide high-resolution printing; however, these printers are often expensive. Extrusion printers print cells layer-by-layer, just like 3D printing to create 3D constructs. In addition to just cells, extrusion printers may also use hydrogels infused with cells.
There are several applications for 3D bioprinting in the medical field. An infant patient with a rare respiratory disease known as Tracheobronchomalacia (TBM) was given a tracheal stint that was created with 3D printing. 3D bioprinting can be used to reconstruct tissue from various regions of the body. Patients with end-stage bladder disease can be treated by using engineered bladder tissues to rebuild the damaged organ. This technology can also potentially be applied to bone, skin, cartilage and muscle tissue. Though there are several examples of tissue engineering using 3D bioprinting technology and the end goal of reconstructing tissue is to reconstruct an entire organ there has been little success in printing fully functional organs due to inefficiency of the medical procedures needed in the process. Israeli researchers constructed a rabbit-sized heart out of human cells in 2019.
3D bioprinting contributes to significant advances in the medical field of tissue engineering by allowing for research to be done on innovative materials called biomaterials. Biomaterials are the materials adapted and used for printing three-dimensional objects. Some of the most notable bioengineered substances are usually stronger than the average bodily materials, including soft tissue and bone. These constituents can act as future substitutes, even improvements, for the original body materials. Alginate, for example, is an anionic polymer with many biomedical implications including feasibility, strong biocompatibility, low toxicity, and stronger structural ability in comparison to some of the body's structural material. Synthetic hydrogels are also commonplace, including PV-based gels. The combination of acid with a UV-initiated PV-based cross-linker has been evaluated by the Wake Forest Institute of Medicine and determined to be a suitable biomaterial. Engineers are also exploring other options such as printing micro-channels that can maximize the diffusion of nutrients and oxygen from neighboring tissues In addition, the Defense Threat Reduction Agency aims to print mini organs such as hearts, livers, and lungs as the potential to test new drugs more accurately and perhaps eliminate the need for testing in animals.
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