From Mechanics to Tissue: Artificial Organ Design

Şehnaz Melisa Acar^A., Tuğçe Binen^E., Esra Avcı^T., Gülnur Uzun^E.

Although medical science is constantly making attempts to treat diseases that have serious and high mortality rates or reduce the quality of life, organ failure is one of the leading causes of death worldwide¹. At this point, the demand for donor organs to replace lost or damaged organs is increasing day by day². Organ transplants have represented the gold standard for saving lives in medical science; however, the growing disparity between the number of donor organs and the number of patients on the waiting list or unplanned donor-related situations limit organ transplantation^3,4. Increasing demands and inability to meet pending demands, costly organ transplantation, ethical problems, and immune response, such as reasons also make organ transplantation disadvantageous^1,2. For this reason, with the development of technology and increasing life expectancy, studies that may be an alternative to possible solutions to the degeneration of some body parts or organ failure have increased³. Regenerative medicine is a sub-branch of medical science that aims to develop various applications to repair, replace, or regrow diseased or damaged cells, tissues, and organs⁵.   

It basically includes tissue engineering, the production of various stem cells for use in treatments, and artificial organ design. Recently, regenerative medicine, which offers the potential to revolutionize the health sector, has become important worldwide with the ongoing studies in this field; however, reliable methods should be applied to develop organs or tissues in the development process of regenerative treatments⁶.

Tissue engineering; basically, aims to produce damaged or diseased tissues in the body in vitro for use onsite. At this point, it constitutes a multidisciplinary research area that has an incredible benefit for patients waiting for tissue or organ transplantation⁷. Artificial organs, which are one of the important applications of tissue engineering, are defined as medical devices that can be surgically implanted and have mechanical and biochemical functions such as heart, kidney, lung, and sensory organs⁸. In this review, artificial organ design, which has opened up a different dimension in regenerative medicine and tissue engineering, will be mentioned.

Since artificial organs can interface with living tissue and be integrated or implanted into the body, they offer the possibility of temporarily or permanently treating or restoring defective tissues/organs. With these opportunities, artificial organs continued to have success in clinical environments in line with the latest developments in biomaterials, innovations in process engineering, increasing the life and durability of devices, and cell-based therapies³. The three-dimensional (3D) printing process for various artificial organ applications is widely used for biosignature which allows the production of bioactive molecules, biomaterials, and cells for the purpose of creating 3D tissues with complex geometries. With the help of 3D printing technology, cartilage, bone, muscles and tendons, nerves, kidneys, lives, and many different tissues or organs continue to be produced⁹. The 3D printing process, which is applied by modeling, printing, culturing, and transplanting the structure espectively, so that the tissues can be re-implanted, has some limitations in terms of practical potential^10,11. 3D printing technology has focused on development in a way that lacks the fundamental factors necessary for properly imitating natural textures. It is known as a factor that lacks the ability to change sharply according to changes in the environment or functional situations.

Due to these limitations, various working groups have proposed four-dimensional (4D) printing as an advanced method for regenerative medicine¹². 4D bioprinting opens the door to a new generation of technology by combining the concept of “time”, defined as the fourth dimension, with 3D printing⁹. The most important feature that distinguishes 4D bioprinting technology from 3D technology is the development of space and time-dependent products that are well controlled by easily separating them from static objects produced in 3D by changing their shapes in response to different external stimuli such as light, heat, pH, humidity, electricity or water, or functional situations.

To ensure the regeneration and repair of natural tissues and conformational changes in tissue structure, it is necessary to include time dependence in 3D-printed tissue structures. For this reason, stimulus-based biomaterials and cell-pulling forces have developed to provide tissues produced with dynamic 4D bioprinting¹⁴. The development of a new generation of applications involving tissue regeneration intelligence materials sensitive to triggers, biological signals, and pathological abnormalities is widely used¹⁵. These materials, which are sensitive to stimulation, undergo conformational changes to certain stimuli such as pH, temperature, magnetic field, electricity, humidity, light, acoustics, or different combinations of these triggers. In this context, stimulus-based biomaterials represent a viable potential for bio-inks used for 4D bioprinting. Stimulus-based biomaterials are generally biocompatible and easily printable. These reasons represent the basic parameters for 4D bioprinting (Fig. 1)¹⁴.

Figure 1. Schematic representation of 3D, 4D printing technologies using traditional materials, cells, and smart materials¹⁴. Cells have been used in 3D or 4D bioprinting processes. At this point, 4D printing technologies have been defined as bio-inks that respond to stimuli or 4D printing of cell-loaded materials that respond to responses originating from internal cell forces.

Heat, among many different stimuli such as pH, electricity, and magnetic field has been one of the most studied stimuli for the exploration of shape memory. At this point, there are many types of polymers known to be temperature-sensitive; however, most of these polymers are synthetic products. Therefore, only a few have the fundamental properties needed to produce ink for the use of 4D printing technology¹⁶. The goal of temperature-based research is to develop biosignature parts that deform based on temperature as an external stimulus.  In the process that is generally followed; first of all, products with form memory are printed. Then, for them to turn into semi-stable temporary shapes, the glassy transition temperature (Tg) is subjected to a deformation step when appropriate conditions are provided, such as cooling after mechanical loading, that is maintained above the critical temperature and applied. After that, the temporarily deformed structure is transformed into the desired shape when sufficient conversion energy is used to ensure the recovery step during application¹⁷. Another parameter that can be used after the temperature is that changes are made to the pH to trigger modifications of hydrogels or polymers. Some biopolymers that are sensitive to pH changes in the environment in which they are located contain ionizable chemical groups in their structure and macroscopically cause changes such as bending, swelling, and shrinking²⁰.

Some studies have also focused on the use of electric and magnetic fields to achieve modifications in biomaterials other than different stimuli such as temperature and pH. In studies where a magnetic field to used as a stimulus, composites usually need to be used for biomaterials to react. Materials that are sensitive to magnetic fields include iron or iron oxide nanoparticles (IONPs)²¹. In addition to being used to induce changes after the printing step, the application of a magnetic field during the bioprinting process by the magnetized bio-ink has been used to determine the direction of the particles. With the determination of the direction of the particles, structures with anisotropic (direction-dependent atomic arrangements) properties that can be oriented according to the foreseen applications are obtained²².

The development of tissues and even organs to be designed and subsequently fabricated using 4D printing technologies embraces the use of materials that have the potential to respond to stimuli as a design procedure to enable scientists to pre-plan a particular shape based on the application envisaged and to activate a functionality change²³. Besides the responsiveness to stimuli such as pH, electricity, and temperature, taking into account the rheological (fluxological) and physicochemical properties of the bio-products to be used during the bio-printing process during the formulation allows the printing process to be performed without compromising the viability of the cell during 3D or 4D printing²⁴. In addition, attention to the formulation of the bio-inks to be used during bioprinting guarantees the stability of 4D structures after printing²⁵. In addition to the potential of the materials to be used to respond to stimuli; they must also meet additional properties such as being mechanically robust, biocompatibility, non-immunogenic, being able to be processed under cell-compatible conditions, being able to undergo or adaptive to shape changes, and biodegradability¹⁷.

Recently, biomaterials used in the production of cells, tissues, and organs, which will primarily have been produced using 4D bioprinting, are based on natural, synthetic, hybrid intelligence polymers. In addition to these, it has been stated by the researchers that polymeric hydrogels sensitive to stimuli should be used; in addition, the injectability of biomaterials and shape memory are among its necessary features (Figure 2)²³.

Figure 2. 4 key components of emerging 4D bioprinting technology²³.

Recently, with the development of 4D bioprinting technology, this technology is used for tissue or organ production in many different areas¹. In a study conducted by Cui et al. in 2020, significant work has been done to engineer heart scaffolds for use in the treatment of myocardial infarction (MI). Due to the difficulty of traditionally used methods, the production of unique heart tissues will be expressed in this study by producing a 4-dimensional heart prototype with physiological adaptability by ray-scanning stereolithography.. Their design uses a flexible microstructure and a proprietary 4D self-modifying capability to enhance the strength of the patches and how well they integrate with a beating heart. Their studies show that natural mechanical stimulation of heart cells in vitro improves their development and blood vessel growth. They also showed that using this method in mice with chronic heart damage increased cell engraftment and blood flow. This research not only offers a promising treatment for heart damage but also offers a new approach to developing complex tissues for organ regeneration²⁷.

Recent advances in tissue engineering have led to the development of 3D scaffolds that provide a natural environment for cells to regenerate; however, constructing these scaffolds remains disadvantageous due to difficulties in encapsulating cells in 3D and controlling their organization¹¹.  In this study, researchers have developed a 4D inkjet printing platform that creates micromodels that can self-fold into 3D microstructures. Cells can be easily seeded into these micropatterns before they self-fold, resulting from the generation of cell-encapsulating 3D scaffolds with uniform cell organization. The method was tested using endothelial cells belonging to the human umbilical vein, and the results showed that the cells grew and attached to the inner wall of the 3D structure. The 4D printing method is an easy-to-use platform that enables the customization of 3D cellularized scaffolds and shows promise for tissue engineering applications²⁸.

The advent of 4D bioprinting has significantly improved tissue engineering and has allowed the creation of complex and dynamic structures that better resemble natural tissues. Stimulus-responsive hydrogels are promising materials for 4D bioprinting due to their ability to support cellular processes and have been modified for different applications²⁹. Natural polymers are being researched and developed for use in the productivity of bio-inks due to their biocompatibility, biodegradability, and ability to adjust their properties³⁰. The use of 4D bioprinting has allowed the incorporation of new features into engineered tissues, such as vascularization and the ability to perform biological functions. More studies on bioprinting techniques, materials, and clinical trials are needed for the widespread application of this technique³¹.

Artificial organ design with 4D bioprinting is a new technology that combines 3D bioprinting with the ability to create materials that can change shape over time in response to various stimuli. The purpose of using 4D printing to create artificial organs is to develop more advanced, functional, personalized organs that are more suitable for the human body. The ultimate goal is to produce more effective solutions for organ implantation that can improve patient outcomes and quality of life.

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