The process of combining cells, growth factors, bio-inks, and/or biomaterials to create biomedical components that mimic the properties of natural tissues, create functioning biofilms, and help remove pollutants is known as three-dimensional bioprinting. The three standard printing methods used for bioprinting are inkjet, laser-assisted, and extrusion bioprinting. Inkjet bioprinters primarily used for tissue engineering applications are of thermal and piezoelectric type. Bio inks, the main constituent of bioprinting, consist of hydrogels. Hydrogel’s choice is due to its printability, biocompatibility, crosslinking ability, and high swelling capacity. Hydrogels can have any natural or synthetic source 8. Figure 1 shows different applications of 3D printed bioprinting.

Surgeons rely on their earlier training, experience gained during earlier surgery, and image data obtained from techniques such as CT, MRI, and others. In some cases, the complex anatomical nature of the surgical site demands more information to plan the surgery. Examples are aneurysms and congenital heart defects. In such cases, the 3D-printed organ models become helpful for better preoperative planning. These models can be fabricated from the information obtained from CT or MRI images. Training new surgeons with these organ models can increase the success rate of the surgeries and drastically reduce the time required to perform the surgery. The surgeon gets a better understanding of the organ and its surrounding region. The tactile feature of the organ, surgical guidance, and other features greatly help the surgeon 13.

The brain is one of the delicate parts of the human body. So, the safest surgical procedure is required to minimize tissue damage 14. The virtual simulation obtained from MRI or CT data and the 3D printed model provides an edge for preoperative planning. The 3D models are also helpful for preoperative planning of differ cardiac related issues 15.

A permanent bioinert implant is a type of medical device that is implanted into the body and is not intended to be removed. Examples are Maxillofacial, craniofacial, dental, load bearing, spine fusion, sternocostal, and cardiovascular stents used as permanent implants. The porosity, grain structure, and surface are the essential parameters of permanent implants comprising titanium alloys, tantalum, cobalt-chromium (Co-Cr) alloys, and gold 16. The advantage of 3D-printed implants is that they can be formed specifically for a patient. It increases the probability of a successful implant.

The fabrication of tissues and organs is one of the prime applications of bioprinting. There are two basic methods to fabricate organs. In the first method, which is indirect cell assembly, a 3D scaffold is first formed, and then the cell seeding process occurs. The second method, direct cell assembly, creates a composite structure using cells and materials 4. The main requirements of scaffold materials are good biodegradability and biocompatibility. The tissue material and scaffold reaction enhance the tissue regeneration, and the hydrogels speed up this process 17. The scaffold should create an interconnected network with porosity required for cell growth, metabolic waste, and transport of nutrients. The mechanical properties, structural features, and other properties of scaffolds should match the tissues required to be implanted 18. The scaffolds can be classified according to the technique used to form them in indirect cells. These are (a) inkjet-based, (b) extrusion-based, (c) laser-assisted, and (d) microvalve-based. Microvalve-based printing is a type of drop-on-demand bioprinting. It has high throughput compared to inkjet bioprinting 19. When cells are directly seeded into 3D-printed scaffolds, it causes a low inoculation rate and increases cell distribution. To resolve the problem, the cell mixture was encapsulated into scaffolds composed of another gel type. It provides better shape and strength for scaffolds. Researchers encapsulated cells directly into the hydrogels for better printing cells/gel results 20. Using bioprinting, Xue et al. manufactured scaffolds using human dental pulp cells with alginate/gelatin gel. It controlled the cell density and adherence of highly viable cells with the 3D network 2. Researchers are developing these techniques to help in minimally invasive surgery. The enhancement of the mechanical properties of the scaffold is also an essential parameter of concern.

The utilization of engineering and medical principles for tissue regeneration is known as tissue engineering. The living cells were produced in vitro environments on specific biomaterials. Then, these were placed in the damaged area, the artificial 3D tissues and organs were produced, and the injured tissues were replaced. Scaffold materials were used in tissue engineering to move cells to specific locations. The newly formed tissues get structural support from them. 3DP provides controlled pore parameters required for a scaffold. The stem cells were also used for the regeneration of the tissues. As a host cell, they renovate themselves. Their growth provides one or more cells in the tissue. The cell renewal process provides a repairing action during an injury or illness. This technique is helpful in deadly diseases like cancer 21.

A literature survey shows that stem cells and tissue engineering techniques show promising results. Researchers have developed a cross-linkable gelatin hydrogel that permits extrusion-based AM into the porous scaffolds 22. The Electrohydrodynamic 3D bioprinting approach is suitable for generating bacterial cellulose /polycaprolactone composite scaffolds with high biocompatibility for tissue engineering 23. They discussed the role of this application in spinal cord injury treatment. Deo et al. 24 evaluated the criteria for designing the bioink used in fabricating complex structures. They have discussed the effect of different processing factors on the biochemical and biophysical properties of bioinks. Jaidev et al. 25 presented a surface modification approach that increases the scaffold's bioactivity for bone tissue regeneration. Jang et al. 26 studied the role and future scope of 3DPT in the background of stem cell fabrications. They identified thirteen different applications of stem cells with 3DPT. Madrid et al. 27 discussed the current state-of-the-art 3DPT for bone tissue engineering-based scaffolds, whereas Ozbolat et al. 28 have suggested the need for tumor tissue models and their placement in bioprinted tissues to study their growth. According to Parak et al., the main limitation of 3D bioprinting technology for tissue engineering is the shortage of bioinks with the required properties 29. They described and critically compared the stated functionalization methods, focusing on their effects on bioinks. Tasmin et al. 30 have discussed examples of 3D-printed stem cell tissues such as blood vessels, cardiac tissue, adipose tissue, heart valves, cartilage, neural tissue, pancreas, and liver muscle. Wu et al. 31 reviewed the current techniques for fabricating neural tissue engineering scaffolds.

Organ transplant is one of the main requirements to save the life of patients. However, it depends on the availability of the donors, who, in most cases, cannot fulfil the urgent organ demand. The 3DPT can meet this extra demand. The 3DP technique produces tissues such as multilayer heart structure, bone structure, and others. However, when more complex tissues are produced, several difficulties also arise. The organs printed with this technology are the liver, skin, cartilage, bone tissues, and heart 32. According to Bozkurt et al., an organ can be printed with a 3D printer, but the organ's functionality is challenging to implement 21. Moreover, the reason is an assembly of vascular structures. Javed et al. identified eight significant AM tissue and organ printing advancements. However, bioprinting-based organs are not as efficient as normal organs for implants.

Nevertheless, different research teams are working in this direction to make it possible in the future, and these organs are very much used for preoperative planning. Sirota et al. provided a strategy to overcome the difficulties arising due to a lack of integration of electronics and biology 33. They generated a 3D-printed bionic ear 34. A cell-seeded hydrogel matrix is 3D printed in anatomical human ear geometry. Park and Agarwal et al. discussed the advancements and challenges of 3D-printed tissue and organ technology 35. Mironov et al. suggested a fully integrated organ bio-fabrication line to develop organ printing technology commercially 36. Fedorovich et al. have discussed the role of 3D organ printing in bone regeneration. The present bone size that is possible to repair with this technology is small, but larger bones can be fixed in the future. According to them, the critical factors are vascularization, architecture that supports good bone growth, and stem cell osteogenic differentiation 37. Zhang et al. reviewed the 3DPT for organ printing with the latest advancements, challenges, and their use for toxin testing and drug delivery 3. Fan et al. highlighted that 3D-printed anatomical models would be helpful for surgeons to understand tumor and kidney anatomy better 38.

A medical device is a tool, apparatus, or other article used to diagnose, prevent, monitor, treat, or alleviate a human disease. It can include components, parts, or accessories. Conventional methods like casting and injection molding were used to make medical devices. However, several difficulties arise in this process. The use of 3DP techniques provides several advantages compared to conventional methods. The devices having complicated shapes can be efficiently designed and fabricated with the help of the 3DP technique in a lesser amount of time. The making of patient-specific medical devices is another advantage of using 3DP. An example is the hearing aid designed according to the patient's specific ear size. This technique is also suitable for making patient-specific eye lenses, stethoscopes, and others 28. Figure 2 shows the 3D-printed prostheses and orthosis devices.

Persons with disabilities (due to old age or an accident) require assistive devices. These are known as orthoses and prostheses. According to the World Health Organization report, more than 35 million people around the globe will require orthotics and prosthetic services soon 41. Furthermore, the demand increases rapidly due to the ageing population and vascular-related diseases 42. These devices improve the quality of the life of the patients. A prosthesis is a device that replaces a missing or damaged body part. A prosthesis can be limbs, feet, toes, fingers, artificial eyeballs, hearing aids, ears, noses, and others 43. Earlier casting methods were used to manufacture prostheses, but 3DP methods are becoming popular. Manero et al. reviewed the role of 3DPT in the field of prosthetics. According to their findings, persons with a difference in limb length generally have low self-esteem, and 3DP-based prosthetics improved their confidence. The prime benefit of 3DP is rapid prototyping with desired mechanical characteristics according to a specific patient. However, some limitations are associated with the prostheses. They have more intricate textures than the original skin, are non-adjustable, and have more weight 21.

The permanent prostheses (hip, knee, dental, cranial) and temporary prostheses (pins, plates, rods, and screws for the fixation of bone fractures) generally use metallic implants like Tantalum, gold, stainless steel, titanium, shape memory, and Co-Cr alloy. One major issue with these prostheses is the proper matching of strength and elastic modulus with bone tissues. In addition, there is a stress shielding effect for poor matching characteristics and prosthetic loosening. Polyetheretherketone (PEEK) with reinforced carbon fibre is also used for artificial hip joint prostheses 44. Birbara et al. suggested using prostheses for cardiovascular medicine to replace heart valves 45. Weber et al. reviewed the current neural technologies to activate sensory information in prosthetic devices 46.

The function of orthoses is to modify and support the features of human musculoskeletal and structural systems. Orthoses provide the biomechanical needs of the patients. It helps maintain body alignment, restore mobility, and perform other tasks. The main advantage of 3D printed orthoses is their relatively lesser time requirement than conventional techniques. Hasibuzzaman et al. reviewed the current literature on 3DP for the orthosis of different body parts. According to them, there are different types of orthoses available today: upper limb, lower limb, spinal, and some others. Upper limb orthosis is based on the arm and wrist/hand regions. Lower limb orthosis is based on knee orthosis brace, foot, ankle-foot, and hip regions. The types of spinal orthosis are based on the spine, cervical, and thoracic regions 47. Choo et al. performed a meta-analysis and investigated the efficacy of ankle-foot orthosis to improve gait parameters such as mobility and walking speed in patients with a stroke and gait disturbance 48. Quaresma et al. presented a support tool to aid the therapists/physicians decide the type of orthosis suitable for a patient. They developed a platform called OrthoRehab and tested it for different conditions 49.

We thank the Vignan's Foundation for Science, Technology & Research, Guntur for providing the infrastructure support for making the review paper.