In this new era of technological advancement, three-dimensional (3D) printing has emerged in medicine, promising to revolutionize surgical practices. Three-dimensional printing could be defined as “translating” a digital image into a 3D solid object by printing consecutive thin layers of materials. The fusion of tissue engineering and 3D printing has given rise to bioprinting. This technique employs biocompatible printers and "bio-ink" to create intricate tissue structures, while the complete fabrication of functional organs remains a research objective. 3D bioprinting has already shown promising results, especially in the field of microfluidic devices with the development of tissues demonstrating proximal tubules, glomerulus, and tubuloinerstitium functions. Such models could be applied in renal disease modeling and during drug development for nephrotoxicity investigation.
1. Introduction
Renal transplantation constitutes the most commonly performed solid organ transplantation. Specifically, the Global Observatory on Donation and Transplantation estimated there were 80,926 renal transplantations (32% from living donors) conducted in 2020, accounting for 62.4% of global transplantation activity
[1]. For patients with end-stage kidney disease (ESKD), renal transplantation with a living or deceased donor transplant remains the treatment of choice when compared with peritoneal dialysis or hemodialysis since it provides substantially greater quality of life and is associated with lower long-term morbidity and mortality
[2]. Nevertheless, renal transplantation is still associated with various postoperative complications, including urological complications (urine leak and urinary obstruction), peritransplant fluid collections (hematomas, lymphoceles, urinomas, and abscesses), vascular complications (renal artery stenosis, renal artery thrombosis, arteriovenous fistulas and pseudoaneurysms, renal vein thrombosis), calculous disease, neoplasms, gastrointestinal complications, and herniation complications
[3]. The introduction of novel technologies and the improvements in medical imaging and surgical techniques have significantly lowered the prevalence of these complications, ameliorating their negative impact on the surgical outcome.
In this new era of technological advancement, three-dimensional (3D) printing has emerged in medicine, promising to revolutionize surgical practices. Three-dimensional printing could be defined as “translating” a digital image into a 3D solid object by printing consecutive thin layers of materials
[4]. Originally, 3D printing materialized in non-medical disciplines to serve the pressing demands of rapid engineering of prototypes. However, it has since expanded to other disciplines, including surgery, where 3D printing has been used for educational purposes to facilitate the comprehension of complex anatomy, for preoperative planning, and particularly for operations involving complex vasculature, for crafting customized surgical tools, and for patient counseling
[5][6].
Despite the expansion of selection criteria, including “marginal” renal grafts from substandard donors, renal transplantation is limited by the shortage of transplants
[7]. Specifically, in the US, only 25% of the waitlisted patients receive a transplant within five years, with patients being removed from the list due to deterioration of health or premature death
[7]. Thus, the lack of donors worsens the already vast healthcare burden associated with ESKD patients on dialysis. Therefore, justifiably, kidney regeneration has been a long-standing challenge for tissue engineering. The fusion of tissue engineering and 3D printing has given rise to bioprinting
[8]. This technique employs biocompatible printers and “bio-ink” to create intricate tissue structures, while the complete fabrication of functional organs remains a research objective. Bioprinting achieves the fabrication of structures of precise internal and external architecture that provide high cell viability and imitation of natural tissue features (biomimicry)
[9][10].
2. 3D Bioprinting Is Employed in Renal Regenerative Medicine
Renal transplantation, despite being the gold standard, intriguingly it is also a halfway measure since it does not address the underlying disease while, at the same time, it does not cure the patient rather than transforming and lessening the morbidity from that of chronic dialysis to the morbidity of long-term immunosuppression therapies. Except for leaving patients vulnerable to opportunistic infections and predisposing to malignancy development, immunosuppression therapy is a main alloantigen-independent factor in renal chronic allograft nephropathy
[11][12]. Up to 50% of kidney transplanted patients lose the graft due to chronic allograft nephropathy within ten years from transplantation
[13]. Kidneys are particularly complex organs with more than 20 different cell types
[14]. Bioprinting a kidney whose cell lines retain their viability and functionality long-term is a herculean task. The 3D bioprinting approach holds potential due to its ability to achieve detailed structures, which may lead to better biomimicry
[15]. For organ 3D bioprinting, two different strategies have emerged: scaffold-based and scaffold-free
[16]. While revolutionary, 3D bioprinting is still in its foundational stages, especially concerning the production of complex structures. Some of the primary challenges include ensuring vascularization, creating a functional nephron unit, and addressing the intricate balance of cellular interactions. Additionally, the issue of scalability and reproducibility across different bioprinting platforms poses significant hurdles. Currently, the field is seeing advancements primarily in microfluidic device development that demonstrate renal function, which represents a more immediate and tangible step towards replicating kidney function. In this section, the related literature where 3D bioprinting is employed in developing renal structures has been identified and presented.
Table 1 summarizes the identified studies where 3D bioprinting was employed in the development of renal cultures/tissues. The first study utilizing 3D bioprinting to develop convoluted renal proximal tubules in vitro was published in 2016
[17]. Homan KA Et al. developed perfusable microfluidic-based chips that housed renal proximal tubules that were fully embedded in an extracellular matrix
[17]. The proximal tubules were characterized by an open-lumen architecture, which was circumscribed by proximal tubule epithelial cells that maintained cell viability and functionality for over two months
[17]. During printing, a fugitive ink (containing a triblock copolymer of polyethylene-polypropylene-polyethylene and thrombin) was used that was then removed before cell seeding. Gene expression analysis of 33 key proximal tubule epithelial cells genes revealed cells that these cells were transcriptionally similar to primary renal proximal tubule epithelial cells
[17]. Finally, the researchers demonstrated how their model could be used to investigate drug-induced tubule damage mechanisms by successfully inducing dose-dependent tubular damage using cyclosporine A
[17]. Notably, their model lacked vasculature, limiting its application in renal reabsorption studies. In 2019, researchers from the same department published a study aiming to develop a 3D bioprinted a microfluidic-based vascularized proximal tubules model, embedded in extracellular matrix, to investigate the reabsorption of solutes via tubular-vascular exchange
[18]. Notably, the markers observed confirmed the presence of endothelial tissue and the perfused model demonstrated active reabsorption of albumin and glucose
[18]. Additionally, the researchers explored the role of the model in disease modeling by inducing hyperglycemic conditions and monitoring endothelial cell dysfunction
[18].
Table 1. Studies developing 3D-bioprinted renal models.