The reconstruction of the lower urinary tract is one of the most delicate challenges in regenerative surgery. The bladder is not a simple reservoir but a dynamic organ that must store urine at low pressures, preserve renal function and ensure coordinated voiding through a stratified wall in which urothelium, smooth muscle, vascularisation and innervation work in synchrony. When this architecture is compromised by congenital conditions such as bladder exstrophy, by neurogenic bladders, by oncological sequelae or by severe urethral strictures, restoring functional tissue becomes a problem that medicine still addresses with imperfect tools. It is on this ground that silk fibroin has established itself in recent years as one of the most studied and most promising biomaterials.
The surgical standard and its limits
The clinical reference for augmenting a poorly compliant bladder remains enterocystoplasty, that is the use of autologous gastrointestinal segments to increase the capacity of the organ. For extensive urethral strictures the equivalent is onlay urethroplasty with autologous tissue. Both procedures work, yet they carry a set of complications that arises precisely from the nature of the transplanted tissue. The intestinal epithelium continues to behave like intestine even after implantation, and from this stem mucus production, metabolic imbalances, an increased risk of stone formation, recurrent infections and, over the long term, a non-negligible oncological concern. Added to this are the morbidity of the donor site and the limited availability of healthy tissue to harvest. It is within this space of clinical dissatisfaction that research began to look for an off-the-shelf biomaterial, a readily available construct able to guide regeneration without reproducing the shortcomings of autologous tissue.
Why silk fibroin meets the requirements of urinary tissue
Fibroin is the structural protein that forms the core of the thread produced by the Bombyx mori silkworm, once the outer layer of sericin has been removed. Its characteristics make it particularly suited to an environment as hostile as the urinary one. It possesses an uncommon combination of mechanical strength and elasticity, two properties that rarely coexist in biomaterials and that the bladder wall instead requires simultaneously, having to expand and contract thousands of times without failing. Its degradation is slow and tunable, which makes it possible to synchronise the disappearance of the scaffold with the regrowth of host tissue. It is biocompatible and poorly immunogenic, tolerates sterilisation and prolonged exposure to urine, and lends itself to being processed into films, sponges, tubes and porous matrices with controlled geometries. This versatility of processing is what has made it possible to design not a generic material but a structure conceived to mimic the organisation of the bladder wall.
The bilayer architecture as an engineering principle
The design that has dominated urological research on silk is the bilayer scaffold, a double-layered matrix that translates the logic of the native bladder wall into engineering. On one side sits a porous sponge, obtained through solvent-casting and salt-leaching techniques, which offers host cells a three-dimensional framework in which to migrate, proliferate and organise into new muscular and vascular tissue. On the other side a compact, impermeable fibroin film acts as a fluid-dynamic barrier, preventing urine leakage during the early phases of healing and providing the mechanical support needed to maintain organ integrity before the regenerated tissue is able to do so on its own. This dual nature, porous and occlusive at the same time, mirrors the functional separation between the urothelial barrier and the underlying muscular layer, and it is the reason the bilayer matrix has established itself as the reference configuration across much of the research.
From rodents to large animal models
Preclinical validation followed a path of increasing scale, from the small animal to the large model, according to the translational logic that precedes any hypothesis of human use. In murine models of augmentation cystoplasty, scaffolds of fibroin alone proved able to support the regeneration of urothelium and muscle, while also allowing researchers to study how the fabrication parameters of the matrix influence the final outcome. The decisive step came with the porcine model, biologically and dimensionally much closer to humans. By implanting bilayer matrices of six centimetres by six in juvenile swine, the group at Boston Children's Hospital observed within three months the regeneration of vascularised and innervated smooth muscle tissue and of a multilayered urothelium extending across the entire surface of the graft, with structural, mechanical and functional properties comparable to those of native tissue. No less important, the animals showed an increase in bladder capacity and compliance over preoperative values and voluntary voiding, which are the indicators that truly matter when discussing an organ whose function is mechanical before it is biological.
Urethral reconstruction and the comparison with intestinal submucosa
Alongside the bladder, fibroin has been studied for urethral repair, a context in which the impermeable barrier of the film takes on an even more evident value. In a model of ventral onlay urethroplasty in the rabbit, bilayer silk scaffolds were compared directly with small intestinal submucosa, one of the most widely used naturally derived biomaterials. Both supported the formation of smooth muscle bundles and of a neo-urothelium, with evidence of innervation and vascularisation, and in all animals voluntary voiding was maintained with wide urethral calibres and no sign of stricture, fistula or stones. The substantial difference emerged at the level of the immune response, because intestinal submucosa triggered a diffuse chronic inflammatory reaction at the implantation site, whereas silk showed markedly superior biocompatibility. Subsequent studies pushed the model a step further, repairing urethras that had been damaged beforehand in order to approach the real condition of the patient, and even in this more severe scenario the fibroin matrices favoured a constructive tissue remodelling compatible with micturition.
Beyond the healthy bladder, disease models
One of the greatest merits of this line of research is that it did not stop at proof of principle on healthy organs. The clinical relevance of a biomaterial is measured by its ability to function under pathological conditions, and it is here that silk scaffolds have been tested in the most demanding ways. They have been evaluated in porcine bladders subjected to partial bladder outlet obstruction, a condition that reproduces the overactivity and wall remodelling typical of many obstructive disorders, and in models of neurogenic bladder associated with spinal cord injury, where the neural component plays a decisive role in the outcome. Research has also extended beyond the bladder in the strict sense, exploring the use of tubular matrices for ureteral reconstruction in porcine models with temporary stenting and the fabrication of urinary conduits for diversion, in a perspective that looks at the entire urinary tract as a continuum reconstructable with the same biomaterial platform.
Cells, factors and the generation of composite constructs
The first generation of silk scaffolds was deliberately acellular, designed to be immediately available and to entrust regeneration solely to the host's cells, with the pragmatic aim of a ready-to-use product free of the costs and regulatory complexity of cell therapy. More recent research is, however, exploring a complementary direction, in which the matrix becomes the support for a biologically more active construct. Silk fibroin scaffolds seeded with adipose-derived stem cells have appeared, and in models of bladder regeneration they have shown effective integration into the wall of the organ. More sophisticated composite architectures are being developed, such as triple-layered matrices combining a fibroin film, a decellularised amniotic membrane and a silk sponge populated by stem cells encapsulated in collagen hydrogel, conceived to reproduce more faithfully both the mechanical properties of human bladder tissue and the microenvironment required by the different cell populations of the wall. It is a trajectory that shifts attention from the material as such to the engineered construct as a whole, where silk remains the framework but converses with cells, factors and other biological matrices.
Essential references
The foundations of this review rest on a body of literature that is by now well established. The reference review on the subject is that of Sack, Mauney and Estrada on fibroin scaffolds for urological tissue engineering (Current Urology Reports, 2016). On the bladder side, the works of Seth and colleagues on the murine cystoplasty model (Biomaterials, 2013) and of Tu and colleagues on the porcine bladder augmentation model (Biomaterials, 2013) are central. For the urethra, see Chung and colleagues on onlay urethroplasty in the rabbit (PLOS One, 2014) and Algarrahi and colleagues on the repair of previously damaged urethras (Journal of Surgical Research, 2018). The extension to pathological conditions and to other segments of the urinary tract is documented by Affas and colleagues on obstructed porcine bladders (Tissue Engineering Part A, 2019) and by Gundogdu and colleagues on tubular ureteroplasty in swine (Frontiers in Bioengineering and Biotechnology, 2021). Among the most recent contributions on cellularised and composite configurations are the 2025 studies on silk fibroin scaffolds seeded with adipose stem cells and on triple-layered matrices with a decellularised amniotic membrane.
