Urinary Bladder vs Gastrointestinal Tissue: A Comparative Study of Their Biomechanical Properties for Urinary Tract Reconstruction

OBJECTIVE

To evaluate the mechanical properties of gastrointestinal (GI) tissue segments and to compare them with the urinary bladder for urinary tract reconstruction.

METHODS

Urinary bladders and GI tissue segments were sourced from porcine models (n = 6, 7 months old [5 male; 1 female]). Uniaxial planar tension tests were performed on bladder tissue, and Cauchy stress-stretch ratio responses were compared with stomach, jejunum, ileum, and colonic GI tissue.

RESULTS

The biomechanical properties of the bladder differed significantly from jejunum, ileum, and colonic GI tissue. Young modulus (kPa-measure of stiffness) of the GI tissue segments was on average 3.07-fold (±0.21 standard error) higher than bladder tissue (P < .01), and the strain at Cauchy stress of 50 kPa for bladder tissues was on average 2.27-fold (±0.20) higher than GI tissues. There were no significant differences between the averaged stretch ratio and Young modulus of the horizontal and vertical directions of bladder tissue (315.05 ± 49.64 kPa and 283.62 ± 57.04, respectively, P = .42). However, stomach tissues were 1.09- (±0.17) and 0.85- (±0.03) fold greater than bladder tissues for Young modulus and strain at 50 kPa, respectively.

CONCLUSION

An ideal urinary bladder replacement biomaterial should demonstrate mechanical equivalence to native tissue. Our findings demonstrate that GI tissue does not meet these mechanical requirements. Knowledge on the biomechanical properties of bladder and GI tissue may improve development opportunities for more suitable urologic reconstructive biomaterials.

On the effect of computed tomography resolution to distinguish between abdominal aortic aneurysm wall tissue and calcification: A proof of concept

PURPOSE

The purpose of this study is to determine the optimal target CT spatial resolution for accurately imaging abdominal aortic aneurysm (AAA) wall characteristics, distinguishing between tissue and calcification components, for an accurate assessment of rupture risk.

MATERIALS AND METHODS

Ruptured and non-ruptured AAA-wall samples were acquired from eight patients undergoing open surgical aneurysm repair upon institutional review board approval and informed consent was obtained from all patients. Physical measurements of AAA-wall cross-section were made using scanning electron microscopy. Samples were scanned using high resolution micro-CT scanning. A resolution range of 15.5-155μm was used to quantify the influence of decreasing resolution on wall area measurements, in terms of tissue and calcification. A statistical comparison between the reference resolution (15.5μm) and multi-detector CT resolution (744μm) was also made.

RESULTS

Electron microscopy examination of ruptured AAAs revealed extremely thin outer tissue structure <200μm in radial distribution which is supporting the aneurysm wall along with large areas of adjacent medial calcifications far greater in area than the tissue layer. The spatial resolution of 155μm is a significant predictor of the reference AAA-wall tissue and calcification area measurements (r=0.850; p<0.001; r=0.999; p<0.001 respectively). The tissue and calcification area at 155μm is correct within 8.8%±1.86 and 26.13%±9.40 respectively with sensitivity of 87.17% when compared to the reference.

CONCLUSION

The inclusion of AAA-wall measurements, through the use of high resolution-CT will elucidate the variations in AAA-wall tissue and calcification distributions across the wall which may help to leverage an improved assessment of AAA rupture risk.

Tissue engineered extracellular matrices (ECMs) in urology: Evolution and future directions

Autologous gastrointestinal tissue has remained the gold-standard reconstructive biomaterial in urology for >100 years. Mucus-secreting epithelium is associated with lifelong metabolic and neuromechanical complications when implanted into the urinary tract. Therefore, the availability of biocompatible tissue-engineered biomaterials such as extracellular matrix (ECM) scaffolds may provide an attractive alternative for urologists. ECMs are decellularised, biodegradable membranes that have shown promise for repairing defective urinary tract segments in vitro and in vivo by inducing a host-derived tissue remodelling response after implantation. In urology, porcine small intestinal submucosa (SIS) and porcine urinary bladder matrix (UBM) are commonly selected as ECMs for tissue regeneration. Both ECMs support ingrowth of native tissue and differentiation of multi-layered urothelial and smooth muscle cells layers while providing mechanical support in vivo. In their native acellular state, ECM scaffolds can repair small urinary tract defects. Larger urinary tract segments can be repaired when ECMs are manipulated by seeding them with various cell types prior to in vivo implantation. In the present review, we evaluate and summarise the clinical potential of tissue engineered ECMs in reconstructive urology with emphasis on their long-term outcomes in urological clinical trials.

Towards the characterisation of carotid plaque tissue toughness: Linking mechanical properties to plaque composition

The morphological manifestation of calcification within an atherosclerotic plaque is diverse and the response to cutting balloon angioplasty remains an elusive target to predict in the presence of extensive calcification. This study examines the resistance of plaque tissue to blade penetration by characterising the underlying toughness properties and stratifying the upper and lower scale toughness limits based on the strong mechanical influence of calcification. Mechanical toughness properties of the common, bifurcation and internal carotid artery (n=62) were determined using guillotine-cutting tests measuring the energy required to pass a surgical blade through a unit length of plaque tissue. The corresponding structural composition of the dissected plaque segments was characterised using Fourier transform infrared analysis, electron microscopy and energy dispersive x-ray spectroscopy. Mechanical results reveal a clear distinction in toughness properties within each region of the carotid vessel with significantly tougher properties localised in the bifurcation (p=0.004) and internal region (p=0.0003) compared to the common. The severity of the intra-plaque variance is highest in plaques with high toughness localised in the bifurcation region (p<0.05). Structural examination reveals that the diverse mechanical influence of the level of calcification present is characteristic of specific regions within the carotid plaque. The energy required to overcome the calcific resistance and propagate a controlled cut in the calcified tissue at each region varies further with the degree of plaque progression. The identification of the localised calcification characteristics is a key determinant in achieving successful dissection of the severely toughened plaque segments during cutting balloon angioplasty.

STATEMENT OF SIGNIFICANCE

Calcification plays a fundamental role in plaque tissue mechanics and demonstrates a diverse range of material moduli properties. This work addresses the characterisation of the toughness properties in human carotid plaque tissue using a fracture mechanics approach. Toughness determines the energy required to propagate a controlled cut in the plaque material. This parameter is crucial for predicting the cutting forces required during endovascular cutting balloon angioplasty intervention. Results demonstrate that a strong relationship exists between the structural calcification configurations, fracture mechanisms and associated toughness properties that are characteristic of specific regions within the carotid artery plaque. The identification of the morphological characteristics of localised calcification may serve as a valuable quantitative measure for cutting balloon angioplasty treatment.

Uniaxial tensile testing approaches for characterisation of atherosclerotic plaques

The pathological changes associated with the development of atherosclerotic plaques within arterial vessels result in significant alterations to the mechanical properties of the diseased arterial wall. There are several methods available to characterise the mechanical behaviour of atherosclerotic plaque tissue, and it is the aim of this paper to review the use of uniaxial mechanical testing. In the case of atherosclerotic plaques, there are nine studies that employ uniaxial testing to characterise mechanical behaviour. A primary concern regarding this limited cohort of published studies is the wide range of testing techniques that are employed. These differing techniques have resulted in a large variance in the reported data making comparison of the mechanical behaviour of plaques from different vasculatures, and even the same vasculature, difficult and sometimes impossible. In order to address this issue, this paper proposes a more standardised protocol for uniaxial testing of diseased arterial tissue that allows for better comparisons and firmer conclusions to be drawn between studies. To develop such a protocol, this paper reviews the acquisition and storage of the tissue, the testing approaches, the post-processing techniques and the stress-strain measures employed by each of the nine studies. Future trends are also outlined to establish the role that uniaxial testing can play in the future of arterial plaque mechanical characterisation.