Biomechanical Properties of Airway Stents: Implications for Clinical Practice

A single-tube-braided stent for various airway structures

Background: Airway stent has been widely used in airway procedures. However, the metallic and silicone tubular stents are not customized designed for individual patients and cannot adapt to complicated obstruction structures. Other customized stents could not adapt to complex airway structures with easy and standardized manufacturing methods.

Object: This study aimed to design a series of novel stents with different shapes which can adapt to various airway structures, such as the “Y” shape structure at the tracheal carina, and to propose a standardized fabrication method to manufacture these customized stents in the same way.

Methods: We proposed a design strategy for the stents with different shapes and introduced a braiding method to prototype six types of single-tube-braided stents. Theoretical model was established to investigate the radial stiffness of the stents and deformation upon compression. We also characterized their mechanical properties by conducting compression tests and water tank tests. Finally, a series of benchtop experiments and ex vivo experiments were conducted to evaluate the functions of the stents.

Results: The theoretical model predicted similar results to the experimental results, and the proposed stents could bear a compression force of 5.79N. The results of water tank tests showed the stent was still functioning even if suffering from continuous water pressure at body temperature for a period of 30 days. The phantoms and ex-vivo experiments demonstrated that the proposed stents adapt well to different airway structures.

Conclusion: Our study offers a new perspective on the design of customized, adaptive, and easy-to-fabricate stents for airway stents which could meet the requirements of various airway illnesses.

Effect of Bronchoscopic Spray Cryotherapy and Ultra-low Temperature on Physical Properties of Metallic and Silicone Airway Stents

BACKGROUND

Bronchoscopic spray cryotherapy (SCT) is utilized in the field of interventional pulmonology for treating benign and malignant airway stenosis as a standard tool to maintain airway patency. Stent-related complications include tumor overgrowth, granulation tissue, and epithelialization. Thermal ablation can have a limited role in such scenarios due to the risk of airway fire and damage to the existing stent. SCT is a potential therapy using ultra-low temperatures that can allow stents to remain in place during treatment. However, there has been no study demonstrating the safety of SCT on the integrity and physical properties of tracheobronchial stents. We report the results of the first study demonstrating the safety of SCT utilized to treat stent-associated granulation or malignant airway disease. The aim of this study was to demonstrate the effects of SCT on the physical properties of airway stents in an ex vivo environment.

METHODS

Various types of airway stents were subjected to multiple intervals of SCT for up to 30 seconds, and then the cycle was repeated 3 times. After every cycle, we compressed the stents to 60% of their original size, and compression and expansion force data was collected immediately after, at 3-minute and 5-minute intervals, and compared with the baseline readings.

RESULTS

There was no significant change in the association between diameter and compression/expansion force, including any derangement in returning to the original diameter or any physical damage to any of the stents even after 3 prolonged SCT sessions of 30 seconds.

CONCLUSION

Our study provides the first evidence that the use of SCT in conjunction with existing silicone/metal stents is feasible and does not cause any physical damage to the stents or alters their ability to maintain the original diameter.

In Vivo Molding of Airway Stents

Like ready-to-wear clothing, medical devices come in a fixed set of sizes. While this may accommodate a large fraction of the patient population, others must either experience suboptimal results due to poor sizing or must do without the device. Although techniques have been proposed to fabricate patient-specific devices in advance of a procedure, this process is expensive and time consuming. An alternative solution that provides every patient with a tailored fit is to create devices that can be customized to the patient's anatomy as they are delivered. This paper reports an in vivo molding process in which a soft flexible photocurable stent is delivered into the trachea or bronchi over a UV-transparent balloon. The balloon is expanded such that the stent conforms to the varying cross-sectional shape of the airways. UV light is then delivered through the balloon curing the stent into its expanded conformal shape. The potential of this method is demonstrated using phantom, ex vivo and in vivo experiments. This approach can produce stents providing equivalent airway support to those made from standard materials while providing a customized fit.

3D printing for airway disease

It has been 30 years since the first commercial three-dimensional (3D) printer was available on market. The technological advancement of 3D printing has far exceeded its implementation in medicine. The application of 3D printing technology has the potential of playing a major role within interventional pulmonology; specifically, in the management of complex airway disease. Tailoring management to the patient-specific anatomical malformation caused by benign or malignant disease is a major challenge faced by interventional pulmonologists. Such cases often require adjunctive therapeutic procedures with thermal therapies followed by dilation and airway stenting to maintain the patency of the airway. Airway-stent size matching is one key to reducing stent-related complications. A major barrier to matching is the expansion of the stent in two dimensions (fixed sizes in length and diameter) within the deformed airway. Additional challenges are created by the subjective oversizing of the stent to reduce the likelihood of migration. Improper sizing adversely affects the stability of the stent. The stent-airway mismatch can be complicated by airway erosion, perforation, or the formation of granulation tissue. Stents can migrate, fracture, obstruct, or become infected. The use of patient-specific 3D printed airway stents may be able to reduce the stent airway mismatch. These stents allow more precise stent-airway sizing and minimizes high-pressure points on distorted airway anatomy. In theory, this should reduce the incidence of the well-known complications of factory manufactured stents. In this article, the authors present the brief history of 3D printed stents, their consideration in select patients, processing steps for development, and future direction.

A new structure-property connection in the skeletal elements of the marine sponge Tethya aurantia that guards against buckling instability

We identify a new structure-property connection in the skeletal elements of the marine sponge Tethya aurantia. The skeletal elements, known as spicules, are millimeter-long, axisymmetric, silica rods that are tapered along their lengths. Mechanical designs in other structural biomaterials, such as nacre and bone, have been studied primarily for their benefits to toughness properties. The structure-property connection we identify, however, falls in the entirely new category of buckling resistance. We use computational mechanics calculations and information about the spicules' arrangement within the sponge to develop a structural mechanics model for the spicules. We use our structural mechanics model along with measurements of the spicules' shape to estimate the load they can transmit before buckling. Compared to a cylinder with the same length and volume, we predict that the spicules' shape enhances this critical load by up to 30%. We also find that the spicules' shape is close to the shape of the column that is optimized to transmit the largest load before buckling. In man-made structures, many strategies are used to prevent buckling. We find, however, that the spicules use a completely new strategy. We hope our discussion will generate a greater appreciation for nature's ability to produce beneficial designs.