Optimizing the Next Generation of Vascular Implants with Biomaterial Coverings

Vascular intervention is evolving at a dynamic pace, with invasive open surgical approaches increasingly being avoided, when possible, in favor of transcatheter intervention and minimally invasive implant delivery.

The vast majority of vascular implants are developed either on balloon-expandable or self-expandable metal frames, with the most commonplace of these being the stent. In order to treat a greater range of indications, however, metal-based vascular implants are being integrated with an increasingly diverse range of biomaterials. Textile-covered endovascular grafts, pericardium valves for aortic valve replacement and polytetrafluoroethylene (PTFE)-covered stents, are just some of the biomaterials growing in prominence within minimally invasive vascular intervention.

As this market continues to evolve, next-generation devices are being modified to lower their profile, reduce manufacturing costs, and improve efficacy and durability. To meet this demand, new synthetic and resorbable biomaterials, as well as functional coatings, are being introduced that offer more effective solutions for an increasing range of potential applications.

Biomaterial Coverings
Biomaterial coverings are used to perform a wide variety of functions ranging from occlusion, filtration, flow diversion and inhibiting restenosis to containing tumor growth, maintaining vessel integrity and supporting heart function. The application of these devices now has evolved beyond traditional coronary and peripheral indications to include endovascular, structural heart, neurovascular, carotid, below the ankle and even venous indications. Similar devices also fulfill solutions outside of vascular, within pulmonary, gastrointestinal and urinary fields of use. Despite the wide range of device applications, biomaterial coverings typically function as porous or nonporous barriers. There are pros and cons attributed to each biomaterial covering option, which determines their suitability for a given application. New innovations, however, are signaling the direction of future product development.

Textile Biomaterial Solutions
Textile fabrics are among the most well-established biomaterial implant coverings—used for many years in grafts and valves. Such devices traditionally were used in open surgery, before being modified for transcatheter delivery. Fabric is extremely versatile and can be formed around diverse implant designs while being thin and flexible enough to deliver through a catheter. Polyethylene terephthalate fiber is the most common material used in textile vascular applications due to its proven biocompatibility, inherent hydrophobicity and established clinical history. The fabric  typically is woven to ensure shape stability, conformability and durability, while minimizing blood permeability.

Over the years, advances in high-density woven endografts have helped to reduce type IV endoleaks, resulting from low-level porosity. (Editor’s note: According to the Cleveland Clinic, endoleaks often occur during endovascular aneurism [EVAR] repair. During EVAR, a fabric-covered stent graft is put in place to reinforce the areas of the aorta that are weak due to an aneurysm. The stent provides a new path for blood flow, which keeps blood from reaching the aneurysm. An endoleak is a complication that affects about 15-25 percent of patients who have EVAR. It means that some amount of blood flow still remains in the aneurysm cavity. A type IV leak happens through graft fabric as a result of graft porosity.) In addition, higher-strength sutures—using material such as ultra-high-molecular-weight polyethylene—allow for lower profile suturing, while supporting the introduction of increasingly lower profile and durable graft systems. Although mechanical fixation and bonding can offer support, suturing remains the primary mode to affix fabric to vascular implants. Unfortunately this process requires highly skilled technicians and is extremely time consuming. As a result, many companies have sought to overcome this challenge with the use of alternative biomaterial solutions. A large number of companies within the endovascular space are using PTFE. The completely hydrophobic nature of the material and how the fabric is formed help to reduce porosity and subsequent risk of endoleaks, while the material can be applied to a metal frame without the need for suturing. In addition, novel solutions using injectable biomaterials and foams are helping to overcome other forms of endoleak.

Synthetic Biomaterial Coverings
For several decades synthetic biomaterials have been used to cover stents, primarily to treat nonvascular indications such as biliary, esophageal and ureteral strictures. Polyurethane and silicone are the most common materials used for these applications. Certain limitations, however, were evident in early versions of these covered stents. There were questions over mechanical performance, including strength, durability and deliverability, as well as the adherence of proteins and formation of biofilms after implantation.

Polymeric coatings and coverings have advanced significantly in recent years, fueled in part by the interest in improving performance of drug-eluting stents. Novel material grades, combined with advanced processing capabilities, can now facilitate micro-thin polymer layers, added to stents as a covering or applied only to the stent struts. Robust polymer coverings can be applied from 20-micron wall thickness, with additional coatings applied to enhance biocompatibility as well as functional performance. Due to these advancements, synthetic biomaterials, including grades of polyurethane and silicone, are being reconsidered as implant coverings or membranes in the vascular market, offering low-profile solutions in areas such as carotid stenting, thrombus retrieval, occlusion and filtration.

As mentioned, PTFE recently has been employed to cover endovascular stent grafts, and also is well established as a biomaterial stent covering for arteriovenous access treatment and renal stenting. New advances in material and processing technology are leading to a greater adoption of this material in peripheral vascular indications, as evident in the launch of the Viabahn stent made by W.L. Gore & Associates to treat stenosis of the superior femoral artery. PTFE offers a versatile covering option for balloon and self-expanding stent systems. It is likely we will see greater proliferation of its use, not only in the peripheral vascular market, but potentially in structural heart applications.

Resorbable Biomaterial Coverings
There is a noticeable trend in the vascular market toward the development of resorbable biomaterial solutions, reflecting a similar pattern in other treatment areas such as sports medicine and general surgery. Resorbable polymeric coverings now are widely used on drug-eluting stents, such as Boston Scientific Corp.’s Synergy BP-DES stent, resulting in a bare metal stent remaining after the drug has been released. The same principle can apply for stents and other implantable devices covered or integrated with resorbable materials. Following the progress of the Absorb stent by Abbott Laboratories Inc., a considerable number of companies are developing resorbable stent scaffolds with performance improvements focused on thinner struts, more comparable to metal stent dimensions. Introducing implants into the vascular system that disappear once its functional use is obsolete undoubtedly is a desirable outcome, and resorbable materials are being considered more than ever for a diverse range of next generation vascular devices. Indications such as trauma or perforations would benefit from resorbable coverings or resorbable implantable solutions, while devices required for short-term filtration and occlusion could resorb into the body without requiring subsequent removal. It is highly likely that we will see the proliferation of these devices in the years to come.

Surface Modification
In order to overcome the shortcomings of certain biomaterials and improve performance, functional coatings often are employed. Such coatings modify the surface characteristics of the implant. Typical examples include lubricious hydrophilic coatings or hydrophobic coatings to facilitate implant delivery, or antimicrobial coatings to reduce risk of infection. Other “polymer-free” coatings can make an implant more biocompatible, while more diverse coatings can be used to reduce the porosity of biomaterial-covered implants and membranes. Recent advances in processing capability also can facilitate surface modification by mechanical means, to offer textured surfaces or integrate a specific pore structure.

Future Implant Design
Advancements in materials and conversion processing techniques are resulting in more sophisticated vascular implantable solutions. New material grades and functional coatings are leading to more effective implants in terms of treatment, biocompatibility and delivery. As we look to the future, resorbable biomaterials also are going to play a greater role in vascular implants, either as coatings, components or finished devices. Innovative processing techniques are facilitating more effective material integration, while delivering solutions in a cost effective manner. As a result, next-generation devices are being developed with lower profiles, while maintaining—if not increasing—durability. Taking all of this into account, we are entering an exciting era of prolific innovation for vascular implantable solutions, making it possible to effectively treat more patients, for an even greater range of indications. 

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Stephen Duffy is the business development manager of Proxy Biomedical Ltd., an Irish company that specializes in biomaterial solutions for medical implantable products, with a proven track record in innovative design and quality-assured manufacturing. Proxy is a supplier of biomaterial coverings for stents and other implants, using materials such PTFE, polyurethane, silicone and resorbable biomaterials, and also provides comprehensive textile conversion capabilities for medical implantable products.