The cardiovascular device market is growing, with research forecasting that the cardiac implant medical device market alone will exceed $27 billion by the year 2017, according to the May 2013 Renub Research study “Global Cardiac Implant Devices Market Forecast, Clinical Trials, M&A”. There is tremendous opportunity in this industry, but with it comes intensifying pressure on device OEMs to innovate on the traditional implant model. As a result, engineers are increasingly required to explore new and different solutions to achieve in vivo drug delivery, implant success, fewer revisions, and improved patient outcomes.
Biomaterials for Cardiovascular Applications
As device OEMs look to the next phase of performance, fibers have the capability to deliver the same level of stability as traditional metals and ceramics with the more biomimetic form of natural materials. From traditional textile fibers such as nylon, polyethylene terephthalate (PET), and polyether ketone (PEEK) to breakthrough composites, biomaterials are evolving and have the potential to revolutionize medical device capabilities.
To aid in both support and repair, as well as in the regrowth of natural cells within the body for recovery from conditions affecting the heart and circulatory system, high-density, non-stretch biomaterials have become increasingly important to device design.
There are a multitude of cardiovascular applications that can be well supported by these knitted and woven fabrics, as well as by non-woven felt scaffolds. Examples of applications include: cardiovascular grafts; heart valve repair; annuloplasty rings; containment devices; tethers; pledgets (absorbent pads); hemostasis applications; vascular repair/replacement conduits; correcting and/or creating occlusions; stent support products; as well as a multitude of combination products.
Textile Engineering for Clinical Precision
Specialized textile engineering techniques enable device developers to capitalize on the unique properties of different materials. Processes such as braiding, weaving, knitting, and needle-punching non-wovens help magnify strength, texture, flexibility, and many other performance characteristics for customized device requirements. (See Figure 1) For engineers, the possibilities for improved mechanical performance, anatomic accuracy, and subsequent clinical benefit are limitless.
Braiding is one of the most commonly used processing techniques for creating fabric-based implants. Braided structures are easily customizable, which means tubes, flat braids, and other geometries with very specific dimensions are possible. The braiding process can produce structures with great strength in a small surface area. Thanks to their strength, ability to expand and compress, and the possibility to customize other performance capabilities, braided materials are ideal for many applications.
For cardiovascular applications, such as sewing threads for aortic repair grafts, strength requirements are usually more precise. Small diameter braids are particularly useful for these kinds of applications, because they can be created in a very thin form factor without sacrificing strength. A secure stitch is critical to graft success, which means braids must meet very precise specifications to deliver the necessary performance.
Using Non-Wovens in Regenerative Medicine
The potential for regenerative medicine in cardiovascular device applications has grown exponentially. Replacing cartilage with synthetic materials has been of interest—and practice—for decades, but OEMs are now recognizing the benefits of not only building artificial implants, but also using materials that enable the regrowth of damaged tissue and help the body heal itself. Absorbable fibers, such as polyglycolide (PGA), poly(L-lactic acid) (PLLA), polydioxanone (PDO), and other copolymers, help to aid in repair and regrowth of native tissue before they are absorbed by the body and completely replaced with natural cells. For absorbable applications in particular, correctly selecting the most effective polymer is critical to device performance.
Developers must be able to engineer degradation profile and total lifespan alongside characteristics like strength, abrasion resistance, elongation, and pore size to ensure the successful growth of new tissue with the correct mechanical properties.
To deliver the high degree of performance required for cardiovascular in vivo applications (as well as applications in many other areas of the body), nonwoven absorbable scaffold that combines the benefits of traditional 3D non-woven scaffold technology with advanced manufacturing techniques is an ideal choice. Composed of a variety of synthetic, absorbable polymer fibers, the 3D structure provides a fibrous platform with high surface area and superior void volume to promote natural tissue in-growth and cellular regeneration at the site of surgery or damage. These customized fabrics can be shaped in a variety of geometries, including flat felts, tubes, cuffs, cones, and more. Because they are most commonly composed of absorbable biomaterials that enable regrowth of natural cells, non-woven structures such as tubular conduits are well-suited for vascular replacement technologies and heart valve repair. (See Figure 2)
To successfully deliver a device that performs as intended in the body, consideration of lifespan is essential. Tissue regeneration of a heart valve, for example, will require support for cell regrowth for a specific duration of time in order to ensure optimal healing and the recovery of as natural a function as possible. Wound treatments, on the contrary, must disintegrate more quickly to keep from hindering the growth of new cells on a particular surface. Degradation profiles can range from days to more than a year depending on the polymer type, so engineering to deliver to device specifications is critical for performance.
Non-wovens can be precisely engineered to maintain material integrity for the required life of the device. These have seen their greatest use in tissue engineering applications as absorbable scaffolds, but these kinds of textiles are increasingly being incorporated into cardiovascular devices using fibrous components, such as suture fasteners, pledgets, and a variety of reconstruction procedures throughout the body.
Drug Eluting Technology to Support Cardiovascular Applications
Another approach of significant OEM interest has been to try to enable drug delivery via implants. However, combining a biomimetic form factor and performance has proven to be a significant challenge. Where lifelike biomaterials can provide performance, their drug delivery capabilities have typically come up short. But now, this goal is attainable through groundbreaking new technology options that enable fiber extrusion that preserves the biological activity of drugs and therapeutic agents.
New fiber extrusion technologies enable biodegradable fibers to be loaded with biological and pharmaceutical agents that can then be directly incorporated into implantable medical devices. For the first time, device developers can take advantage of the sophisticated drug delivery capabilities of fibers incorporating high levels of manufacturing tolerance to provide tailored release kinetics for in vivo support and regeneration applications. And, with the right processing techniques, they can engineer textile structures of precise chemical composition and mechanical properties such as size, shape, strength, and porosity to the needs of the specific application or anatomical requirement.
This customized scientific approach enables single fiber-based structures that can satisfy both physical and pharmaceutical performance requirements without requiring additional material support for implantation. Designed to provide drug delivery and aid in repair or regrowth of natural tissues all at once, this capability is particularly important for applications where the strength is critical to performance, such as vascular grafts and stents.
Alongside support and repair functions, textiles can also aid in drug delivery. Today, drug-loaded fibers are a growing delivery approach, and the right processing technique can maximize the biomechanical properties of those fibers to create textile structures with the power to administer drugs in vivo or even aid in healing using an implantable device. Unique new extrusion technologies that preserve the biologic activity of pharmaceutical compounds and other biologic agents under a breadth of conditions are enabling a greater range of drugs to be loaded into fibers than ever before.
The result is the ability to create textile structures composed of these drug-loaded fibers and provide not only a means of drug delivery, but also functional support at the same time. Because implant sites often contain damaged tissue, loaded fibers can also act as a potential healing aid by releasing agents such as thrombin or vascular endothelial growth factors to help heal natural cells even as they provide support or repair functions.
An absorbable textile platform allows for the engineering of both chemical composition and mechanical properties such as size, shape, and porosity to the specific application, which means that a single structure can satisfy both physical and pharmaceutical performance requirements without requiring additional material support for implantation. In these cases, polymer selection is once again critical to successful performance, and engineers must work closely with textile scientists to ensure structural viability before any delivery method is possible. The latest breakthroughs in drug-eluting technology are ideal for:
Covered stents: Development of a drugeluting sleeve for a balloon-expandable cardiac stent. This can be fully expandable with the stent to enable maximum performance in a reduced form factor.
Self-expanding, biodegradable, peripheral stents: Development of a coil-based, drug eluting stent specifically designed for the high mechanical demands of the peripheral vascular system.
Anti-arrhythmia treatment: Development of a new technology called Over- The-Wire extrusion to allow for incorporation of a drug-loaded polymer coating on wires. The ability to apply a thick polymer coating allows for very high drug loads and is designed for long-term delivery where mechanical support is important.
Vascular grafts: Development of vascular grafts that have the ability to release drugs to reduce thrombus formation, encourage endothelial cell coverage, and provide good mechanical support.
The advanced scientific expertise of today’s medical textile developers combined with advancements in biomaterial engineering is presenting today’s OEMs with the opportunity to revolutionize and redefine what medical devices can do for cardiovascular applications. From heart valves to implantable drug delivery, cardiovascular device applications are entering a new phase of development. It is up to the industry to leverage these engineering advancements of medical textiles and drug-eluting technologies for next-generation clinical benefit.
This article was written by Todd Blair, Biomedical Structures, Warwick, RI. For more information, Click Here .