Along with the biocompatibility, a device must be durable enough to withstand the mechanical loads placed upon it. These forces can be exerted upon the device by implantation, cardiac motion, or external events such as getting hit in the chest or falling on the ground (specifically for implantable devices). Potential situations like these must be considered when designing the device, to ensure it has the robustness needed to withstand such environments as well as an active lifestyle.

Major factors that any design engineer must take into consideration are the repetitive movements and strains that will be placed on a device within a patient’s body. If you were to imagine the environment in which a coronary stent is placed, the arteries will have a cyclic pressure placed on them which, in turn, will flex and relax the stent with each heartbeat. This requires the stent’s material to either: (1) oscillate with the artery while being durable enough to withstand flexing for the lifetime of the stent or (2) provide enough opposing force to the artery that it does not move during these times of increased pressure.

Cardiac pacing and defibrillation leads are also exposed to repetitive mechanical stresses that can cause faults or failures in their integrity. Two individual incidents were observed within defibrillation leads from two separate companies—St. Jude Medical (St. Paul, MN, USA) and Medtronic, Inc. (Minneapolis, MN, USA)—that were competing to get a small defibrillation lead on the market. The two companies produced small-diameter leads, the Riata (St. Jude Medical) and Sprint Fidelis (Medtronic, Inc.), that gained FDA approval and were then appropriately implanted clinically [5]. Before the release of these smaller leads, >8F defibrillation leads were the standard of care, but in 2002, St. Jude Medical released their 7.6F Riata defibrillation lead. In response, Medtronic, Inc., released their 6.7F Sprint Fidelis in 2004, shortly followed by the 6.3F Riata ST by St. Jude Medical. The perception was that a smaller lead would decrease the blockage caused within the venous system at the level of the subclavian vein. However, it was found that these systems were unable to perform at adequate levels of durability within the body [5]. Both companies ended up having issues where their leads broke or eroded to the point where the conductor wires could penetrate and cause misreading of the cardiac signals and potentially lead to excessive shocks being delivered to the patient (or even worse, no shocks were delivered when they were needed).

In 2007, Medtronic, Inc., sent a letter to physicians informing them that they had seen failures in the field both at the distal portions of these leads and at points near the anchoring sleeve tie-down. Further, they reported that the distal failures were significantly affected by the bending of the lead body due to tortuous anatomy in the veins. The failures at the tie-down location were potentially caused by the way physicians implanted and secured the leads within the body. Each of these failure modes suggested that the way in which the lead was implanted could make a significant impact on the longevity of these devices and their potential for failure. Ultimately in late 2007, Medtronic, Inc. issued a voluntary market suspension that the FDA formally considered to be a recall. During this time, St. Jude Medical was also seeing similar issues with their leads. They reported that portions of their leads were being eroded to the point where the wires were being externalized in both styles of the Riata leads. It was also reported that the primary reason for this externalization was due to abrasion of the lead insulation with either the anatomy (e.g., tricuspid valve) or the internal components of the lead (e.g., the pacing/sensing wires). In one study, they found that a common defect was due to the internal abrasion of either the pace/sense coils or the shocking coils against the silicone insulation. This would be in response to the repetitive motion of the lead and the ability for the internal components to move relative to the external insulation. This caused what they deemed to be an inside-out abrasion, where these wires were wearing away at the insulation and become externalized and readily visible under fluoroscopy [6]. Due to these failures, St. Jude Medical ended up removing the Riata family of leads from the market in 2010 and recalling them in 2011. Since these recalls, the defibrillator lead market has stayed at >8F, due to the perception that smaller leads could be associated with potential failures in patients.

The ability to put together a device, or manufacturability, is something that should not be overlooked during the design processes. By incorporating early feedback about manufacturing the product, the risks of not being able to build the device efficiently enough or to produce adequate quantities to fulfill demand will be greatly reduced. Failure in either case could cause the product to be stifled, no matter how successful or clinically helpful it is. As such, design for manufacturability (DFM) has become a common practice in the cardiac device industry, a practice that emphasizes how a successful design should ensure the highest-quality products while decreasing manufacturing costs. This is accomplished by making manufacturing cost estimates to proactively guide and prioritize cost reduction efforts involved with design. Consequently, DFM should have significant effects on product lead times, development costs, and ultimate product quality. As such, DFM specifically requires input from a multidisciplinary team, including manufacturing engineers, cost accountants, and production personnel, in addition to the design engineers [7]. Yet, when applying these principles to cardiac device design, it is imperative to understand that the quality of care impacted by the device must not be compromised by the need for a more cost-effective manufacturing process. One needs to consider that production costs can be controlled by using existing technologies and established manufacturing techniques. There are many methods for ensuring that the requirements for manufacturing are being taken into consideration, and there are several numerical methods of design and testing (also known as in silico testing) that can be used to streamline the design for the manufacturing process.

There are often two types of development referred to by designers—iterative and disruptive. Those that fall into the iterative category are devices that generally are perceived as logical next steps in the development chain. For example, this would include the addition of a third and fourth electrode to a left-sided lead that would give the physician more vectors to choose from when programming a pacemaker, as discussed in Chap. 30. The ideas and devices that are considered to be disruptive are those that medical industry professionals would consider as game changers . These are devices like transcatheter-delivered cardiac valves. The option prior to these transcatheter valves was open-heart surgery, which often involved placing the patient on cardiac bypass and fully stopping the heart; this procedure limited the population of patients who could receive the treatment to those that were able to physically undergo an invasive surgery and required weeklong recovery periods in the hospital. In other words, these devices have effectively provided a means to treat more of the patient populations which were underserved by prior medical management.

Although there may be a valid unmet clinical need, oftentimes there are points along the product development pathway that may ultimately put a halt to a device or therapy. This can be as simple as the product not working as intended or a business decision to not pursue a particular therapy because it does not align with their internal objectives. This, along with other reasons, is generally why larger companies will often focus their efforts on incremental improvement of devices as opposed to disruptive technologies. The incremental improvements are much more predictable and lower risk, since the market is already known and the needs often come directly from their voice of customer (VOC) . Disruptive technology offers a greater amount of risk and, conversely, often a higher reward. This volatility of the device can be a deterrent for larger, publicly held companies that rely on stability, but it provides an opportunity for smaller companies willing to take the risk.

Whether it is an iterative or disruptive technology, the development process generally follows a similar pathway (also utilized in nonmedical device development), with the hope of producing a marketable product. Often, a problem is identified within the field, and to better understand the clinical needs, the designers must empathize with those experiencing the problem. This can be the physician, patient, payers, or any other stakeholders. While studying and understanding these situations, the designers gain invaluable insights as to the root cause(s) of the clinical problem or need, as well as what confounding factors are present. Then, by defining which portion of the issue they are trying to address, designers can begin to hone in on a potential solution. In turn, this will spark ideas and a multitude of solutions (partial or complete) will be generated. These solutions (as you will see in Sect. 42.3.3) will be prototyped and selected based on technical and market feasibility. Promising options will then be tested for usability and functionality; if an acceptable option is produced, the design will be frozen to move on to preclinical animal testing and eventual clinical testing (Fig. 42.2) (see Chap. 43). The generalized overarching progression of how cardiac devices develop from concept to market will be presented in the following sections.