Artificial Heart Valves

1. INTRODUCTIONThe cardiovascular system is a closed circulatory systemthat ensures blood ﬂow through the human body. Bloodcirculates through two systems; the ﬁrst is responsible forpushing blood from the heart to the lungs to capture ox-ygen, and the second is responsible for distributing oxy-genated blood to bodyorgans. This system is composed ofadouble pump with branched tubes that leads blood fromthe heart to the organs (arteries) and returns it to theheart (veins). The heart is essentially a muscular pumpwith four rooms, two for receiving incoming blood (atria)and two for pushing blood toward the organs (ventricles).In order to maintain unidirectional ﬂow of blood, 3-Dopen-close ‘‘gateways’’ (valves) are optimally located be-tweenatriaand ventricles,andbetween theventricles andthe emerging arteries. The workload of heart valves isnothing short of extraordinary. The valves open and closeonce per second totaling more than 3 billion times in alifetime. These movements take place under constantpressure and in ﬂowing blood, a very viscous ﬂuid richin minerals, proteins, lipids, and cells.Valves may progressively become defective and criti-cally inﬂuence the performance of the heart. These defectsare collectively named valve diseases. As the properties ofnormal and diseased valves are not sufﬁciently under-stood, no drugs are available to treat and cure valve dis-eases. The only curative solution is to perform open-heartsurgery to replace diseased valves with artiﬁcially engi-neered devices. These valve substitutes are nonliving ma-terials fashioned in the form of valves that reestablishinitial mechanical functions and restore the functionalityof the heart. However, these devices may eventually failbecause of imperfections in design, composition, or bio-compatibility. Once a device shows signs of failure, thepatient requires a second open-heart surgery for replace-ment of the defective device. Reoperations are generallyrisky and pose additional problems for children who nat-urally outgrow their implants.Thus, the long-term goal of biomedical engineering is toﬁnd better methods for treating valvular diseases andthus impact lives of millions of patients worldwide. Thisdaunting effort bringstogether a multidisciplinary team ofspecialists in medicine, biology, engineering, and mechan-ics. This chapter describes the structure, function, biology,and pathology of heart valves; information on current re-placement devices; and the necessary prerequisites forconstructing an ‘‘ideal’’ replacement valve. Ongoing re-search is aimed at improving existing devices by enhanc-ing biocompatibility, as well as pioneering work on noveltissue-engineering approaches, which would facilitatecomplete regeneration of valve tissues.2. NATURAL HEART VALVES ARE SPECIALIZEDCARDIOVASCULAR TISSUESThe mechanical action of the human heart is similar tothat of the two-cylinder engine. Four natural heart valvesmaintain the direction of blood ﬂow within the circulatorysystem (Fig. 1). The tricuspid and mitral valves (collec-tively called the atrioventricular valves) open to allowblood toﬁlltheventricular cavities.Thesetwo‘‘admission’’(inﬂow) valves are connected to the subjacent heart mus-cle via ﬁbrous extensions (chordae tendinae). When theinﬂow valves are closed, blood is forced to ﬂow through theaortic and pulmonary valves (collectively called the semi-lunar valves). These two ‘‘ejection’’ (outﬂow) valves openin response to blood ﬂow and immediately close after ejec-tion. Although not initially apparent, a structural andfunctional correlation between the heart muscle and thevalves exists (Fig. 1). A 3-D network of extra cellular ma-trix (mainly collagen ﬁbers), formally known as the ‘‘car-diac skeleton,’’ maintains the spatial structure andfunction of the heart muscle. This cardiac skeleton in-cludes large structures (macro-skeleton), such as the