**1. Introduction**

A biomaterial is any material that comprises whole or part of a living structure or biomedi‐ cal device which performs, augments, or replaces a natural function to improve the quality of life of the patients [1]. Over the past fifty years biomaterials has been developed as a sci‐ ence with various forms of implants/medical devices, and have been widely used to replace and/or restore the function of traumatized or degenerated tissues or organs. As a life-saving and life-improving option for countless patients, biomaterials have been paid more and more attention during the last decade. Only in the United States, more than 13 million im‐ plant/medical devices implanted annually. As a result, the impact factor of the journal of "Biomaterials" has boomed from 2.489 to 7.404 from the year 2001 to 2012.

The implant/medical device scope of biomaterials ranges from simple implants like intraoc‐ ular lenses (which restore sight to millions of cataract patients every year), sutures, wound dressings, decellular matrices, bone plates, joint replacements to more complex materials like biosensors, catheters, pacemakers, blood vessels, artificial heart (that provide both me‐ chanical and biological functions in a body), left ventricular assist devices and prosthetic ar‐ terial grafts. According to the resources and properties biomaterials can be assorted into autografts, allografts, organic polymers, such as natural collagen, fibrin, chitosan, hyaluron‐ an, heparin, cellulose, and synthetic polyurethane (PU), polyester, metal, such as alumini‐ um, steel, titanium, inorganic salts, such as calcium phosphate, hydroxyapatite, and their compounds or derivatives. There are more than one hundred different biomaterials which have been applied *in vivo*. All biomaterials when implanted into a body initiate a host re‐ sponse that reflects the first steps of tissue repair. The host/biomaterial interactions which follow implantation of any prosthesis or device are a series of complex events that have not been well defined. Generally, host reactions following implantation of biomaterials include

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injury, blood–material interactions, provisional matrix formation, acute inflammation, chronic inflammation, granulation tissue development, foreign body reaction, and fibrosis/ fibrous capsule development [2]. There are numerous types of host responses to a broad spectrum of biomaterials.

When considering a biomaterial for implantation or medical use, the first and most impor‐ tant requirement is nontoxic, nonimmunogenic, chemically inert/active, and acceptable by the human body. Biocompatible in most cases means that the biomaterials must not form thrombi in the blood system, result in tumors in the surround tissues, or be immediately at‐ tacked, encapsulated, or rejected by the body [3]. According to the host responses to im‐ plantable biomaterials, there are many different kinds of biocompatibilities, including local tissue responses, such as necrosis, repulsion, infection, inflammation, calcification, scar, cyst, amalgamation, thrombus, tumor, cancer, and whole body responses, such as fever, toxicity, circulation impediment, nerve anesthesia, malformation, etc. The overall biocompatibilities including cyto-compatibility, hemo-compatibility, and tissue-compatibility, are often evalu‐ ated using histological sections, cell markers, and metabolite measurements. Sometimes, polymers with similar chemical characteristics behave differently in certain situations. For example, polyethylene and ultrahigh molecular weight polyethylene behave differently as orthopedic biomaterials for knee and hip replacement [4]. Until present, most of the im‐ plantable biomaterials trigger acute or/and chronic inflammatory responses in the body. These reactions can totally black a biomaterial and even lead to huge disasters or personal misfortunes. Among the numerous types of host responses, early interactions between im‐ plants and inflammatory cells are probably mediated by a layer of host proteins on the bio‐ material surface. Franz and coworkers have described several typical host responses of implantable biomaterials (Figure 1) [5]. This model can be used as a reference for evaluation of an implantable biomaterial when it is implanted shortly *in vivo*.

In this chapter, I will focus on the *in vivo* host responses about twenty common used bioma‐ terials which cover nearly every tissue and organ in human body. Advanced biologic techni‐ ques have been employed in determining the mechanisms behind observed macroscopic or microscopic responses. An understanding of the molecular and cellular events which occur in response to implantable biomaterials may allow us to manipulate responses and design more biocompatible, bioactive and functional biomaterials for clinical applications, such as regenerative medicine and controlled releasing drugs.
