A biomaterial “can be defined as any material used to make devices to replace a part or a function of the body in a safe, reliable, economic, and physiologically acceptable manner” [1]. Biomaterials are generally made with metals and alloys, polymers, ceramics or composites [2]. Owing to a suitable combination of wear resistance, tensile strength, fracture toughness and fatigue resistance, metallic materials are particularly relevant candidates in a wide range of applications, including in orthopedic implants and fixation devices (e.g. joint replacement, bone plates and screws), orthodontics and dental implants, suture wires, stents, cardiovascular and neurosurgical devices [3,4]. From a historical perspective, the first attempts for the implantation of metals dates back to the late 18th century where iron, gold, silver and platinum were used for the repair of long bones [1]. These implantations were successful thanks to the discovery of the aseptic surgical technique by J. Lister (1860), which limited the infection after implantation. Later, and since the 1960s, the use of metallic biomaterials has been extended to a wide range of orthopedic, cardiovascular, spinal, and other medical devices [5].

The biocompatibility of metallic biomaterials, which can be considered as the ability to stimulate minimal inflammatory responses, has mostly been associated with their corrosion and wear resistance and, thus, their low tendency to release metallic debris or ions in the body. From this point of view, metallic biomaterials can be divided into two main categories on the basis of their thermodynamic stability in physiological conditions, as predicted by Pourbaix diagrams [6]. On one hand, passive-type metals and alloys (e.g. Ti and Ti-based alloys, Co-Cr alloys, stainless steels), commonly called “inert” or “non-degradable” biomaterials, are characterized by the instantaneous formation of thin oxide layer, which is able to inhibit the anodic dissolution of the metal. In practice, this oxide layer, commonly called passive film, is expected to limit the anodic reaction at a low rate (typically few nA.cm−2). On the other hand, active-type materials, typically Mg, Fe, Zn and their alloys, are called “biodegradable” or “absorbable” materials, for which the anodic dissolution rates is higher than passive-type materials by several orders of magnitudes [[7], [8], [9]]. They are used for implants with a temporary function or act as a temporary support in reparative and regenerative medicine [7,[10], [11], [12], [13]]. In many situations, these biodegradable metals and alloys may be more suitable than permanent implants as they reduce the need of costly and risky additional surgeries for either implant replacement or removal. Accordingly, the corrodibility of these active metallic materials is a major advantage for their application as biodegradable implants. By contrast, the stability of passive materials toward corrosion or mechanical wear is required in other applications such as hip, joint or dental implants.

The host responses toward metallic biomaterials result in a variety of physicochemical processes and a cascade of biological events initiated at the blood-material interface mainly involving plasma proteins, macrophages and other cells [14,15]. Recently, extensive investigations have been dedicated to the mechanisms by which host tissues respond to implanted biomaterials [[16], [17], [18], [19]]. This leads to a better consideration of the factors regulating the immune system responses, in which proteins play a pivotal role, and their diversity for different tissues [15,16]. Following this logic, the surface modification approach appears as a suitable way to control the host-material interface and possibly stimulate specific biochemical signaling pathways. This explains the extensive research interest in surface modification techniques for metallic biomaterials, and the reason why widespread strategies have been and will continue to be developed for this purpose [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]].

Surface (bio)-chemical functionalization is among the main strategy used for the modification of metallic biomaterials. This strategy consists in using bifunctional molecules anchoring to the metallic surface on one side, and bearing selected chemical groups on the other side. This offers the possibility to tune the surface properties (charge, wettability, adhesion, etc) or to retain biochemical entities, mostly proteins and peptides, which are specifically recognized by cells. This is because, in vivo, proteins and peptides have been identified as key players to mediate temporarily and spatially the interaction between cells and their biological environment, and they reproduce this behavior when they come in contact with biomaterials surfaces [38,39]. Moreover, the possibility to preserve the intrinsic properties of these biomolecules in vitro have motivated extensive research studies for the development of biomimetic strategies that allows their permanent immobilization on the surfaces of metallic materials and tentatively improve the host responses. The most common procedures used for the functionalization of metallic surfaces involve the self-assembly of alkane thiols on gold, silver, copper or platinum. However, these substrates have limited use as biomaterials, even they may have great interest in other biomedical applications dealing with diagnosis. For metallic biomaterials, and due to the unavoidable presence of an oxide layer on their surfaces, the main organic molecules used for the anchoring are organosilanes, phosphonates and catechols [40]. These procedures are usually preceded by chemical and/or physical treatments for cleaning or tuning the surface properties (topography, reactivity, bearing surface groups, etc).

A reliable evaluation of surface bio-functionalization procedures requires coping with the multiplicity of interfacial processes. This issue may, indeed, be scrutinized by considering separately each side of the biointerface. On the one hand, metals and alloys may be subjected to significant chemical and morphological changes due to their reactivity in aqueous solutions. These processes can considerably change if the material surface is chemically modified. On the other hand, proteins interact with solid surfaces in different ways, depending on their intrinsic properties (e.g. 3D structure) and extrinsic parameters (e.g. physicochemical conditions), so that their behavior at the interface is difficult to predict (adsorption/desorption dynamics, competitive adsorption, structure and orientation in the adsorbed state, supramolecular organization, etc) [[41], [42], [43], [44], [45], [46]].

Through the main strategies extensively used for surface (bio)-functionalization of metallic biomaterials during the last two decades, this review addresses the issues related to the reality of the produced interfaces, i.e. their non-ideality, and their fate in biologically relevant media. Examples are given to illustrate the concepts beyond the “biological identity of materials” and the state of “real interfaces”. At the heart of these interfacial processes lies the need for the development of a methodology to examine the surfaces in a reliable way (surface (re)-contamination processes, discrimination between biomolecules of interest and other bio-organic compounds, depth distribution of adsorbed compounds, etc). Through a large amount of results obtained on different metallic materials, a particular interest of a quantitative utilization of X-ray photoelectron spectroscopy (XPS) analyses is thoroughly described, including peak decomposition, angle-resolved measurements and correlations between spectral information on sets of data. This approach shows the possibility to acquire sharp information regarding the chemical composition of the interface with nanoscale depth resolution, and to analyze data with comprehensive guidelines and critical eye.