Surface Tailoring of Inorganic Materials for Biomedical Applications

Biomaterials’ applications and performances are strongly related to their surface properties, such as surface chemistry, surface energy, roughness, surface charge, topography, hydrophobicity or hydrophilicity. In this chapter, the fundamental aspects of some of these properties – in particular wetting, adhesion, roughness and topography – will be elucidated. The relation between such surface features and the biocompatibility issues will also be discussed briefly. The three main classes of biomaterials – polymers, metals and ceramics – will be separately analyzed, listing a series of examples of the most commonly employed surface modifying techniques for that class.

Calcium phosphates with apatitic structure (or apatites) constitute the mineral part of hard tissues in vertebrates. The structure and properties of apatites open several possibilities in medical sciences and also play an important role in living organisms for biomineralization processes. One of the most interesting characteristics of apatitic calcium phosphate is its surface reactivity and capability to exchange mineral ions and small molecules upon interaction with surrounding fluids. Such surface properties can be exploited and tailored in materials science to obtain nanostructured and bioactive biomaterials, in particular in view of drug delivery and tissue engineering.

In this chapter, biogenic calcium phosphates will be first presented, giving details about the principal characteristics of bone, tooth and pathological calcifications where calcium phosphates are present. We will then expose a general presentation of calcium phosphates used as biomaterials, especially in terms of ceramics, cements and coatings. Then the surface chemistry of synthetic and biogenic calcium phosphates will be examined in detail, summarizing the principal surface characteristics of biomimetic nanocrystalline apatites and of some other calcium phosphates of biological interest. In a subsequent part of the chapter, we will discuss surface interaction processes between calcium phosphate and molecules of biological interest: on one hand in the case of biomolecules (including peptides and proteins) for biomaterials development and biomineralization aspects, and on the other hand in the case of bioactive molecules for specific applications in drug delivery. Finally, we will conclude this chapter by presenting some in vitro and in vivo results obtained on functionalized calcium phosphates with the aim to develop innovative drug delivery devices.

The interaction between ultrasound and bubbles in liquids has led to the development of a number of applications. The scattering of sound waves by bubbles has been utilized in ultrasound contrast imaging. The absorption of sound energy by bubbles that exist in liquids can lead to acoustic cavitation, the growth and violent collapse of microbubbles. Extreme temperature conditions and highly reactive radicals are generated during acoustic cavitation, which have been utilized in a number of processes that include the synthesis of a variety of nanomaterials. Acoustic cavitation also generates strong shear forces and streaming effects in liquids that have been used for the deactivation of pathogens and modification of the properties of bioactive molecules. This chapter aims to provide an overview of the recent work that has been carried out in our laboratory that includes the ultrasonic and sonochemical synthesis of biofunctional metal nanoparticles, polymers, and microspheres.

This chapter describes some of the most important spectroscopic methods to study the materials surface. Special IR and Raman configurations providing information from the surface will be reviewed highlighting the main advantages and/or drawbacks for their applications in the study of biomaterials. Additionally, some examples are presented to demonstrate their usefulness for the surface characterization of materials for biomedical applications as well as to understand the complexity of the chemical interactions between biomaterial surface and biological molecules.

In this chapter, the possibilities to highlight atomic/molecular details of the surface of biomaterials by IR and Raman analysis of their interaction with molecules in controlled environments are reviewed. For this aim, information on the experimental setups and procedures is also provided, as the use of vibrational spectroscopies in such conditions is rather new in the field of biomaterials. Particular attention has been devoted to the use of probe molecules. Such molecules can be quite different from those in interaction with biomaterials in actual functional conditions, but, when adsorbed in controlled conditions, become useful “molecular tools”, because of the sensitivity of their vibrational features to the physical-chemical properties of surface sites.

Control of surface morphology impacts greatly upon the performance of nano-structured devices. This is particularly true in the case of biomaterials and medical devices, because cell responses are highly affected by topography. This chapter describes some of the most up to date microscopy principles and methods, offering a flavor of the broad range in which AFM, SEM and TEM techniques have impacted upon advances in understanding. Additionally, some applied examples of biomedical materials surface characterizations are described in order to point the way to gain useful information about the interaction between biomaterials and biological systems.

There has been a transition in the definition of biocompatibility from a material that was once required to be inert and simply act as ‘filler’ or scaffold, to one which is dynamic, interactive and has a beneficial effect. In addition to the definition changing, the actual role of the biomaterial has also changed. More is demanded from biomaterials and also a greater understanding about the way cells in the body interact with biomaterials on a three dimensional level. Biomaterials are expected to be used in many different forms and devices in medicine; blocks, fillers, scaffolds that provide structure and shape and if possible be completely resorbable and remodeled. A biomaterial is required not only to be non-toxic but it should also allow the cells in contact with it to benefit from the interaction. The testing of all these functions is quite difficult in vivo but can be studied in detail with a sensitive panel of in vitro tests. There is an increasing pressure from society to reduce animal experimentation which has led to more sensitive and sophisticated methods and today in vitro testing is an important and major part of testing biomaterials and in some cases has been able to replace in vivo testing completely. In this chapter, we will discuss these tests and how they can be used to evaluate a range of materials used for clinical application.

The in vivo evaluation of a material to be used in clinics is a long trial and requires a well planned sequence of steps. In vivo tests are complementary to those performed in vitro that provide necessary and useful results to be added to those found in the in vivo testing, and strongly reduce the number of animals used. Animal studies require first of all the compliance to the ethical and legal rules on animal experimentation. The procedures are standardized at an international level. In particular the rules of ISO 10993 are the guidelines for these investigations. In the procedure of validation of a material, the choice of more than one animal species for the performing of the tests, namely a small/medium and a large size animal should be appropriate. Considering the research on biomaterials, the use of small animals can be acceptable in the early stage of testing, but for the evaluation of prototypes biofunctionality large animals should better approximate the human environment. According to the European and International rules, the experiments must be performed in authorized laboratories, and surgical interventions in well equipped operatory rooms and in general anesthesia. Clinical monitoring and X ray investigations allow the scheduled follow-up on living animals. After the sacrifice histology and histomorphometry provide useful information about the interactions between the material and the tissues, together with biomechanical test. More recently computerized microtomography, a non destructive procedure, is added to the methods of study on laboratory animals allowing high resolution images and the visualization of the internal structures of the objects.

Nowadays long bone healing is still a major socio-economic concern. In fact, although in many fields of bone surgery suitable clinical solutions addressed to the regeneration of bone are achieved, for the healing of load-bearing bones the clinical solutions still consist of bioinert prostheses which only provide a mechanical sustain without restoring the complex biomechanical functionality of bone. As a consequence, the recourse to additional surgery is often unavoidable, especially for the younger patients; additionally the bone surgery in orthopaedics is often very invasive and patients are forced to spend long time in hospital with strong impact on the psychological and physical wellbeing, as well as on the healthcare costs. In the last decade new regenerative solutions based on the use of bioactive materials and composites and on the set up of innovative synthesis techniques to achieve bone scaffolds endowed with hierarchical organized morphologies are being developed. Such scaffolds exhibit high chemical biomimesis and interesting anisotropic physical and mechanical properties, such as the native bone, so to potentially pave the way for realizing prosthetic devices closer to the extraordinary performance of human tissues addressed to the regeneration of long and load-bearing bones for which no acceptable solutions exist so far.

Metallic materials are widely employed for bone contact applications (dental and orthopaedic ones) because of their good mechanical properties and load bearing ability. Titanium and its alloys are the most diffused biomedical metals due to their biocompatibility, on the other hand bare metals are almost inert and cannot stimulate tissue-integration processes. In order to improve bone integration ability of metallic implants several solutions have been proposed both in the scientific literature and also among the commercial clinical applications. A synthetic review of these techniques is described in the present chapter. Surface modification strategies have been divided in morphological, chemical and biological, considering their main aim: realization of a particular surface topography, introduction of chemical elements/reactive groups or, finally, addition of specific biomolecules.

The cell–material interaction plays an important role in the biomaterial design. Biomaterials, such as diamond-like carbon (DLC), titanium (Ti), and stoichiometric titanium nitride (TiN) as well as titanium carbo-nitrade (Ti(C, N)), seem to be good candidates for future blood-contact applications. These materials were deposited as thin films by the hybrid pulsed laser deposition (PLD) technique to examine the influence of such surfaces on cell behavior. The cell-material reactions were examined in static conditions with Dictyostelium discoideum cells and then subjected to a dynamical test to observe the cell detachment kinetics. For a given cell, detachment occurs for critical stress values caused by the applied hydrodynamic pressure above a threshold which depends on cell size and physicochemical properties of the substrate. Tests revealed differences in behavior with respect to the applied coating material. The strongest cell-biomaterial interaction was observed for the carbon-based materials compared to the titanium and titanium nitride. The research activity was performed on fabrication and diagnostics of materials characterized by reduction or erasing of thrombogenicity. Development of surfaces that both favor endothelial cell monolayer reconstruction and prevent platelets aggregation and binding was under examination. Research study was performed on soft polyurethane surfaces (PU) by application of thin inorganic coatings. Experiments have been performed on surface functionalization with application porous materials produced by electrospinning. Research on scaffolds for precursors of tissue analogs and cell migration channels was presented.

In this chapter, the possibilities to improve in vitro diagnostic tests by providing more sensitive detection technologies and better nano-labels devices will be reviewed.

New therapeutic approaches based on the miniaturization of devices and targeted delivery systems are discussed as well.

Particular attention has been devoted to the development and use of “theragnostic” platforms and molecular imaging technologies which allow to systematically monitor the care process.

The improvement of the sol-gel processes has provided a new generation of silica-based ordered mesoporous materials (OMM) for biomedical applications and bone tissue engineering. These materials, indeed, have been suggested as matrices for sustained drug release, showing that both small and large molecular drugs can be entrapped and released from the mesopores through several different processes. Many emerging biotechnologies can benefit from these OMM-based drug delivery systems. For instance, bone tissue engineering is a growing area directed towards the design of materials able to improve the bone regeneration capacity by recovering both its structure and function. In this area, controlled drug delivery from biocompatible and bioactive mesoporous materials with sufficient mechanical strength could favor the cellular growth and bone regeneration. Ordered mesoporous silica (OMS) present a high drug loading capacity and a controlled sustained release which could be helpful for this purpose, however their poor bioactive behaviour and mechanical properties drove to the development of new bioactive systems. On one hand mesoporous bioactive glasses (MBGs) have been extensively investigated, on the other hand the combination of OMS and bioactive macroporous scaffolds has been proposed in order to obtain hierarchical systems for bone substitution.

The formation of so-called bioactive bond in which specific compositions of glasses and glass-ceramics strongly bond with bone, is a complex process highly influenced by the material surface reactivity when in contact with the biological fluids and cells. Indeed, non bioactive materials elicit a foreign body reaction and are isolated by a non adherent fibrous capsule responsible for the failure of many prostheses. It is clear that understanding the complex mechanisms that form the bioactive bond can help to design new prostheses and improve the present, increasing their survival rates with the consequent economic and social benefits. An important role in the bioactive bond formation is played by biomolecules and cells, which can only be investigated in complex in vitro cell cultures studies, aided by bioreactors, or in vivo, in animal models. However, the reason why an implanted material is isolated from the body by a fibrous capsule or strongly bonded to osseous tissue is its surface reactivity that can be evaluated under in vitro conditions in artificial solutions with certain blood plasma characteristics like pH, temperature, inorganic ion concentration, etc. These solutions were designed to evaluate new candidates to implantable materials and have also led to great advances in the study of the mechanisms of bioactivity. This chapter presents the methods used to study the mechanism of formation of bioactive bond.

Biomaterial choice and porosity features play a crucial role in scaffolds design. Bioactive glasses are ideal candidates for manufacturing bone tissue engineering scaffolds as they can bond to bone and stimulate bone cells towards osteogenesis. A 3-D network of highly interconnected macropores is essential to allow cells migration into the scaffolds, as well as the in vivo vascularization of the implant. An ordered structure of nanopores can impart an added value to the scaffold, enhancing its bioactive properties and providing drug uptake/release abilities to the system. This chapter highlights the great potential carried by hierarchical glass scaffolds, exhibiting pore scales from the meso- to the macro-range, and their impact in the field of bone tissue engineering.

Bioactive glasses are widely studied in the biomaterial field because of their ability to chemically bind to living tissues and to positively stimulate cells through the release of specific ions. They are extremely versatile materials, in fact their composition can be tailored in order to improve particular properties (e.g. bioactivity, resorbability, mechanical resistance …). In this research work, different strategies aimed to modify the surface of bioactive glasses have been reviewed. The main idea of these treatments is to combine the conventional properties of bioactive glasses (bioactivity, ion release …) with more specific ones, by the addition of particular ions or the grafting of biologically active molecules (e.g. proteins or drugs). At first a brief introduction on bioactive glasses is reported, then surface modification by the introduction of active ions is described and finally the surface functionalization with biomolecules (proteins or drugs) is discussed.