Biomolecules – biomaterials interactions: IR study of the adsorption, in model conditions, of glycine on Hydroxyapatite and TiO2

Chapter 1 – Biomaterials 
 3 
 
 
 
 
 
 
 
Chapter 1 
 
Biomaterials Biomaterials Biomaterials Biomaterials    
 
 
Biomaterials are materials of natural or man-made origin that are used to direct 
supplement, or replace the function of a living tissues of the human body. Use of 
the biomaterials dates far back into ancient civilizations. Artificial eyes, ears, teeth, 
and noses were found on Egyptian mummies, Chinese and Indians used waxes, 
glue, and tissues in reconstructing missing or defective parts of body.
[1]
 The 
technological advances achieved over the centuries and particularly in recent 
years in areas such as biotechnology, molecular and cell biology, tissue  
engineering, materials science and other related fields, has resulted in significant 
improvement of biomaterials.
 [2]
  
Today many devices are used with these purposes, and over the years many 
definitions have been proposed for the term biomaterial. For example, a 
biomaterial can be simply defined as "a synthetic material used to replace part of a 
living system or a vital function in intimate contact with the living tissue."
[3]
 The 
Clemson University Advisory Board for Biomaterials held that a biomaterial is a 
"systemically and pharmacologically inert substance designed to plants or 
incorporated into living systems"
[3]
. J. Black defines biomaterials as "non-living 
materials used in medical devices intended to interact with biological systems" 
[4]
 
or “materials of synthetic as well as of natural origin in contact with tissue, blood, 
and biological fluids, and intended for use for prosthetic, diagnostic, therapeutic, 
and storage applications without adversely affecting the living organism and its
Chapter 1 – Biomaterials 
 4 
components”
 [5]
. Still another definition of biomaterials is stated as “any substance 
(other than drugs) or combination of substances, synthetic or natural in origin, 
which can be used  for any period of time, as a whole or as a part of a system 
which treats, augment, or replace any tissue, organ, or function of the body”
 [6]
. 
According to these definitions, one must have a broad knowledge of various 
disciplines and collaborate with people from a wide variety of specialized fields to 
work out and develop successfully biomaterials for use in the medical field
 [3]
. 
The key feature of a biomaterial is the biocompatibility, i.e. the ability to establish 
favourable interactions with living systems with which it comes into contact: this is 
a fundamental requirement, linked to the need to improve or restore a particular 
biological function, without interfering or interacting in a harmful way with the 
body's physiological 
[7]
.  
The degree of biocompatibility of a material depends on many factors, including 
the shape, structure and chemical composition, physical, mechanical and electrical 
properties, as well as being influenced by the site's location in the body and the 
type of application which is intended.  
Last, but of basic importance in determining the biocompatibility of the material, we 
must consider the surface properties, because the surface is the direct interface 
with tissues and biological fluids and on it depends all the favourable or 
unfavourable interactions that  can be established with the body. 
 
1.1. Classification of biomaterials 
 
For biomaterials are possible several criteria of classification 
[8]
: 
 
• On the basis of the effects produced by material itself: i.e. if after implantation 
in the body the materials undergo substantial chemical and / or physical 
changes, they are called "biodegradable", otherwise if they keep their own 
characteristics are classified as "biostable" . 
 
• On the basis of body-material interactions: one can distinguish between 
"biotoxic" materials, which induce a reaction of rejection by the biological tissue 
due to unfavourable chemical interactions (as for example, some alloys 
containing nickel or some steels with peculiar carbon content); "bioinert"
Chapter 1 – Biomaterials 
 5 
materials, kind of stable materials that have little interaction with the 
surrounding tissues and allow good coexistence between organism and 
implant (like oxides of titanium or aluminium), and finally, the class more 
interesting is represented by "bioactive" materials, which promote direct 
biochemical interaction with biological tissue, which can grow on the surface of 
the material by establishing a solid bond between prosthesis and natural tissue 
(e.g. certain types of hydroxyapatite and bioglasses)
[7]
. Within the bioactive 
materials it is possible to distinguish a subclass represented by 
"bioabsorbable" materials, or materials that undergo a progressive degradation 
in the body without toxic effects and are gradually replaced by the biological 
tissue (some examples may be particular types of calcium phosphate and 
porous hydroxyapatite). 
 
• Based on the chemical nature: the materials are classified as: 
 
- "Metal", suitable for the construction of structures capable of supporting 
heavy loads without the risk of significant elastic and plastic deformation 
but with a low degree of biocompatibility.  
- "Polymer", consisting of molecules with high molecular weight 
concatenated to form fibres, fabrics, film or viscous liquids (polyamides, 
polyesters, polyacrylates, polycarbonates, etc ...).  
- "Ceramic" or "bioceramic", organic compounds with wide variation of 
composition, and structures that have the advantage of a high degree of 
biocompatibility (eg hydroxyapatite, bioglasses and some types of 
calcium phosphate).  
- "Composite" materials that contain two or more phases of the 
constituents (such as fiberglass and reinforced plastic). And, at last, 
materials derived from biological sources, like cells and organic tissues 
produced in vitro and then grafted into the body of the patient. 
 
The classification based on the chemical nature is the most important and 
interesting for a chemist, and then for that reason it deserves further consideration 
in the frame of this thesis, with a more accurate description of the characteristics 
of different classes of materials.
Chapter 1 – Biomaterials 
 6 
1.1.1. Metallic materials 
[3]
 
The metals are used as biomaterials for their excellent thermal and electrical 
conductivity and the optimal mechanical properties. Since some electrons in 
metals are independent, they can easily transfer an electrical charge and conduct 
heat. The free moving electrons act as bond strength to hold together the positive 
metal ions and this attraction is very strong, as evidenced by the close hanged 
aggregation of atoms, that is the cause of the high specific weight and melting 
point of metals. Since the metallic bond is essentially non-directional, the position 
of metal ions within the binder electronic "cloud" can be changed without 
destroying the crystalline structure, resulting in plastic deformation of the solid. 
Several metals are used with passive functions for the replacement of hard tissues 
of the body, such as total hip prostheses and knee joints, as aids for the healing of 
bone fractures such as plates and screws, or as devices to support the spinal 
column and dental implants, because of the excellent mechanical properties and 
resistance to corrosion of these materials. Some metal alloys are used instead for 
more active roles in devices such as vascular stents (mesh of cylindrical structures 
that are introduced into the lumen of the artery in case of atherosclerotic disease 
and they are expanded in the region of obstruction until their diameter is equal to 
the original vessel), guide catheters, coils, bridges and dental devices or cochlear 
implants. 
The first metal alloy developed specifically for use in humans was " Vanadium 
Steel " which was used to manufacture support for bone fractures as plates 
(Sherman’s plates) and screws. However, many metals such as iron (Fe), 
chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), thallium (Ta), niobium (Nb), 
molybdenum (Mo) and tungsten (W), used in alloys for the construction of 
prostheses and implants, can be tolerated by the body only in small quantities. 
Many times, even metallic elements naturally present in the body, such as iron, 
essential for the functions of red blood cells, or cobalt involved in the synthesis of 
vitamin B12, may be poorly tolerated in large quantities and develop toxic effects. 
The biocompatibility of metal implants is a subject of fundamental importance, 
since these materials in an “in vivo” environment may be subject to corrosion and 
as a consequence may result (in addition to the weakening or disintegration of the 
implant) in toxic effects on tissues and surrounding organs as a result of corrosion 
products released in the biological system.
Chapter 1 – Biomaterials 
 7 
To date, most metallic materials used in the biomedical field, for their mechanical 
properties, stability and biocompatibility are: carbon stainless steel, CoCr alloys, 
titanium alloys, TiNi alloys and some alloys for dental use made of silver, tin, 
copper and zinc. 
1.1.2. Polymeric materials 
[8,9]
 
Synthetic polymeric materials have been widely used in medical disposable 
supplies, prosthetic materials, dental materials, implants, dressings, extracorporeal 
devices, encapsulants, polymeric drug delivery systems, tissue engineered 
products, and orthodontics like those of metal and ceramics substituents. The 
main advantages of the polymeric biomaterials compared to metal or ceramic 
materials are the ease of manufacturability to produce various shapes (latex, film, 
sheet, fibres, etc.), ease of secondary processability, reasonable cost, and 
availability with desired mechanical and physical properties. The required 
properties of polymeric biomaterials are similar to other biomaterials, that is, 
biocompatibility, sterilizability, adequate mechanical and physical properties, and 
manufacturability to obtain the desired forms.  
The sterilization of polymers is an important aspect in the choice of these materials 
for biomedical applications, because the polymers have lower thermal and 
chemical stability compared to other materials such as ceramic and metal, and 
therefore require special and more complex methods than conventional ones. 
The sterilization techniques commonly used in the case of polymers are: dry 
heating, autoclaving, high-energy radiation and gasses of ethylene oxide. 
However, choices must be adequate, because may give rise to problems of 
stability of the polymer. 
In the dry heat sterilization, the temperature is variable between 160 and 190 ° C, 
higher than the point of melting and softening of many linear polymers such as PE 
and PMMA, and moreover, also polymers such as polyamides that have higher 
melting temperatures may manifest problems of oxidation, whereby this technique 
is applicable only to PTFE and silicone rubbers. Autoclaving requires more 
moderate temperatures (125-130 ° C) as it uses high pressure, however, if the 
polymer is subject to attack by water vapour, such as PVC, Polyacetals, LDPE and 
polyamide, this technique can not be used. Chemical agents such as ethylene or 
propylene oxide are the most widely used for sterilization, but it is necessary to 
remove them effectively after the treatment, because they can cause toxicity in the
Chapter 1 – Biomaterials 
 8 
body. Finally, the high-energy radiations have the problem to be ionizing and are 
able to degrade the polymer by bond breaking, that may produce a decrease of 
the molecular weight and mechanical properties. This tecnique requires to carry 
out the treatment in oxygen-free atmosphere to avoid the oxidative degradation, 
which can go on even after implantation of the prosthesis, causing premature 
failure. 
1.1.3. Ceramic materials 
The ceramics are materials polycrystalline, refractory, usually made up of 
inorganic compounds such as phosphates, silicates, metal oxides, carbides, 
hydrides, sulphides and selenides; can contain oxides of metallic and non-metallic 
elements, such as Al
2
O
3
, TiO
2
, MgO, SiO
2
 and ZrO
2
, and ionic salts such as NaCl, 
CsCl and ZnS. Some exceptions to the above include covalently bonded ceramics 
such as diamond, graphitic like carbon structure and pyrolytic carbon. Mainly 
special relevance is owned by Hydroxyapatite an Titanium dioxide that will be 
object of studies exposed in this thesis work 
[3]
. 
Ceramics are used by humans for thousands of years to produce objects of 
manufacture, but until a few decades ago their use was rather limited due to their 
intrinsic fragility, susceptibility to cracks and microfracture and poor tensile and 
impact strength. However, in the last century, innovative manufacturing techniques 
have led to the use of ceramic materials as "high tech". The development and 
discoveries in the field of materials science and bioengineering, have shown that 
ceramics and their compounds can be used successfully to repair or replace 
various parts of the body, particularly bone tissue. So, a category of materials was 
born, and it was classified as “bioceramics” 
[8]
. 
Their relative inertness to body fluids, the high compressive strength and the 
aesthetically pleasing appearance has led to the use of ceramics in dentistry. 
Some carbides have found applications in implants especially for employments 
that require interfacing with the blood such as heart valves. Thanks to their high 
specific strength in the form of fibres and their biocompatibility, ceramic materials 
have also been used as components of reinforced composite materials and 
systems for applications like load tensors such as artificial tendons and ligaments. 
Unlike metals and polymers, ceramics are difficult to plastic cut, due to the nature 
of the predominantly ionic bonds. This feature makes ceramics non-ductile and 
subject to frequent nicks or cracks because, instead of undergoing plastic
Chapter 1 – Biomaterials 
 9 
deformation, this type of material tends to fracture as a result of cracks 
appearance. In the crack region, in fact, is concentrated mechanical stress that 
can be more superior than stress that the material undergoes in the intact areas 
and this leads to a significant weakening of the product. However, if the ceramic is 
fully intact, appears to be very durable even when subjected to tensile stress: for 
example, glass fibres without defects have a tensile strength twice than high-
strength steel (~ 7 GPa) 
[3,4]
. 
Ceramics are generally very hard and therefore their hardness is determined in 
relation to other ceramic materials, classified on a scale called Mohs scale, in 
which the limits are represented by the diamond, with a hardness of 10, and talc 
(Mg
3
Si
3
O
10
COH), with hardness 1. Other typical ceramic materials such as 
alumina (Al
2
O
3
), quartz (SiO
2
) and fluoroapatite (Ca
5
P
3
O
12
F) are found at 
intermediate values, respectively, 9, 8 and 5. 
Other characteristics of ceramic materials, due to the type of chemical bond, are 
the high melting temperatures and low conductivity of electricity and heat. 
To be classified as bio-ceramic, a material must also satisfy some properties such 
as non-toxic and non carcinogenic activity, should not give rise to allergic or 
inflammatory processes, and must ensure a good biocompatibility and a 
biofunctionality for its lifetime in the host body 
[5]
.  
For their promising characteristics currently a large number of ceramic-type 
materials are in use or being studied for biomedical applications, and they are 
rising as biomaterials. The cells of bone tissue well tolerate their presence and 
produce less adverse effects (low impact on immuntary system, inflammatory 
answer almost absent and so absence of fibrous capsule around non-self implant) 
and moreover, in some cases, is stimulated the bone mineralization. The strength 
of bond with bone and artificial implant for this kind of materials is more higher 
than others, producing an increase in resistance of orthopaedic device and a 
gradual substitution of its material with neo-formed bone tissue 
[10]
. 
The main problem in employment of these bioceramic materials is due to low 
tensile strength respect to metals, and to and the difficulties of handling during 
surgery.
Chapter 1 – Biomaterials 
 10 
1.1.4. Composite materials 
[3]
 
The composite materials are solid compounds that contain two or more 
constituents in different phases over the atomic scale. The term "composite" is 
generally reserved for those materials whose properties are significantly altered 
compared to a homogeneous material. Therefore, a plastic reinforced with glass 
fibres, as well as bone or cartilage are classified as composite materials, while 
alloys can not be regarded as such. Foam can also meet the definition of 
composite, in which one phase is represented by empty space. The natural 
biological materials tend to be composite, such as bone, wood, dentin, cartilage, 
skin and many other foamy tissues such as lung and spongy bone. 
Composite materials offer a variety of advantages compared to homogeneous 
materials that include the ability to control, if necessary, the properties of the 
manufacts. Strong materials, rigid and lightweight, or flexible and resistant, can be 
obtained combining the properly constituent phases. 
In using these materials with great potential as biomaterials, along with 
appropriate mechanical properties, should always be aware that each component 
of the composite should be biocompatible. In addition, the interface between the 
components must not undergo changes inside the body. 
Some consolidated applications of composites as biomaterials are: dental filling, 
bone cement reinforced with methylmethacrylate or high molecular weight poly 
ethylene and orthopaedic implants with porous surfaces. 
 
 
1.2. Applications 
[11]
 
 
One of the main applications of biomaterials is to physically replace hard or soft 
tissues that have become damage or destroyed through some pathological 
process. Although the tissues and structure of body perform for an enhanced 
period of time in most people, they do suffer from a variety of destructive 
processes, including fracture, infection, and cancer that cause pain, disfigurement, 
or loss of function. 
Under these circumstances, it may be possible to remove the disease tissue and 
replace it with some suitable synthetic materials.
Chapter 1 – Biomaterials 
 11 
Fig.1.1. representative image of some of the many possible applications of biomaterials 
in the human body (image freely available on the web) 
Depending on the features of the tissue that they are substituting, different 
materials can be used in different districts of the body, as is shown in Fig.1.1 and 
listed below. 
 
 
 
• Cardiovascular Applications (heart valves and arteries replaced by using 
ceramics, metals and polymeric materials) 
• Ophthalmic (intraocular lens made in polymeric materials to replace the 
natural ones and heal disease like cataracts) 
• Wound Healing (polymers like suture materials for soft tissue and metals in 
form of plates, screws, nails, rods and wires and other devices for fracture 
treatment of hard tissues) 
• Dental Applications (teeth in their entirety and segment of teeth both can be 
replaced and restored by ceramics, metallic and composites materials to 
heal dental caries or dissolution due to demineralization)
Chapter 1 – Biomaterials 
 12 
Fig.1.2. representation of  biomaterial – bone interface 
(image freely available on the web) 
• Orthopaedic (polymeric, metallic and ceramics materials can both be 
employed for realization of prosthesis of joints, very frequently used in case 
of osteoarthritis and reumatiod arthritis, for the relief of pain and restoration 
of mobility) 
 
Certainly the most prominent application areas for biomaterials is for orthopaedic 
and dental implant devices, and the bone substitution represent a significant share 
of the world market.   
 
1.2.1. Employment in bone substitution 
Biomaterials for bone substitution are employed mainly as fillers, structural 
substitutes or as coating for screws and similar devices 
[12]
. The ultimate aim of 
this kind of materials is to restore the structural integrity of the damaged bone. In 
addition to in vivo performance, however, a material must also satisfy mechanical 
properties, the practical manufacturing, reasonable cost, ability to be fabricated 
and ease of surgical application 
[13]
. The biomaterial scientist is 
expected to develop implants of 
optimum design that take fullest 
advantages of the materials 
available. But the design of that 
kind of biomaterials is very 
challenging, because the tissue 
that they have to substitute is a 
nanocomposite exceptionally 
complex, characterized by 
impressive mechanical and 
biological features. Moreover, 
the materials studied for this aim 
are expected to endure our 
body’s internal environment, 
which can be very aggressive 
[1]
. For example the pH of body fluids in various 
tissues varies in the range from 1 to 9. During daily activities bone are subject, in
Chapter 1 – Biomaterials 
 13 
normal condition, to a stress of approximately 4 MPa, rising higher levels during 
load actions or jump, dependents on patient conditions and activities 
[13]
.  
A “perfect” biomaterial for bone substitution would have to meet surface and 
structural compatibility: surface compatibility meaning the chemical, biological and 
physical (including surface morphology) suitability of an implant to the host tissue. 
Structural compatibility is the optimal adaptation to the mechanical behaviour of 
the host tissues. Therefore it refers to the mechanical properties of the implant, 
such as elastic and strength modulus, design and optimal load transmission at the 
implant/tissue interface. Optimal interaction between biomaterials and host is 
reached when both surface and structural compatibility are met 
[14]
.  
Today a large number of materials are available for bone tissue replacements: 
metals (mainly Titanium, Steel and Gold), ceramics (Alumina, Titania, Bioglasses, 
Carbon and Hydroxyapatite) and polymers (PE, PEB, PET, PTFE,…), but none of 
them can, at the same time, match the extraordinary mechanical properties of the 
natural bone and exhibit the surface compatibility and bone bonding capability 
needed. The choice of a biomaterial is therefore a very complex process, also 
because the success of the implant depends on many other factors such as 
surgical techniques, health conditions and activities of the patient 
[13]
.    
 
 
1.3. The basic role of the surface 
 
Limited in vivo functionality and longevity of an implant is a critical issue, resulting 
either from the normal response to the implantation injury, tissue or bone/device 
interface interaction or even to a lack of biocompatibility. A foreign body response, 
based on non-specific protein adsorption, immune, and inflammatory cells occurs 
under normal physiological conditions in order to protect the body from the foreign 
object. Reactions of both the implant on the host bone/tissue and of the host on 
the implantable device must be understood to avoid health complications to the 
patient and/or device failure 
[15]
.  
A common characteristic of biomaterials is a time dependent, kinetic modification 
of the implant surface that occurs upon implantation. Fig.1.3 and 1.4 summarize 
the sequences of interactions which occur on the surface of a biomaterial as a 
bond with bone is formed 
[7]
.
Chapter 1 – Biomaterials 
 14 
Fig.4. Schematic illustration of the sequential 
reactions that take place after the 
implantation of an biomaterial into a living 
system. (from reference [3]) 
 
The first molecules that reach the surface are water ones. Water is known to 
interact and bind very differently depending on the surface properties (see section 
1.4). The properties of the surface water shell are an important factor influencing 
proteins and other molecules that arrive 
at little later 
[16]
. These water-soluble 
biomolecules also have hydration shell 
and the interaction between them and 
water surface layer influences the 
fundamental kinetic processes at the 
interface. The protein–surface interaction 
is linked to the primary structure of 
proteins, dependent to the aminoacidic 
sequences. Bigger proteins interact more 
with the surface of the materials, because 
of their higher number of functional 
groups, (aminoacidic residues). Charged 
side chains, due to their hydrophilic 
behaviour, are usually exposed to the 
aqueous medium of biological fluids, 
hence can much influence the interaction 
with material surface 
[16]
.  
Fig.1.3. Schematization of the interfacial interaction occurring when a material is put in contact with 
a living tissue (image freely available on the web).