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Vol. 44 Supp. SCIENCE IN CHINA (Series A) August 2001
Tribological applications of biomaterials: an overview
Bing Shi & Hong Liang
Department of Mechanical Engineering, University of Alaska Fairbanks, Fairbanks, AK, USA
Received July 5, 2001
Abstract Tribological research is the study of lubrication, friction, and wear. Tribology of biomate-rials is to study how the materials work and fail. This will help us to produce better biomaterials.Tribology plays a very important role in improving the design and making successful biomaterialsfor medical purposes. Joints of human body, such as hip, knee, jaw, dental parts etc., all need toconsider the wear and lubrication problem. In this paper, we give a general introduction of bioma-terial research in tribological applications. Materials, the synthetic characterization, and their failureare introduced.
Keywords: Biomaterials, Tribological Materials.
The earliest successful implants were bone plates, introduced in the early 1900s to stabilize
bone fractures and accelerate their healing. As early as in the late 1950s, the first total hip re-
placement was introduced using polytetraflouroethylene (PTFE) as the cup bearing surface[1].
However, the PTFE undergoes aseptic loosening.
Early in late 1960s, tribology was introduced into the living and artificial human joints[2]. The
development has been seen in many areas. For examples, the tribological behavior of synovial
joints and their replacement, wear of human dental tissues, the lubrication by plasma of red blood
cells in narrow capillaries, the wear of replacement heart valves, the wear of screws and plates in
and on bone in fracture repair, the tribology of skin and the friction of hair, biosensors, artificial
fingers, etc., are all the topics of study.
In the late 1970s, a temporomandibular joint (TMJ) replacement using PTFE as the bearing
counterface was invented. In 1983, the PTFE implant, which was called the Interpositional Im-
plant (IPI), was approved to the market. Because PTFE exhibits a low coefficient of friction and
has been used extensively as a bearing surface in other engineering applications, it would seem an
appropriate choice for an implant material in theory, However, of the more than 25,000 PTFE
TMJ implants received by patients, most failed. With the development of the research on IPI, evi-
dence did exist that PTFE was not an appropriate implant material[1].
It is estimated that 85 to 95% of approximately 600,000 total joints are performed each year
in the United States (More than 120,000 artificial hip joints are being implanted annually) will still
be functioning after ten years, based on research reported in orthopedic medical journals[3, 4]. Arti-
ficial joint replacement is performed when the cartilage that lines the joint deteriorates, resulting
in bones grinding against each other (shown in Fig. 1)[5]. If non-surgical treatments, such as
anti-inflammatory drugs, walking aids, and support braces do not offer relief.
298 SCIENCE IN CHINA (Series A) Vol. 44
Joint replacement surgery is most commonly performed
for hips, knees, and shoulders(See the joint replacements in Fig.
2)[5], although toes, fingers, and elbows have been successfully
replaced. Most artificial joint replacements are designed to re-
move the diseased areas of the joint and replace them with
metal- and-plastic implants designed specifically to restore that
joint's function and stability.
For biomaterial design, engineers must consider the
physiologic loads to be placed on the implants sufficient struc-
tural integrity. Material choices also must take into account
biocompatibility with surrounding tissues, the environment and
corrosion issues, friction and wear of the articulating surfaces, and implant fixation either through
osseointegration (the degree to which bone will grow next to or integrate into the implant) or bone
cement. One of the major problems plaguing these devices is purely materials-related: wear of the
polymer cup in total joint replacements. A summary of tribo-biomaterials was introduced in Table
1.
Fig. 2. Joint replacements.
Table 1 Summary of Tribo-biomaterials
Material Application Major Properties Description
Alloy: Titanium Alloys[6], Titanium Alumi-num Vanadium Alloy[7], Cobalt ChromiumAlloy[8], Cobalt Chromium Molybdenum
Alloy[9]
Total joint replacement Wear and corrosion resistance
Inorganic: diamond-like carbon, Biocompatible coatings,Reduced friction and increased wear resis-
tance
Ceramics[10]: Al2O3, ZrO2, Si3N4, SiC, B4C,quartz, bioglass(Na2O-CaO-SiO2-P2O5),
sintered hydroxyapatite (Ca10(PO4)6(OH)2)Bone joint coating Wear and corrosion resistance
Polymers: Ultrahigh molecular weight poly-ethylene,
Polytetraflouroethylene (PTFE)
Poly(glycolic acid)
Joint socket
Interpositional Implanttemporomandibular
joint(Jaw)Joint bone
Wear and corrosion resistanceLow coefficient of friction
Elastic with less wear
Composites: Specialized silicone polymers Bone joint Wear, corrosion, and fatigue resistance
Fig. 1. Knee joint deteriorate.
Supp. TRIBOLOGICAL APPLICATIONS OF BIOMATERIALS: AN OVERVIEW 299
As advances have been made in the medical sciences, aging population has become impor-
tant. More organs, joints, and other critical body parts will wear out and must be replaced if peo-
ple are to maintain a good quality of life. Biomaterials now play a major role in replacing or im-
proving the function of every major body system (skeletal, circulatory, nervous, etc.). Some
common implants include orthopedic devices such as total knee and hip joint replacements, spinal
implants, and bone fixators; cardiac implants such as artificial heart valves and pacemakers; soft
tissue implants such as breast implants and injectable collagen for soft tissue augmentation; and
dental implants to replace teeth/root systems and bony tissue in the oral cavity.
1 Synthesis and characterization
Biomaterials used to date, as mentioned earlier, are metals, ceramics, polymers, composites,
and biopolymers. The first four classes are conventional methods adapted from traditional materi-
als processing. The biopolymers, on the other hand, involve materials synthesis and bioengineer-
ing. There are three basic approaches: DNA linked material building, tissue culture, and biopoly-
merization.
Recently, scientists developed a method to link DNA to gold, a noble metal[11]. There are bio-
chemical approaches to molecular structures of precisely defined dimensions ranging from 1 nm
to 10 cm in length. Conversely, the synthesis of precisely defined unnatural molecular archi-
tectures beyond 25 nm in length is often unattainable due to solubility, material through-put, and
characterization constrains[12]. Therefore, as nanotechnological needs advance, syntheses could
rely upon self-assembling strategies using natural scaffolds as templates for the construction of
synthetic nanostructures[13]. Most studies involving DNA self-assembly have focused on the du-
plex interactions between complementary DNA strands, the insertion of synthetic units through
covalent tethering to specific nucleotides involving the assembly of materials through electrostatic
interactions on the phosphate groups along a DNA backbone[14].
The goal of tissue culture is to develop a synthetic alternative to orthopedic tissues such as
bone, ligament, and cartilage. Once implanted, the presence of cells and growth factors will initi-
ate bone regeneration throughout the 3-D pore
network.As regeneration continues, the matrix is
slowly resorbed by the body. Upon complete deg-
radation, the implant site is filled with newly re-
generated bone and free of any residual polymer.
Examples are given about tissue enginereered bone
(shown in Fig. 3), tissue engineered ligament
(shown in Fig. 4), tissue engineered cartilage
(shown in Fig. 5), and Cell Culture (shown in Fig.
6)[15]. In studies examining cell growth on these
matrices, osteoblast cell lines have been utilized asFig. 3. Tissue engineered bone.
300 SCIENCE IN CHINA (Series A) Vol. 44
well as bone cells isolated from rat calvaria to
create model systems for cell growth on bio-
erodable materials. This image shows os-
teoblasts growing on the surface of the 3-D
microsphere matrix. It is interesting to note that
the cells are growing in a circumferential pat-
tern due to the structure of the matrix.
Biopolymerization has been used to gen-
erate intelligent and smart materials. Very often
one can find them in sensoring and electronic
devices.
Fig. 5. Tissue engineered cartilage. Fig. 6. Cell culture.
Materials characterization methods are mechanical testing, chemical analysis, microstructural
characterization, explant analysis, coatings evaluation, development of special test techniques and
component design and analysis include structural analysis, reliability and probabilistic modeling,
engineering design optimization, failure analysis. Characterization is particularly difficult when
the materials are used for human body that needs in vivo test conditions.
An atomic force microscope (AFM) image is widely used in materials analysis at nanometer
scale[16, 17]. It is also used in biomaterial research. Fig. 7 is an AFM image, which shows the sur-
face structure of a hydrated Staphlococcus epidermis biofilm formed on an implant surface[18]. The
channels observed are believed to provide nourishment that sustains the biomaterials-associ- ated
infection. Using an AFM, cortical bone is imaged at ultrahigh magnification (shown in Fig. 8)[19].
Advanced in situ microscopy testing devices allow biomaterials and biological materials to be
observed at high magnification with the AFM while being subjected to controlled levels of stress
or deformation.
Scanning electrochemical microscopy (SEM) is used to characterize the reaction rate and ion
release imaging modes of metallic biomaterials[20]. The scanning potential microscope (SPM) can
Fig. 4. Tissue engineered ligament.
Supp. TRIBOLOGICAL APPLICATIONS OF BIOMATERIALS: AN OVERVIEW 301
also be used in measuring the micro-corrosion processes on metallic biomaterial surfaces. The
SPM has imaged localized preferential corrosion areas on the samples. Localized corrosion poten-
tial is clearly associated with topographic features[21].
Fig. 7. AFM image of a hydrated. Fig. 8. AFM image of bone Staphlococcus
epidermis biofilm.
Confocal laser-scanning microscopy (CLSM) is a rapidly advancing technique used to pro-
duce crisp and precise images of thick specimens in fluorescent and reflective light modes by re-
jection of out-of-focus light via a confocal pinhole. This feature of confocal microscopes, known
as optical sectioning, makes it possible to scan a sample at various x-y planes corresponding to
different depths, and, by ordering these planes into a vertical stack, reconstruct a three-dimen-
sional image of the specimen. Because it does not require physical sectioning of thick samples,
and precludes the need for extensive specimen processing, CLSM is one of the most efficient
methods available to gain three-dimensional information on living biological specimens and bio-
materials.
Auger electron spectroscopy (AES) is then used to identify and quantify calcium and phos-
phate, the major inorganic components of bone. The deposited mass depends on the surface
chemistry and a clear correlation between surface pretreatment and calcium/phosphate ratios is
observed. This can in some cases also be correlated to macroscopic properties such as the surface
wetability[22].
Materials Scientists continue to develop novel surface modifications to enhance textile per-
formance. Wettability is one important characteristic of a surface. Wettability of textile fibers can
be enhanced to assist the dyeing operation during processing; and the fibers can be made to be
wettable for application in, for example, materials of tissue culture.
Wear test is a basic method to evaluate the durability of biomaterials. Pin-on-disc experi-
ments are carried out on different material combinations using systematically varied tribological
conditions (lubrication, loading, contact geometry, sliding speed). Novel candidate materials will
be investigated, as well as current materials (UHMWPE) that have been modifies by different
methods (e.g. plasma technology, microfabrication, and polymer processing). The chemical and
302 SCIENCE IN CHINA (Series A) Vol. 44
structural properties of the tested materials and wear products are characterized by spectroscopic
and microscopic analysis techniques. Results from multi-axial joint simulator experiments, as well
as analysis of components from retrieved clinical implants constitutes references for the in vitro
simulation studies.
The material properties and the mechanical and electrochemical processes were observed to
control the corrosion attack to the hip replacement. Some electrochemical tests methods list as
following, Short term tests: the effect of cyclic load magnitude on fretting corrosion currents;
Long term tests: rest tests, cyclic mechanical loading, scratch test, etc[7, 8, 9].
Artificial mechanical heart valves are made of pyrolytic carbon to prevent complications as-
sociated with blood clotting, but this material can be subject to cyclic fatigue. An acoustic emis-
sion-based system is developed for detecting crack initiation and the growth of existing flaws
during controlled stress testing of artificial heart valves.
2 Tribological applications and failure
The properties of biomaterials to date still do not meet the application requirements. The part
of many potential causes of failure for the total hip arthroplasty, for example, is[23] deficiencies in
design (size and shape) of the device for a particular patient (e.g., an undersized noncemented
stem); surgical problems (e.g., problematic orientation or problems in wound healing); host ab-
normalities or diseases (e.g., osteopenia); infection; and material fracture, wear, and corrosion[11].
Synovial joints, including the knee joints, are typically found at the ends of long bones and
permit a wide range of motion[24]. This type of joint is surrounded by a fibrous articular capsule,
which is lined by a thin synovial membrane. It is important to note that the surfaces of each bone
are not in direct contact, but are covered by special articular cartilages, which resemble the hyaline
cartilages found elsewhere in the body.
The synthetic biomaterials, such as stainless steel, titanium alloy, polymers, and ceramic
composite, undergo degradation through fatigue and corrosive wear due to load-bearing and the
salty environment of the human body. At the same time, deposits of inorganic salts can scratch
weight-bearing surfaces, making artificial joints stiff and awkward. The lifetime of an implant is,
at most, 10 to 15 years.
Using the methods previously mentioned, the failure mechanisms reported are discussed with
a few examples.
Fracture happens often in bone and orthropedic implants when load-bearing ability is impor-
tant. Once fractured, biomaterials cannot be regenerated as nature bones. Therefore, structure and
properties are particularly important here. It is also very important to induce lubricants to improve
the biomaterial properties in the same time. The inducing of the lubricants could reduce the wear,
which is one of the main problems that could lead to fracture of the materials. Some research
works have been done in investigating the lubricant thin film’s failure[25, 26].
Wear of the articulating surfaces in artificial hip and knee joints gives rise production of wear
Supp. TRIBOLOGICAL APPLICATIONS OF BIOMATERIALS: AN OVERVIEW 303
particles of sizes of submicrons and larger. The negative biological effects of these wear particles
are considered to be one important factor that limits the long term clinical performance of this
type of devices. There is therefore an immediate need for development of novel and improved
material combinations for the articulating surfaces in artificial joints. Such development is in turn
dependent on an improved understanding of the wear processes involved and how these are influ-
enced by different material properties and conditions. Equally important is it to develop reliable
and predictive methods for stimulating the wear processes under in vitro conditions, preferentially
in an accelerated way.
In order to provide a basis for systematic development of novel material combinations and
surfaces with improved wear properties and to develop reliable in vitro screening methods for as-
sessing the wear of different candidate combinations, the wear research is done to obtain an in-
creased understanding of the mechanisms underlying wear in artificial joint protheses. The long
term engineering goal is to develop a material (surface) combination which does not produce bio-
logically harmful wear particles under the physiological conditions that prevail in artificial hip and
knee joints.
A total hip replacement is clinically successful with most designs that utilize an ultrahigh
molecular weight polyethylene (UHMWPE) acetabular surface articulating with either a metallic
or ceramic femoral head component[27]. However, aseptic loosening, often accompanied by oste-
olysis, continues to be a source of long-term clinical failure. It was suggested that the macrophage
response to phagocytosis of particulate wear debris, occurring in interaction between the cement
and bone, was an important causative factor in osteolysis, leading to eventual loosening[28]. The
particulate debris has emerged as a major factor in the long-term performance of joint replacement
prostheses[29]. When present in sufficient amounts, particulates generated by wear, fretting, or
fragmentation induce formation of an inflammatory, foreign-body granulation tissue that has the
ability to invade the bone-implant interface[30�37]. This may result in progressive periprosthetic
loss of bone, a loss that threatens the fixation of prostheses inserted with or without
cement[12, 38, 39].
Studies of wear debris extracted from actual tissue samples of patients whose implants failed
as a result of aseptic loosening have generated significant information regarding wear particle size,
shape, and surface morphology. AFM was used to produce detailed, high resolution images of
wear particles, few hundred nanometers in size. Wear debris studied sometimes exhibits a cauli-
flower-like surface morphology. By combining wear debris and cellular response studies, engi-
neers and biologists will be able to better understand implant failure and to re-engineer implants to
prevent future problems
The wear problem that occurs with an artificial joint implant component (socket) constructed
of UHMWPE is illustrated in Fig. 9. At left is unworn UHMWPE. The sample at right has under-
gone a friction and wear test versus cobalt chromium (artificial joint ball material). The fibrillation
and small particles are characteristic of an adhesive wear mechanism, which can result in sur-
304 SCIENCE IN CHINA (Series A) Vol. 44
rounding bone loss and the need for implant replacement[1].
Fig. 9. SEM micrographs of socket of UHMWPE before and after wear test(20,000X).
Corrosive attack in the taper crevice of modular implants made of similar metals or
mixed-metals combinations[40�44]. The corrosive attack results in metal release and mechanical
failure of the component[45]. Corrosive fatigue, on the other hand, is expected due to cyclic loading
and the corrosive environment of human body. The fractures occurred at the grain boundaries of
the microstructure and appeared to be the result of three factors: porosity at the grain boundaries;
intergranular corrosive attack, initiated both at the head-ceck taper and at free surface; and cyclic
fatigue-loading of stem[46].
3 Summary
Biomaterials are the future of the medicine. From tissue removal, replacement, to future re-
generation, biomaterials will take part the most advancement in the near future. Biomaterials and
implant research will continue to concentrate on serving the needs of medical device manufactur-
ers and recipients, as well as medical professionals. Development of technologies will meet those
needs by designing the materials in strength, shape, function, and behavior. Future biomaterials
will incorporate biological factors (such as bone growth) directly into an implant’s surface to im-
prove biocompatibility and bioactivity. New projects will be directed at materials development for
improved mechanical integrity, corrosion resistance, and biocompatibility. The understanding of
the nature of the human body parts like knee, jaw, dental parts, and hip, etc., will lead to the ap-
plying of these techniques in future materials design.
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