Biomaterials Practice Questions for Exam 2 WITH
Complete Solutions.
Pure Titanium was historically used as a femoral stem. The crystal structure of titanium at
ambient temperature and pressure is close-packed hexagonal (α-Ti). At about 890°C, the
titanium undergoes an allotropic transformation to a body-centered cubic (β) phase which
remains stable to the melting temperature.
a. Draw a unit cell of β-Ti.
b. How many atoms are inside a unit cell of β-Ti?
c. While the femoral part is typically made of titanium alloy, the acetabular liner is made of
Ultra High Molecular Weight Polyethylene (UHMWPE). Explain briefly why. - Correct
Answers b. 1 center atom + 8 corner atoms = 1(1) + 8(1/8) = 2 atoms
c. Ti has poor wear resistance, but is mechanically superior to UHMWPE and thus a better
choice for the load-bearing femoral stem. UHMWPE has better wear resistance and is
thus used in the liner that experiences routine friction between cup and ball.
The two images below show the stress trajectories obtained from a finite element analysis
of a 2D femoral-head-like structure (indicating a large region of stress applied at R and a
point force applied at M, and the other showing the X-ray microarchitectures in a femoral
head. Explain your observations of the bone structure that has adapted in response to this
loading scenario. - Correct Answers The large region of stress applied at R appears to
result in bone reorganization parallel to the direction of the load (red arrow). There
appears to be high bone density around that area to further support the applied load. Point
force applied at M appears to result in bone reorganization to the right of M (blue arrow).
This could be a result of a force at M resulting in a clockwise-direction moment near the
blue region. Increased matrix deposition may result to support this applied moment.
2.5pts for saying bone reorganizes or mentioning Wolff's Law, something along those
lines.
2.5pts for describing how it appears to reorganize. Description doesn't have to be exactly
what is written in the answer key.
Low molecular weight materials display a single value for the molar mass but synthetic
polymers display a spectrum of molar mass. Describe the differences between
monodisperse and polydisperse polymers - Correct Answers Answer: Monodisperse
polymer means a polymer with almost no polydispersity, i. e. PDI = 1, σ = 0. Monodisperse
polymers are composed of building blocks with almost identical molecular weights,
whereas polydisperse polymers have a range of MWs, although the fundamental building
blocks are the same in each polymer.
,What are the main stages in bacterial biofilm formation? In addition, give examples of
approaches that could be used to limit biofilm growth that would work on three of these
different stages (i.e., three different strategies, each intended to block a different stage of
biofilm maturation). - Correct Answers ANSWER: 1. Initial attachment of single cells and
clumps of detached biofilm bacteria, 2. Production of EPS to more firmly adhere cells to
the surface 3. Early development of biofilm clusters by clonal expansion 4. Mature biofilm
in a steady state 5. Dispersion of single cells by swarming dispersion. Different options for
control. Passivate surface to prevent initial attachment, make the surface anti-microbial,
etc etc.
Name two benefits of metallization of a surface. - Correct Answers Allows for very thin
modification, also increased resistance to delamination, also leads to functional groups
that can be used for strong bonding, increases durability in some applications, etc.
How does temperature impact the Young's Modulus of a polymer? - Correct Answers
Higher temperatures lead to lower young modulus of the material.
Explain the effect of crystallinity on the thermal behavior of a polymer. Specifically, as
temperature is increased, how does crystallinity impact the Young's Modulus of a polymer
as it is heated? - Correct Answers Crystallinity is known to increase the values of Young's
modulus and strength for polymers.
Explain how molecular weight of a polymer impacts the tensile strength of a polymer. -
Correct Answers Higher molecular weight polymers are seen as providing a greater
number of the chains, thus giving the fiber a higher tensile strength.
The image given below shows a helical fracture in a femur bone. Explain your
observation. - Correct Answers A helical fracture line is surely the result of torsional
loading. Referring to the Mohr's circle and stress elements, it is known that under a
torsional load, the femur could break either in a shear failure mode or a tensile failure
mode.
If in a shear-failure mode, the fracture surface should be in a cross-cutting plane
perpendicular to the shaft of the femur because the this plane is where the maximum
shear stress lies, and if in tensile-failure mode, the fracture surface will follow a helical line
due to the existence of the maximum tensile stress following a helical pattern. From the
fracture shown, it is believed that the bone fractured under a torsional load. However, the
load was applied very likely at a relatively high speed such that the bone behaves more
like a brittle material. In this situation, the tensile strength of the bone becomes the
weakest link because it will not be able to resist the highest tensile stress generated in the
bone, thereby leading to a tensile-stress induced helical fracture due to the high-speed
torsional load.
This example highlights the fact that regardless the external loading situations, the actual
fracture mode in a material is always dictated by the internal stress state and the nature of
material, as well as the loading speed in the case of materials of viscoelastic nature
, A titanium dental implant was manufactured by the Biomatter Company for the past
8years. It performed well clinically. For economic reasons, manufacturing of the titanium
device was outsourced to Metalsmed, Inc. Early clinical results on this Metalsmed
implant, supposedly identical to the Biomatter implant, suggested increased inflammation.
How would you compare the surface chemistry and structure of these two devices to see if
a difference that might account for the difference in clinical performance could be
identified? - Correct Answers To compare the surface chemistry of the product I would first
use ESCA to identify all the elements that are on the surface of the product. This will
initially confirm that the same material is indeed being used. Next, I would use SIMS
because it provides unique information that is complementary to ESCA and can greatly
aid in understanding the surface composition. After that, I would use Infrared
Spectrometry to reveal the specific functional groups that are on the surface. To compare
the surface structure of the product I would use SEM to obtain a high-resolution image of
the surface (material microstructure). Next, I would use either AFM or STM to provide
another image of the surface. Since titanium is a poor conductor of electricity, AFM would
likely be the superior choice in this case.
How is critical surface tension related to wettability? - Correct Answers Contact angles
directly measure surface wettability. Surface tensions are calculated using the measured
contact angle. Poor wettability (hydrophobic surfaces) is seen with higher surface
tensions, while good wettability (hydrophilic surfaces) is seen with lower surface tensions.
Critical surface tension is 27 dyne/cm which is found by extrapolating to zero on a surface
tension vs. surface angle graph.
List all the surface analysis methods learned. - Correct Answers (1) Contact Angle (2)
Electron Spectroscopy for Chemical Analysis (ESCA)/ X-ray Photoelectron Spectroscopy
(XPS) (3) Secondary Ion Mass Spectroscopy (SIMS) (4) Scanning Electron Microscopy
(SEM) (5) Fourier Transform Infrared (FTIR) Spectrometry (6) Atomic Force Microscopy
(AFM)/ Scanning Tunneling Microscopy (STM)
Explain the principle of surface analysis method: Contact Angle - Correct Answers An
equilibrium is established between surface tension forces contracting a liquid drop to a
spherical shape and forces interacting the drop with the surface. The force balance
between the liquid-vapor surface tension of a liquid drop and the interfacial tension
between a solid and the drop, manifested through the contact angle of the drop and can
be used to quantitively characterize the surface-vapor interfacial tension. Contact angles
directly measure surface wettability and indirectly probe surface energy, roughness,
heterogeneity, contamination, and molecular mobility. A high contact angle indicates poor
wetting and means that the surface is hydrophobic. A low contact angle indicates good
wetting and means that the surface is hydrophilic.
Explain the principle of surface analysis method: Electron Spectroscopy for Chemical
Analysis (ESCA)/ X-ray Photoelectron Spectroscopy (XPS) - Correct Answers ESCA is
one of the most important and common tools and is based on the photoelectric effect. X-
rays are focused upon a specimen, and the interaction of the X-rays with the atoms in the
