Kenneth E. Gonsalves, Craig R. Halberstadt, Cato T. Laurencin, Lakshmi S. Nair. Biomedical Nanostructures
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ion implantation has been widely used to improve the performance of metallic
implants. This section will review studies involving ion implantation of hard
metals.
2.4.1 Orthopedic Applications
As mentioned above, metals are used in orthopedic applications because of
their good biocompatibility and their excellent mechanical and tribological
properties. However, there are still limitations associated with their applica-
tions. For example, a typical concern with titanium is its relatively poor wear
resistance. Among various procedures used to enhance the performance of
metallic orthopedic products, ion implantation has been the most commonly
used treatment. Ion implantation can harden the surface and reduce the
friction coefficient, ultimately improving the wear resistance [43]. Buchanan’s
work [44] demonstrated that impregnation of Ti6Al4V with nitrogen ion
significantly improved the material’s resistance to wear-accelerated corrosion
in both saline and serum solutions at all applied stress levels. Other researchers
observed order-of-m agnitude improvements in the wear resistance of nitrogen
ion implanted titanium alloys [45, 46], using traditional pin-on-disk wear tests.
Besides nitrogen, several other elements have also been used to improve wear-
corrosion and corrosion behavior of metals. Tan et al. [47] modified
nickeltitanium with oxygen at three levels of implantation doses (5 10
16
,
1 10
17
, and 3 10
17
ions/cm
2
). Topographical changes were induced by
oxygen ion implantation. Grooves that wer e originally present on the NiTi
surface were smoothed by low and medium dose oxygen ion implantation, and
the medium dose oxygen-implanted samples (1 10
17
ions/cm
2
) showed the
best corrosion resistance; however, nanopores appeared on the surface of the
modified materials at the high dose (3 10
17
ions/cm
2
), which acts as the
source of pitting corrosion and impairing the resistance of pitt ing co rrosion.
Krupa et al. [48] studied how the modification of titanium surfaces by calcium
ion implantation affects the corrosion resistance of titanium. It was found that
calcium ion implantation at a dose of 1 10
17
ions/cm
2
increases the corrosion
resistance of titanium under stationary conditions, but, at longer exposure
times, the susceptibility of calcium ion implanted titanium to pitting corrosion
under anodic polarizations increases. In a subsequent study [49], phosphorous ion
implantation was performed on titanium surface at a dose of 1 10
17
ions/cm
2
.
The process led to amorphization of the surface layer and the formation of TiP.
It also increased the corrosion resistance after short-term as well as long-
term exposures, suggesting the advantageous effect of phosphorous ion
implantation.
Great effort has also been devoted to improve the integration of metallic
implants with the surrounding bone tissue. It is widely known that ceramic
materials bind well to bone tissue due to the advantageous effect of their
components CaO and P
2
O
5
, which are capable of nucleating hydroxyapatite
36 BIOMEDICAL NANOSTRUCTURES
(HA). HA has many crystallographic features similar to those of the natural
apatite present in bone, and therefore has been frequently used as coatings to
create the bone-like surface chemistry that will be beneficial for osseointegra-
tion. Metals such as titanium do not support precipitation of HA intrinsi cally.
In order to endow the metal surface with factors capable of being actively
involved in the osseointegration process, researchers have studied how calcium
ion implantation and phosphorous ion implantation modify surface chemistry
and bioactivity both in vitro and in vivo. Hanawa et al. [50] demonstrated
in vivo that the implantation of Ca
+
ions into Ti surfaces enhanced
osseointegration and the formation of new osteoid tis sue, possibly as a result
of more rapid calcium phosphate precipitation on the implanted surface. Pham
et al. [51] studied surface-induced reactivity for Ti by ion implantation of Ca
and P, respectively, at several implantation energy steps. Needle-like HA
crystallites were found on all implanted surfaces, but absent on the con trol
samples without any ion implantation, suggesting that Ca and P ion
implantation rendered the Ti surface active in the biomineralization process.
In vitro culturing of bone cells on the Ca ion implanted Ti samples showed that
the cellmate rial interaction was altered and was dependent on the ion
implantation dosage [52–54]. High dose of Ca ions (1 10
17
ions/cm
2
)
significantly enhanced cell spreading, formati on of focal adhesion plaques,
and expression of integrins, despite the initial decrease in cell adhesion. Several
groups [55–57] also studied the effect of dual ion implantation of Ca and P into
Ti and its alloys. It was fou nd that the binding energies of the Ca 2p, P 2p, and
O Is peaks of the modified surface layer about 100 nm thick are the same as the
hydroxyapatite standard, indicating the formation of an apatite-like nanoco at-
ing on the surface by the successive implantation of calcium and phosphorous.
The improved biocompatibility was further confirmed by the growth of bone
marrow cells on the treated surfaces.
Sodium has also been implanted into titanium to improve its bone
conductivity [58–60]. The implantat ion induced a modification in chemistry
and morphology, producing morph ologically rugged surfaces and showing
sodium titanate incorporated within the surface layer with concentration,
depth distribution, an d morphology depending on the parameters of the ion
implantation. The growth of bone-forming cells on the Na ion implanted
surfaces was significantly improved from that on the untreated samples,
evidenced by the high alkal ine phosphatase activity and better spreading of
cells [60].
Amino groups (NH
2
+
) have been implanted to modify Ti surfaces [61, 62].
Because of the formation of TiN during the implantation process and the
ability to create depths as deep as 61 nm, properties of the Ti, such as wear,
corrosion, and fatigue resistance, have been improved. In addition, the
attachment and spreading of cultured osteoblasts onto NH
2
+
implanted Ti
was better than on hydroxyapatite and there was also an increased calcification
on its surface.
MICRO/NANOMACHINING AND FABRICATION OF MATERIALS 37
2.4.2 Dental Implants
Dental applications use metallic implants for several therapies, including root
form implants, endodontic stabilizers, and dental blades. Similar to orthopedic
applications, it is also desirable to have faster and stronger bone bonding to the
dental implant. Surface modification by ion implantation has been used to
enhance the efficacy of these metallic implants. De Maeztu et al. [63] evaluated
bone integration of dental implants habitually used in clinical practice and
subjected to ion implantation surface treatment. Both commercially pure
titanium dental implants and Ti6Al4V alloy implants, untreated or treat ed with
either one of the following ions, carbon oxide (CO
+
), nitrogen (N
+
), carbon
(C
+
), and neon (Ne
+
), were examined. The study found that surface treatment
of dental implant surfaces by ion implantation to improve bone integration not
only quantitatively (higher percentage contacts between bone and implant), but
also qualitatively (with strong and more stable binding). There was a statistical
difference found between the treated implants and controls, especially for C
+
ion implantation in Ti, and for CO
+
implantation in Ti6Al4V where the
existence of covalent bonding between the TiOC atoms was confirmed by
X-ray photoelectron spectr oscopy. Lee et al. [64] tried to successfully improve
the surface hardness of endodontic Nitinol (nickeltitanium alloy) roo t canal
instruments using boron implantation. Alternatively, Rapisarda et al. [65]
implanted nitrogen ions into nickeltitanium end odontic instrument at
an implantation dosage of 2 10
17
ions/cm
2
and demonstrated that the ion
implantation process makes NiTi endodontic instruments more resi stant to
wear and, therefore, able to shape more pulp canals before being discarded. It
has also been shown that the ion implantation of nitrogen onto the surface of
orthodontic wires decreases the frictional forces produced during tooth
movement, thereby improving the efficiency of tooth movement [66].
2.4.3 Blood-Contacting Devices
Metals can also be used in blood-contacting devices. For example, coronary
stents are mostly made from 316L stain less steel. Carbon ion implantation on
the stainless steel stents was suggested to have the potential of reducing
restenosis caused by inflammatory reaction after coronary revascularization
[67]. Recent work by Yang et al. [68] demonstrated that phosphorous doping is
an effective way to improve the blood compatibility of titanium oxide film, and
it is related to the changes of electron structure and surface properties caused
by phosphorous doping. On the contrary, ion implantation can also be used to
improve thrombogenicity. Murayama et al. [69] modified Guglielmi detachable
platinum coils (GDCs) that are used for endovascular treatment of wide-
necked aneurysms, using a combination of protein coating and Ne
+
ion
implantation at a dose of 1 10
15
ions/cm
2
. They found an intensive blood
cellular response on ion-implanted coil surfa ces, indicating that ion impl anta-
tion combined with protein coating of GDCs improved cellular adhesion and
38 BIOMEDICAL NANOSTRUCTURES
proliferation. This technology may provide early wound healing at the necks of
embolized, wide-necked, cerebral aneurysms.
2.4.4 Other Applications
As discussed above, ion implantation can also gen erate antibacterial properties
on the metallic implant surfaces. It has been shown that F
+
implanted titanium
significantly inhibited the growth in vitro of both bacteria Porphyromonas
gingivalis and Actinobacillus actinomycetemcomitans than nonion implanted
control polished titanium samples [70]. Other ions such as silver and copper
also have favorable effects on antibacterial property of the surfaces. Dan et al.
[71] demonstrated that implantation of copper ions into AISI 420 stainless steel
with a dose of 5 10
17
ions/cm
2
greatly improved its antibacterial property
against both Escherichia coli and Staphylococcus aureus.
2.5 NOVEL BIOCOMPATIBLE PHOTORESISTS
As mentioned earlier, the application of conventional photolithography for
cellular and biomolecular patterning is greatly hampered due to the fact that
organic solvents are normally utilized to dissolve the photoresist in order to
reveal the pattern on the surface of the material. Such processes can lead to
the denaturation of biomolecules, and thus, it is not suitable for application
in biomolecular surface patterning. For example, the commonly used
negative photoresist SU8, a multifunctional epoxy derivative of a bis-
phenol-A novolac, requires the use of developers such as ethyl acetate or
diacetone alcohol. Therefore, in order to create specific biocompatible
surfaces using photolithography, a new class of photoresists will need to be
developed. After all, photolithography is the current workhorse for the
microelectronics industry, and it is a very mature and developed technology,
with the capability of generating very precise, highly reproducible, and
large-scale patterns. To date, two types of new biocompatible photoresist
materials have been developed: one is based on poly(t-butyl acrylate) [72–74],
and the other is based on poly (3-(t-butoxycarbonyl)-N-vinyl-2-pyrrolidone)
[75, 76].
Both photoresists share the chemical amplifi cation mechanism; that is, the
photogenerated acid deprotects the t-butyl ester groups resulting in changes in
solubility/hydrophilicity. It has been demonstrated that with the poly(t-butyl
acrylate) type photoresist, it is possible to pattern biomolecules directly while
the resist is being processed because the biomolecules can tolerate the low
temperatures and the concentrations of the developer applied. Instead of the
commercially common developer of base concentration in the range of 0.27N,
diluted aqueous base developers of 0.0026N or lower base concentration were
sufficient to reveal the pattern, making the processes both environm ent and
MICRO/NANOMACHINING AND FABRICATION OF MATERIALS 39
biomolecular friendly. On the contrary, the development of poly(3-(t-
butoxycarbonyl)-N-vinyl-2-pyrrolidone) type photoresists has further im-
proved the biocompatibility of the lithographic patterning processes, as the
usual step of feature development by organic solvents that normally denature
biomolecules was eliminated. Upon exposure to UV light, the t-BOC groups
undergo photolysis and become carboxylic groups, resulting in a change of
surface property from hydrophobic to hydrophilic. Consequently, upon seeding
fibroblast cells onto these patterned surfaces, the cells aligned along the
hydrophilic patterned grooves. The preferential adhesions to the patterns are
most likely due to the preferential adsorption of serum proteins from culture
media onto the UV exposed area, which is more hydrophilic. The newly formed
carboxyl groups also provide possibilities to couple specific peptides or proteins
that will guide cell interaction with the substrate in a specific manner. In
addition, these photolithographic techniques can control the depth and the
pitch of the surface structures at the nanometer scale. As shown in Fig. 2.5, the
patterned surface presents topography of nanogrooves with resolution around
200 nm, which may contribute to the observed oriented cellular growth. These
important components coupled with the ability to mask different patterns of cell
adhesive and nonadhesive peptides may allow for the tailoring of complex cell
adhesive patterns onto the surface for the creation of complex tissues and
organs.
FIGURE 2.5 AFM micrographs of patterned poly(MMA-co-t-BOC-NVP) surface
(25 mm line 50 mm space) showing section analysis.
40
BIOMEDICAL NANOSTRUCTURES
2.6 THREE-DIMENSIONAL LITHOGRAPHY
A major interest in the field of tissue engineering is creating three-dimensional
scaffold that will mimic the extracellular matrix (ECM) and accommodate
mammalian cells by guiding their growth in three dimensions. ECM consists
of numerous topographical features at the nanoscale, such as nanofibers,
pores, bands, and bridges [77]. Thus, it is important to replicate these features
to ensure the creation of functional tissues in vitro. Currently, techniques that
can produce nanopatterns on biomedically relevant surfaces are limited.
Ideally, such techniques should satisfy the requirements of nanoscale
resolution, high throughput, reproducibility, high controllability of 3D
nanostructures, low cost, and ability to pattern large area [78]. Nanoimprint
lithography (NIL) has emerged as a versatile technique to meet these
requirements. NIL can generate sub-10 nm resolution features on either flat
or nonflat surfaces and has been widely used in nanodevice fabrications
ranging from electronic to biological applications [79, 80]. Recently, Hu et al.
[78] have demonstrated that NIL can be applied to fabricate three-dimensional
scaffolds consisting of multiple-layer nanostructures on biomedically
relevant substrates. Motivated by the natural 3D nanostructures on collagens,
which have a 400 nm fibrillar width and 70 nm cross-striation, they developed
a multiple-NIL process to generate similar 3D nanostructures on tissue-
culture polystyrene. Bovine pulmonary artery smooth muscle cells were
cultured on the scaffolds and the results show that these imprinted polymer
scaffolds with nanotopographical features can effectively direct the cell
orientation.
The other approach to generate 3D nanostructures is UV nanoembossing.
UV embossing offers many advantages; for example, it is usually performed at
room temperature and low pressure. This is crit ical to the patterning of delicate
substrates such as encapsulated protein-in-polymer or water-containing
hydrogels for biomedical applications [81]. Yue’s group has demonstrated
the use of UV embossing to create high aspect ratio submicrometer-scale 3D
structures on biocompatible polymers, that is, poly(ethylene glycol) diacrylate
hydrogel, with faithful replication, and the feasibility of incorporating proteins
into the patterned structures that will have potential in many biological
applications such as protein and drug delivery, as well as tissue engineering
[81, 82].
2.7 CONCLUSIONS
Advances in the lithographic processes are providing powerful and versatile
tools for biomedical en gineering research. In a recent review by
Khademhosseini et al. [83], the impact of micro/nanoscale engineering enabled
by the widespread use and availability of lithographic technologies has been
MICRO/NANOMACHINING AND FABRICATION OF MATERIALS 41
summarized, especially for tissue en gineering and biological applications. An
impressive body of research has demonstrated that surface structures even
down to the nanometer scale can regulate cell behavior and fate both in vitro
and in vivo. Therefore, the ability of generating biologically relevant structures
with dimensions ranging from nanometer to millimeter and providing exquisite
control over the composition and properties at the interface is very attr active
for biomedical engineering research. Because of its unique capabilities of
generating surface chemistry alterations and micro/nanoscale topography
simultaneously, ion implantation/lithography are becoming increasingly
important in enhancing biomaterialcell interactions. It can be envisioned
that micr o/nanoscale technol ogies will play a critical role in advancing many
clinical applications.
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