1. Introduction
The bioactivity of implantable biomaterials can be strengthened by the
formation of biologically active bone-like apatite coatings on its surface.
Several methods have been used to prepare such apatite coatings on
implantable biomaterials [1], [2], [3], [4] and [5]. Among them, a biomimetic
process, which enables the formation of a dense and uniform layer of
bone-like apatite on various kinds of materials in different shapes, has been
widely used. In this process, the substrate was soaked in a simulated body
fluid (SBF) with ion concentrations nearly equal to those of human body
plasma at 36.5 °C [6]. The use of the biomimetic method to form an apatite
layer on implantable materials was very attractive because of its low
formation temperature, uniform thickness, excellent adhesion and
inexpensive production cost. To date, apatite coatings have been prepared
on a large number of materials including surface-modified silk fabrics [7],
surface-modified cotton [8], bioglass [9], silica gel [10], Titanium [11],
Tantalum [12], polyamide films [13], CM-chitin and gellan gum gels [14],
Polyethyleneterephthalate (PET) [15], and polytetrafluoroethylene (PTFE)
membranes [16] by this biomimetic process. In view of the widespread use of
the biomimetic process, it is rather surprising that the literature documents
seldom reported the formation of apatite coatings on conducting polymers by
this method.
Polypyrrole (PPy) is one of the most frequently investigated conducting
polymers for its good stability and high conductivity. In recent years, the
biomedical applications of PPy have been reported because of its good
biocompatibility with the mammalian cells and the ability to modify cellular
activities by electrical stimulation [17], [18], [19] and [20]. Therefore, PPy and
its composite might be used as the next generation implantable biomaterials,
which will be capable of interactive and programmable, and thus capable of
seamless communication with surrounding tissues, to regulate cell
attachment, proliferation, and differentiation through the electrical stimulation.
So it would be very attractive to incorporate electroactive PPy with the
bone-like apatite to prepare a novel bioactive composite which would be a
desirable candidate for the coatings of the metal implants and tissue
engineering scaffolds due to its ability to bond to bone as well as to affect
cellular activities through electrical current or field.
In the present paper, we show that a thin bone-like apatite layer can be
fabricated on PPy substrate doped with heparin sodium and P-TSA by the
biomimetic process using SBF. The result means that PPy itself, when
doped by some chemical substances containing special functional groups,
may exhibit bone-bonding ability, i.e. bioactivity. This work serves as an
important first step toward the development of PPy based biomimetic
implantable composite with enhanced tissue–implant interactions. In order to
reveal the fundamental conditions for obtaining apatite/PPy composite
materials using biomimetic method, the preparation of the apatite coatings
on PPy, which formed with and without dopants, were investigated. The
possible mechanism responsible for the formation of apatite coatings was
also discussed.
2. Experimental
Pyrrole monomer was purified by distillation under nitrogen atmosphere
before use, stored at low temperature and protected from light. All other
reagents were of analytical grade and were used as received. De-ionized
water was used throughout this work.
Pyrrole (0.15 ml dissolve in 20 ml de-ionized water), P-TSA (0.30 g), and
heparin sodium salt (0.05 g) were dissolved in 20 ml de-ionized water and
stirred magnetically in the ice bath. Then an aqueous solution of (NH4)2S2O8
(0.80 g dissolve in 20 ml de-ionized water) was added to the above solution
slowly (0.5 ml/min). The mixture was stirred for another 10 min to ensure
complete mixing. Then the polymerization was allowed to proceed without
agitation for 24 h at ice bath. PPy doped with heparin sodium salt and P-TSA
was obtained after filtration and rinsing with de-ionized water for several
times and dried at 80 °C overnight. Finally, as-obtained PPy was
compressed into pellets of 15 mm diameter and about 1mm in thickness.
Meanwhile, PPy also synthesized without dopants through the same process
for a comparative study.
The obtained PPy pellets were soaked in 30 ml of SBF, which was prepared
by dissolving appropriate quantities of the reagent-grade chemicals of NaCl,
NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2 and Na2SO4 in de-ionized
water and buffered at 7.40 using tris-hydroxymethylaminomethane
((CH2OH)3CNH2) and hydrochloric acid at 36.5 °C [18], for a predetermined
periods. Then the samples were removed from the solution, washed with
de-ionized water and dried at room temperature.
X-ray diffraction (XRD) and energy-dispersive X-ray analysis (EDX) were
used to characterize the elemental composition and surface chemistry of the
SBF-immersed samples. The surface morphology of the specimens was
observed by a scanning electron microscopy (SEM, LEO1530).
3. Results and discussions
Fig. 1 shows SEM photographs of the surfaces of doped PPy samples after
soaking in SBF for 0, 7, and 21 days. It can be seen clearly from these
pictures that particles were formed on the doped PPy. This particle
morphology is similar to that of apatite crystals formed on polyamide films
after having been soaked in SBF [21]. The EDX results distinctly showed that
the particles contained predominantly calcium and phosphorus (Fig. 2).
(114K)
Fig. 1. SEM images of the surfaces of doped PPy before and after soaking
in SBF for 7 and 21 days. Doped PPy without soaking is denoted as “0 d”.
(18K)
Fig. 2. EDX spectra of doped PPy after soaking in SBF for 7 and 21 days.
Fig. 3 shows XRD patterns of the surfaces of doped PPy before and after
immersion in SBF for 7 and 21 days. XRD peaks were observed at
2θ = 26°and 32° after soaking and these peaks were all ascribed to a
crystalline apatite, indicating that the particles formed on doped PPy in Fig. 1
are apatite.
(4K)
Fig. 3. XRD patterns of doped PPy before and after soaking in SBF for 7
and 21 days. Doped PPy without soaking is denoted as “0 d”.
Fig. 4 shows the SEM photographs of the surface of undoped PPy before
and after soaking in SBF for 21 days. As shown in Fig. 4, there is no obvious
difference between the morphologies of undoped PPy even after soaking in
SBF for up to 21 days, which means that apatite could hardly formed on
undoped PPy. Therefore, we considered that the dopants might play an
important role in the formation of the apatite layers on the PPy substrate.
(91K)
Fig. 4. SEM images of the surfaces of PPy before and after soaking in SBF
for 21 days. PPy without soaking is denoted as “0 d”.
It has been reported that some organic polymers containing negatively
charged functional groups, such as carboxyl groups [22], sulfonic groups [21]
and phosphate groups [16] have ability to induce deposition of apatite on
their surfaces in SBF. Thus, it is reasonable to postulate that the apatite layer
formed on doped PPy are induced by the heparin sodium salt and P-TSA
which contained carboxyl groups (
COOH) and sulfonic groups (
SO3H)
and been used as dopants in the synthesis of PPy. Under the induction of
these dopants, apatite nuclei were formed and spontaneously grew to
apatite crystal by consuming the calcium and phosphate ions from the
surrounding fluid.
4. Conclusions
Apatite coatings were fabricated on the surface of PPy doped with heparin
sodium and P-TSA by a biomimetic process utilizing SBF. We propose that
the formation of apatite depended on the presence of carboxyl groups and
sulfonic groups contained dopants. The resulting apatite/PPy composite may
have great potential for the development of conducting polymers based
bioactive coatings on metal implants and tissue engineering scaffolds. We
are currently exploring the factors affecting the formation of apatite coatings
in order to increase the content of apatite in the apatite/PPy composite within
a relatively short period.
Acknowledgements
This work was supported by the Research Foundation of Xiamen University
(No. 2003xdyy28).
References
[1] M. Stewart, J.F. Welter and V.M. Goldberg, J. Biomed. Mater. Res. 69
(2004), pp. A1–A10.
[2] C.C. Almeida, L.A. Sena, A.M. Rossi, M. Pinto, C.A. Muller and G.A.
Soares, Key. Eng. Mat. 254 (2004), pp. 729–732. Abstract-Compendex
[3] H.B. Hu, C.J. Lin, P.P.Y. Lui and Y. Leng, J. Biomed. Mater. Res. 65
(2003), pp. A24–A29.
[4] S. Hirai, K. Nishinaka, K. Shimakage, M. Uo and F. Watari, J. Am. Ceram.
Soc. 87 (2004), pp. 29–34. Abstract-INSPEC | Abstract-Compendex
[5] W.H. Song, Y.K. Jun, Y. Han and S.H. Hong, Biomaterials 25 (2004), pp.
3341–3349. SummaryPlus | Full Text + Links | PDF (513 K)
[6] T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi and T. Yamamuro, J.
Biomed. Mater. Res. 247 (1990), pp. 21–34.
[7] Y. Tamada, T. Furuzono, T. Taguchi, A. Kishida and M. Akashi, J.
Biomater. Sci. Polymer. Edn. 10 (1999), pp. 787–793. Abstract-MEDLINE |
Abstract-EMBASE
[8] M.R. Mucalo, Y. Yokogawa, T. Suzuki, Y. Kawamoto, F. Nagata and K.
Nishizawa, J. Mater. Sci. Mater. Med. 6 (1995), pp. 658–669.
Abstract-Compendex | Abstract-EMBASE
| Full Text via CrossRef
[9] L.L. Hench and O. Andersson, Intro. Bioceramics (1993), pp. 41–62.
[10] P. Li, C. Ohtsuki, T. Kokubo, K. Nakanishi, N. Soga and K. Degroot, J.
Biomed. Mater. Res. 28 (1994), pp. 7–15. Abstract-MEDLINE |
Abstract-EMBASE | Abstract-Compendex
| Full Text via CrossRef
[11] T. kokubo, H.M. Kim, M. kawashita and T. Nakamura, J. Mater. Sci.
Mater. Med. 15 (2004), pp. 99–107. Abstract-INSPEC | Abstract-Compendex
| Abstract-EMBASE | Abstract-MEDLINE
| Full Text via CrossRef
[12] T. Miyazaki, H.M. Kim, T. Kokubo, C. Ohtsuki, H. Kato and T. Nakamura,
Biomaterials 23 (2002), pp. 827–832. SummaryPlus | Full Text + Links | PDF
(272 K)
[13] T. Kawai, C. Ohtsuki, M. Kamitakahara, T. Miyazaki, M. Tanihara, Y.
Sakaguchi and S. Konagaya, Biomaterials 25 (2004), pp. 4529–4534.
SummaryPlus | Full Text + Links | PDF (570 K)
[14] M. Kawashita, M. Nakao, M. Minoda, H.M. Kim, T. Beppu, T. Miyamoto,
T. Kokubo and T. Nakamura, Biomaterials 24 (2003), pp. 2477–2484.
SummaryPlus | Full Text + Links | PDF (352 K)
[15] H.M. Kim, K. Kishimoto, F. Miyaji, T. Kokubo, T. Yao, Y. Suetsugu, J.
Tanaka and T. Nakamura, J. Biomed. Mater. Res. 46 (1999), pp. 228–235.
Abstract-MEDLINE | Abstract-EMBASE
| Full Text via CrossRef
[16] L. Grondahl, F. Cardona, K. Chiem, E. Wentrup-Byrne and T. Bostrom, J.
Mater. Sci. Mater. Med. 14 (2003), pp. 503–510. Abstract-Compendex
Full Text via CrossRef
|
[17] B. Garner, A. Georgevich, A.J. Hodgson, L. Liu and G.G. Wallace, J.
Biomed. Mater. Res. 44 (1999), pp. 121–129. Abstract-MEDLINE |
Abstract-EMBASE | Abstract-Compendex
| Full Text via CrossRef
[18] J.H. Collier, J.P. Camp, T.W. Hudson and C.E. Schmidt, J. Biomed.
Mater. Res. 50 (2000), pp. 74–84.
[19] E.D.E. Giglio, L. Sabbatini, S. Colucci and G. Zambonin, J. Biomater. Sci.
Polym. Edn. 11 (2000), pp. 1073–1083.
[20] H. Castano, E.A. O’Rear, P.S. McFetridge and V.I. Sikavitsas, Macromol.
Biosci. 4 (2004), pp. 785–794. Abstract-EMBASE | Abstract-Compendex |
Abstract-Elsevier BIOBASE | Abstract-MEDLINE
| Full Text via CrossRef
[21] T. Kawai, C. Ohtsuki, M. Kamitakahara, T. Miyazaki, M. Tanihara, Y.
Sakaguchi and S. Konagaya, Biomaterials 25 (2004), pp. 4529–4534.
SummaryPlus | Full Text + Links | PDF (570 K)
[22] M. Kawashita, M. Nakao, M. Minoda, H.M. Kim, T. Beppu, T. Miyamoto,
T. Kokubo and T. Nakamura, Biomaterials 24 (2003), pp. 2477–2484.
SummaryPlus | Full Text + Links | PDF (352 K)