- Email: [email protected]
Affinity of amine-functionalized plasma polymers with ionic solutions similar to those in the human body
Progress in Organic Coatings 64 (2009) 322–326
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Affinity of amine-functionalized plasma polymers with ionic solutions similar to those in the human body E. Colín a , M.G. Olayo a,∗ , G.J. Cruz a , L. Carapia b , J. Morales c , R. Olayo c a
Departamento de Física, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, Col. Escandón, D.F., C.P. 11801, Mexico Departamento de Tecnología de Materiales, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, Col. Escandón, D.F., C.P. 11801, Mexico c Departamento de Física, Universidad Autónoma Metropolitana Iztapalapa, Apdo. Postal 55-534, Col. Vicentina-Iztapalapa, D.F., C.P. 09340, Mexico b
a r t i c l e
i n f o
Article history: Received 3 June 2008 Received in revised form 11 August 2008 Accepted 18 August 2008 Keywords: Plasma Biomaterials Polypyrrole Polyethylenglycol Polyallylamine
a b s t r a c t This work presents a study on the synthesis and on the interaction of allylamine, pyrrole and ethylenglycol polymerized by plasma with solutions of ionic composition similar to those in the nervous system. These polymers are attractive substrates to interact with the ionic pulses of the spinal cord due to their electrical and biocompatible characteristics. The ionic solutions were prepared with aqueous combinations of NaCl, MgSO4 , KH2 PO4 , KCl, CaCl2 , and NHCO3 . The polymers were prepared as thin films on glass substrates. The results indicated that some of the most important physical characteristics in the hydrophilicity of the polymers, roughness, porosity and functional groups, can be controlled with the energy of polymerization. The interaction between polymers and solutions was studied measuring the contact angle at the solid–liquid interface and the electrical conductivity of the polymers wet with these solutions. The contact angles were between 8◦ and 38◦ , and the electrical conductivity was in the 10−8 to 10−9 S/cm interval. The general tendencies indicate that the amine-functionalized polymers of this work are good materials to interact with the spinal cord system. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Plasma processes allow the use of a great variety of materials in biological applications due to the functional groups with N H, C N, C O and O H bonds, common in the human body, which can be added to almost any material to enhance the biocompatible superficial properties [1,2]. Compounds with some of these groups are: pyrrole (C4 H4 NH), allylamine (CH2 = CH CH2 NH2 ) and ethylenglycol (OH CH2 CH2 OH), whose chemical functionality influences the protein absorption, adhesion of cells and provides sites for immobilization of biomolecules [3]. Polypyrrole provides advantages in flexibility and biodegradability [4] and has been also used in studies of artificial muscles, due to its ability to change its volume under electrochemical stimulation [5,6]. Polymers derived from these compounds are considered attractive substrates for implants in the nervous system, because their electrical characteristics can be used to interact with the ions of K+ , Na+ or Ca2+ of this system. Additionally, as the human body contains approximately 60% of weight in electrolytic solutions, the chemical and electrical interaction at the solid–liquid interface can influ-
∗ Corresponding author. Fax: +52 55 5329 7301. E-mail address: [email protected] (M.G. Olayo). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.08.033
ence the reproduction of cells on materials implanted in biological systems and therefore on the regeneration of damaged tissues. For that reason, the aim of this work is to study the interaction of amine-functionalized plasma polymers polypyrrole (PPy), polypyrrole doped with iodine (PPy/I), polyallylamine (PAl) and polyethylenglycol (PEG) with solutions of salt concentrations similar to those in the spinal cord. Polymers formed with pyrrole have heterocyclic segments, while ethylenglycol and allylamine form chains with aliphatic linear arrangements. The objective involves the chemical and electrical interactions, which are related with the H-bonding and the ionic or polar activity between both phases. These polymers and some of their copolymers are being studied in the reconnection between neuronal cells after a severe lesion in the spinal cord [7].
2. Synthesis of polymers PPy, PPy/I, PAl and a copolymer formed with PPy and PEG (PPy/PEG) were synthesized in a 1500 cm3 cylindrical glass reactor with resistive glow discharges during 180 min at 13.5 MHz, 10−1 mbar and power between 10 and 100 W. The reactor has similar configuration reported in a previous work [8] and consists of a glass tube of 9 cm diameter and 30 cm long with Stainless Steel (SS) flanges at both sides. 3 access ports are located in the flanges
E. Colín et al. / Progress in Organic Coatings 64 (2009) 322–326
323
Fig. 1. Morphology of plasma PPy, PPy/I, PPy/PEG and PAl at different power. (a) PPy, 10 W. (b) PPy, 100 W. (c) PPy/I, 10 W. (d) PPy/I, 100 W. (d) PPy/PEG, 10 W. (f) PPy/PEG, 100 W. (g) PAl, 10 W. (h) PAl, 100 W.
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to introduce the monomers, dopants or other chemical regents to the reactor. Each electrode consists of a SS rod with a circular plate at the end and uses one of the access ports in each flange. In the syntheses of this work, the separation between the electrode plates was 10 cm. The polymers were obtained by applying an electrical field to the reactor which forms radicals and ions with the chemical species inside. The charged particles are accelerated in the electrical field and constantly collide with other particles. Depending on the energy of collision, the molecules excite, break or combine to form new structures. As the molecular weight grows; the molecules adhere to the nearest surfaces, where the chemical reactions continue. In this way, the monomers transform in dimers, trimers, and so on, reaching the dimension of polymers. The polymerization stops when the electrical field is turned off; however, some long-life radicals may remain on the surfaces and neutralize later with the after-glow atmosphere. The final polymers are thin coatings on the inner surfaces of the reactor. The film was separated from the surfaces applying acetone and water, which swelled the polymers and afterwards, they were removed with a thin spatula. The polymerization can be controlled partially with the energy applied to the system. However, although the energy promotes the polymerization, applied in excess, it tends to increase the fragmentation and crosslinking. Fragmentation changes the chemical nature of the polymers and crosslinking induces three-dimensional networks. Nevertheless, even if both factors are present to some extent, the important biological groups containing C N, N H, C O and O H bonds would still have an important participation in the final molecules, which in turn influences the interaction with electrolytic solutions.
3. Results 3.1. Morphology of polymers The morphologic analysis was performed by means of a Philips XL30 scanning electron microscope. Roughness and porosity are important factors in biomaterials, because the interstices work in favor of the retention of liquids and provide sites to lodge cells. The polymers of this work grew as consecutive layers with individual thickness related with the variations of pressure during the synthesis. The total thickness of the films increased along with the time of synthesis. The films show a compact appearance at low power. PPy shows compact layers with smooth surfaces, even at high power. The thickness of the PPy layers shown in Fig. 1(a) and (b) is 4 and 8 m, respectively. The surface of PPy/I is characterized by spherical particles and agglomerates at high power, with diameters in the interval of 250 nm to 2 m, randomly mixed. It has been shown that the dimension of particles greatly influence the hydrophobic character of the surfaces [9–11]. The approximate thickness of the single PPy/I layers varied from 3 to 8 m, see Fig. 1(c) and (d). PPy/PEG has a coarse and uneven surface with great internal porosity at high power. The pores have different profiles and depths. The layer thickness in Fig. 1(e) and (f) is 5 and 30 m, respectively. PAl grew as consecutive layers with different morphology, associated with small changes in both, pressure and temperature during the syntheses. The individual layers are compact with thickness of 1 or 2 m, see Fig. 1(g) and (h). As PPy tends to grow as compact layers, the influence of iodine and PEG on their respective polymers produces the agglomerates and porosity described before.
Fig. 2. Contact angles as a function of the energy of synthesis for: (a) PPy, (b) PPy/I, (c) PPy/PEG, and (d) PAl.
E. Colín et al. / Progress in Organic Coatings 64 (2009) 322–326
3.2. Interaction between polymers and solutions An important variable in the interface between polymers and solutions is the force needed to increase the wet area; this is known as the superficial tension and involves the gas–solid, gas–liquid and liquid–solid interfaces. These forces tend to minimize the energy in the surface, making the drop spherical; however, this effect depends on the balance of forces in each specific system. The chemical nature and concentration of the salts, morphology, electrical charges on the surface, pressure, temperature, etc., influence the entire polymer–solution interaction. This effect is studied in this work by means of the contact angles at the solid–liquid interface. The contact angles of solutions and polymers were evaluated with drops of solutions prepared with NaCl, MgSO4 , KH2 PO4 , KCl, CaCl2 , and NHCO3 in the concentrations found in spinal cord fluids. Some of the salts were tested individually (NaCl, 118 mM), others were combined in pairs (NaCl–MgSO4 , 118 and 1.17 mM, respectively), and another solution was prepared including all of them, Krebs–Ringer solution (KB) [12,13]. This last solution had the following composition: NaCl (118 mM), KH2 PO4 (1.3 mM), KCl (4.7 mM), CaCl2 (2.5 mM), MgSO4 (1.17 mM) and NHCO3 (25 mM). The contact angle of water with the polymers was also measured and taken as a reference. During these measurements, pressure and temperature remained constant. The contact angles of the different polymer–solution combinations are plotted in Fig. 2. The general tendency is a linear reduction as the power of synthesis increases. This effect indicates that more hydrophilic surfaces are obtained due to the increase in the roughness and porosity in the polymers, among other factors. From a
325
global point of view, the highest angles were obtained with distilled water and reduce depending on the chemical composition of solutes. PPy and PPy/I have similar tendencies in contact angles. The highest values were obtained with pure water and reduce when NaCl is added to the system. The angles reduce even more with all the salts of the KR solution, see Fig. 2(a) and (b). However, the lowest angles were found with the NaCl + MgSO4 solution, probably due to the electric interference that occurs when several species of ions are mixed in one solution, which is the case of KR. This effect is not as evident in the NaCl + MgSO4 combination. The influence of PEG in PPy is shown in Fig. 2(c), where the KR solution presents the highest angles, even higher than those of water. However, the lowest angles were found with the NaCl solution. The oxygen atoms distributed along the chains in the copolymer have a great influence on the hydrophilicity of this polymer. The effect of oxygen in polymeric surfaces has been studied before, in which the results show an increase of the hydrophilicity [14]. The slopes of contact angles are very similar, indicating that physical predominates over chemical factors in the absorption of solutions, due most surely to the great porosity of PPy/PEG. PAl also showed the lowest angles with NaCl followed closely with the KR solution; see Fig. 2(d). From a spinal cord perspective, represented by the KR solutions, the lowest angles were found with PAl, with values between 11◦ and 12◦ . On the other hand, the highest angles were found in PPy/PEG, 15◦ –18◦ . Nevertheless, the difference between all polymers is not more than 7◦ , which means that all of them can be good materials to interact with the spinal cord fluids.
Fig. 3. Electrical conductivity of amine-functionalized plasma polymers as a function of the added volume of ionic solutions. (a) PPy. (b) PPy/I. (c) PPy/PEG. (d) PAl.
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3.3. Electrical conductivity The electrical conductivity was calculated measuring the resistance of the polymers in a two-probe arrangement which consists of 2 flat Cu electrodes, 10 mm diameter, supported in Nylon plates [15]. The ionic conductivity was calculated with the following procedure. 3 or 4 layers of polymers were deposited in one of the electrodes. Then, one drop of solution (∼0.05 ml) was added to the polymers. 5 min later, the other electrode was put on the wet films and the Teflon plates tightened with screws and bolts to obtain a ‘good’ contact between electrodes and polymers. After that, the resistance of the wet polymers was measured with an Otto multimeter. This procedure was repeated changing the number of drops of solution, 2, 3, until 6. In any case, no liquid was clearly released from the system, although there may have been some evaporation during this procedure. In this configuration, the charges have to travel perpendicular to the film surface. Additional lectures of the polymer resistance without solutions were made in order to calculate the electronic conductivity. The electrical conductivity was calculated as a function of the volume of solution added to the polymers. The level and response of the conductivity influence the biocompatibility of any material implanted in the body modifying the communication among cells [16,17]. This is particularly important among neuronal cells. The electrical response was very similar, composed by electronic and ionic mechanisms. The electronic conductivity was calculated in the polymers in a dry condition (<20% relative humidity) and was in the 10−9 to 10−11 S/cm range, see the first point of conductivity in Fig. 3. The ionic conductivity was obtained after that point. It was promoted by the ions of the solutions and increased up to three orders of magnitude, 10−7 to 10−8 S/cm. In general terms, the ionic conductivity of each system studied in this work augments as the solutions are added to the polymers. With the first drop of solution the biggest step occurred, PPy/PEG increased up to three orders of magnitude. This conductivity was calculated in the 10−9 to 10−8 S/cm range, where the participation of ions from the salts increases the transport of charges up to one order of magnitude. The highest conductivities belong to KR and NaCl + MgSO4 solutions, however, the conductivities in all polymers are very similar. 4. Conclusions Amine-functionalized polymers were synthesized by plasma in order to study the interaction of these materials with ionic solutions
in concentrations similar to the spinal cord. The results indicated that the chemical nature of the salts modifies the hydrocompatibility of the polymers. With water, the contact angles were higher than those obtained when salts were added to the solutions. The greatest affinity was obtained between PPy/I and the NaCl + MgSO4 solution. However, the tendencies indicate that the aliphatic and heterocyclic polymers of this work are good materials to interact with the ionic test solutions. The electrical response was measured as a function of the volume of solution added to the polymers and was very similar, composed by electronic and ionic mechanisms. The electronic conductivity was in the 10−11 to 10−8 S/cm range and the ionic conductivity increased up to three orders of magnitude. This will be the conductivity that the ionic pulses of neuronal cells would have to use to communicate among them through amine-functionalized polymers implanted in the spinal cord system. Acknowledgement The authors want to thank Conacyt for the partial financial support to this work under the project 47467. References [1] F.D. Egitto, Pure and Applied Chemistry 62 (9) (1990) 1699–1708. [2] P. Favia, R. d’Agostino, Surface and Coatings Technology 98 (1998) 1102–1106. [3] R. d’Agostino, P. Favia, C. Oehr, M.R. Wertheimer, Plasma Processes and Polymers 2 (2005) 7–15. [4] E. Poncin-Epaillard, G. Legeay, Journal of Biomaterials Science, Polymer Edition 14 (10) (2003) 1005–1028. [5] B.D. Ratner, Journal of Molecular Recognition 9 (1996) 617–625. [6] T. Otero, M.T. Cortez, Advanced Materials 15 (3) (2003) 279–282. [7] R. Olayo, C. Rios, H. Salgado, G.J. Cruz, J. Morales, M.G. Olayo, M. Alcaraz, A.L. Alvarez, R. Mondragon, A. Morales, A. Diaz, Journal of Materials Science: Materials in Medicine 19 (2008) 817. [8] M.G. Olayo, J. Morales, G.J. Cruz, S.R. Barocio, R. Olayo, Journal of Polymer Science, Part B: Polymer Physics 41 (2003) 1501–1508. [9] W. Ming, D. Wu, R. van Benthem, G. de With, Nano Letters 5 (1) (2005) 2298–2301. [10] R. Furstner, W. Barthlott, C. Neinhuis, P. Walzel, Langmuir 21 (2005) 956–961. [11] N.A. Patankar, Langmuir 20 (2004) 8209–8213. [12] W. Mertz, E.J. Underwood, Trace Elements in Human and Animal Nutrition, Academic Press, Inc., USA, 1987. [13] Y. Tamada, Y. Ikada, Journal of Biomedical Materials Research 28 (7) (1994) 783–789. [14] S.M. Mirabedini, H. Arabi, A. Salem, S. Asiaban, Progress in Organic Coatings 60 (2007) 105–111. [15] G.J. Cruz, J.C. Palacios, M.G. Olayo, J. Morales, R. Olayo, Journal of Applied Polymer Science 93 (2004) 1031–1036. [16] G. Shi, M. Rouabhia, Z. Wang, L.E. Dao, Z. Zhang, Biomaterials 25 (2004) 2477–2488. [17] J.H. Collier, J.P. Camp, T.H. Hudson, C.E. Schmidt, Biomedical Materials Research 50 (2000) 574–584.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Affinity of amine-functionalized plasma polymers with ionic solutions similar to those in the human body E. Colín a , M.G. Olayo a,∗ , G.J. Cruz a , L. Carapia b , J. Morales c , R. Olayo c a
Departamento de Física, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, Col. Escandón, D.F., C.P. 11801, Mexico Departamento de Tecnología de Materiales, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, Col. Escandón, D.F., C.P. 11801, Mexico c Departamento de Física, Universidad Autónoma Metropolitana Iztapalapa, Apdo. Postal 55-534, Col. Vicentina-Iztapalapa, D.F., C.P. 09340, Mexico b
a r t i c l e
i n f o
Article history: Received 3 June 2008 Received in revised form 11 August 2008 Accepted 18 August 2008 Keywords: Plasma Biomaterials Polypyrrole Polyethylenglycol Polyallylamine
a b s t r a c t This work presents a study on the synthesis and on the interaction of allylamine, pyrrole and ethylenglycol polymerized by plasma with solutions of ionic composition similar to those in the nervous system. These polymers are attractive substrates to interact with the ionic pulses of the spinal cord due to their electrical and biocompatible characteristics. The ionic solutions were prepared with aqueous combinations of NaCl, MgSO4 , KH2 PO4 , KCl, CaCl2 , and NHCO3 . The polymers were prepared as thin films on glass substrates. The results indicated that some of the most important physical characteristics in the hydrophilicity of the polymers, roughness, porosity and functional groups, can be controlled with the energy of polymerization. The interaction between polymers and solutions was studied measuring the contact angle at the solid–liquid interface and the electrical conductivity of the polymers wet with these solutions. The contact angles were between 8◦ and 38◦ , and the electrical conductivity was in the 10−8 to 10−9 S/cm interval. The general tendencies indicate that the amine-functionalized polymers of this work are good materials to interact with the spinal cord system. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Plasma processes allow the use of a great variety of materials in biological applications due to the functional groups with N H, C N, C O and O H bonds, common in the human body, which can be added to almost any material to enhance the biocompatible superficial properties [1,2]. Compounds with some of these groups are: pyrrole (C4 H4 NH), allylamine (CH2 = CH CH2 NH2 ) and ethylenglycol (OH CH2 CH2 OH), whose chemical functionality influences the protein absorption, adhesion of cells and provides sites for immobilization of biomolecules [3]. Polypyrrole provides advantages in flexibility and biodegradability [4] and has been also used in studies of artificial muscles, due to its ability to change its volume under electrochemical stimulation [5,6]. Polymers derived from these compounds are considered attractive substrates for implants in the nervous system, because their electrical characteristics can be used to interact with the ions of K+ , Na+ or Ca2+ of this system. Additionally, as the human body contains approximately 60% of weight in electrolytic solutions, the chemical and electrical interaction at the solid–liquid interface can influ-
∗ Corresponding author. Fax: +52 55 5329 7301. E-mail address: [email protected] (M.G. Olayo). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.08.033
ence the reproduction of cells on materials implanted in biological systems and therefore on the regeneration of damaged tissues. For that reason, the aim of this work is to study the interaction of amine-functionalized plasma polymers polypyrrole (PPy), polypyrrole doped with iodine (PPy/I), polyallylamine (PAl) and polyethylenglycol (PEG) with solutions of salt concentrations similar to those in the spinal cord. Polymers formed with pyrrole have heterocyclic segments, while ethylenglycol and allylamine form chains with aliphatic linear arrangements. The objective involves the chemical and electrical interactions, which are related with the H-bonding and the ionic or polar activity between both phases. These polymers and some of their copolymers are being studied in the reconnection between neuronal cells after a severe lesion in the spinal cord [7].
2. Synthesis of polymers PPy, PPy/I, PAl and a copolymer formed with PPy and PEG (PPy/PEG) were synthesized in a 1500 cm3 cylindrical glass reactor with resistive glow discharges during 180 min at 13.5 MHz, 10−1 mbar and power between 10 and 100 W. The reactor has similar configuration reported in a previous work [8] and consists of a glass tube of 9 cm diameter and 30 cm long with Stainless Steel (SS) flanges at both sides. 3 access ports are located in the flanges
E. Colín et al. / Progress in Organic Coatings 64 (2009) 322–326
323
Fig. 1. Morphology of plasma PPy, PPy/I, PPy/PEG and PAl at different power. (a) PPy, 10 W. (b) PPy, 100 W. (c) PPy/I, 10 W. (d) PPy/I, 100 W. (d) PPy/PEG, 10 W. (f) PPy/PEG, 100 W. (g) PAl, 10 W. (h) PAl, 100 W.
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to introduce the monomers, dopants or other chemical regents to the reactor. Each electrode consists of a SS rod with a circular plate at the end and uses one of the access ports in each flange. In the syntheses of this work, the separation between the electrode plates was 10 cm. The polymers were obtained by applying an electrical field to the reactor which forms radicals and ions with the chemical species inside. The charged particles are accelerated in the electrical field and constantly collide with other particles. Depending on the energy of collision, the molecules excite, break or combine to form new structures. As the molecular weight grows; the molecules adhere to the nearest surfaces, where the chemical reactions continue. In this way, the monomers transform in dimers, trimers, and so on, reaching the dimension of polymers. The polymerization stops when the electrical field is turned off; however, some long-life radicals may remain on the surfaces and neutralize later with the after-glow atmosphere. The final polymers are thin coatings on the inner surfaces of the reactor. The film was separated from the surfaces applying acetone and water, which swelled the polymers and afterwards, they were removed with a thin spatula. The polymerization can be controlled partially with the energy applied to the system. However, although the energy promotes the polymerization, applied in excess, it tends to increase the fragmentation and crosslinking. Fragmentation changes the chemical nature of the polymers and crosslinking induces three-dimensional networks. Nevertheless, even if both factors are present to some extent, the important biological groups containing C N, N H, C O and O H bonds would still have an important participation in the final molecules, which in turn influences the interaction with electrolytic solutions.
3. Results 3.1. Morphology of polymers The morphologic analysis was performed by means of a Philips XL30 scanning electron microscope. Roughness and porosity are important factors in biomaterials, because the interstices work in favor of the retention of liquids and provide sites to lodge cells. The polymers of this work grew as consecutive layers with individual thickness related with the variations of pressure during the synthesis. The total thickness of the films increased along with the time of synthesis. The films show a compact appearance at low power. PPy shows compact layers with smooth surfaces, even at high power. The thickness of the PPy layers shown in Fig. 1(a) and (b) is 4 and 8 m, respectively. The surface of PPy/I is characterized by spherical particles and agglomerates at high power, with diameters in the interval of 250 nm to 2 m, randomly mixed. It has been shown that the dimension of particles greatly influence the hydrophobic character of the surfaces [9–11]. The approximate thickness of the single PPy/I layers varied from 3 to 8 m, see Fig. 1(c) and (d). PPy/PEG has a coarse and uneven surface with great internal porosity at high power. The pores have different profiles and depths. The layer thickness in Fig. 1(e) and (f) is 5 and 30 m, respectively. PAl grew as consecutive layers with different morphology, associated with small changes in both, pressure and temperature during the syntheses. The individual layers are compact with thickness of 1 or 2 m, see Fig. 1(g) and (h). As PPy tends to grow as compact layers, the influence of iodine and PEG on their respective polymers produces the agglomerates and porosity described before.
Fig. 2. Contact angles as a function of the energy of synthesis for: (a) PPy, (b) PPy/I, (c) PPy/PEG, and (d) PAl.
E. Colín et al. / Progress in Organic Coatings 64 (2009) 322–326
3.2. Interaction between polymers and solutions An important variable in the interface between polymers and solutions is the force needed to increase the wet area; this is known as the superficial tension and involves the gas–solid, gas–liquid and liquid–solid interfaces. These forces tend to minimize the energy in the surface, making the drop spherical; however, this effect depends on the balance of forces in each specific system. The chemical nature and concentration of the salts, morphology, electrical charges on the surface, pressure, temperature, etc., influence the entire polymer–solution interaction. This effect is studied in this work by means of the contact angles at the solid–liquid interface. The contact angles of solutions and polymers were evaluated with drops of solutions prepared with NaCl, MgSO4 , KH2 PO4 , KCl, CaCl2 , and NHCO3 in the concentrations found in spinal cord fluids. Some of the salts were tested individually (NaCl, 118 mM), others were combined in pairs (NaCl–MgSO4 , 118 and 1.17 mM, respectively), and another solution was prepared including all of them, Krebs–Ringer solution (KB) [12,13]. This last solution had the following composition: NaCl (118 mM), KH2 PO4 (1.3 mM), KCl (4.7 mM), CaCl2 (2.5 mM), MgSO4 (1.17 mM) and NHCO3 (25 mM). The contact angle of water with the polymers was also measured and taken as a reference. During these measurements, pressure and temperature remained constant. The contact angles of the different polymer–solution combinations are plotted in Fig. 2. The general tendency is a linear reduction as the power of synthesis increases. This effect indicates that more hydrophilic surfaces are obtained due to the increase in the roughness and porosity in the polymers, among other factors. From a
325
global point of view, the highest angles were obtained with distilled water and reduce depending on the chemical composition of solutes. PPy and PPy/I have similar tendencies in contact angles. The highest values were obtained with pure water and reduce when NaCl is added to the system. The angles reduce even more with all the salts of the KR solution, see Fig. 2(a) and (b). However, the lowest angles were found with the NaCl + MgSO4 solution, probably due to the electric interference that occurs when several species of ions are mixed in one solution, which is the case of KR. This effect is not as evident in the NaCl + MgSO4 combination. The influence of PEG in PPy is shown in Fig. 2(c), where the KR solution presents the highest angles, even higher than those of water. However, the lowest angles were found with the NaCl solution. The oxygen atoms distributed along the chains in the copolymer have a great influence on the hydrophilicity of this polymer. The effect of oxygen in polymeric surfaces has been studied before, in which the results show an increase of the hydrophilicity [14]. The slopes of contact angles are very similar, indicating that physical predominates over chemical factors in the absorption of solutions, due most surely to the great porosity of PPy/PEG. PAl also showed the lowest angles with NaCl followed closely with the KR solution; see Fig. 2(d). From a spinal cord perspective, represented by the KR solutions, the lowest angles were found with PAl, with values between 11◦ and 12◦ . On the other hand, the highest angles were found in PPy/PEG, 15◦ –18◦ . Nevertheless, the difference between all polymers is not more than 7◦ , which means that all of them can be good materials to interact with the spinal cord fluids.
Fig. 3. Electrical conductivity of amine-functionalized plasma polymers as a function of the added volume of ionic solutions. (a) PPy. (b) PPy/I. (c) PPy/PEG. (d) PAl.
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3.3. Electrical conductivity The electrical conductivity was calculated measuring the resistance of the polymers in a two-probe arrangement which consists of 2 flat Cu electrodes, 10 mm diameter, supported in Nylon plates [15]. The ionic conductivity was calculated with the following procedure. 3 or 4 layers of polymers were deposited in one of the electrodes. Then, one drop of solution (∼0.05 ml) was added to the polymers. 5 min later, the other electrode was put on the wet films and the Teflon plates tightened with screws and bolts to obtain a ‘good’ contact between electrodes and polymers. After that, the resistance of the wet polymers was measured with an Otto multimeter. This procedure was repeated changing the number of drops of solution, 2, 3, until 6. In any case, no liquid was clearly released from the system, although there may have been some evaporation during this procedure. In this configuration, the charges have to travel perpendicular to the film surface. Additional lectures of the polymer resistance without solutions were made in order to calculate the electronic conductivity. The electrical conductivity was calculated as a function of the volume of solution added to the polymers. The level and response of the conductivity influence the biocompatibility of any material implanted in the body modifying the communication among cells [16,17]. This is particularly important among neuronal cells. The electrical response was very similar, composed by electronic and ionic mechanisms. The electronic conductivity was calculated in the polymers in a dry condition (<20% relative humidity) and was in the 10−9 to 10−11 S/cm range, see the first point of conductivity in Fig. 3. The ionic conductivity was obtained after that point. It was promoted by the ions of the solutions and increased up to three orders of magnitude, 10−7 to 10−8 S/cm. In general terms, the ionic conductivity of each system studied in this work augments as the solutions are added to the polymers. With the first drop of solution the biggest step occurred, PPy/PEG increased up to three orders of magnitude. This conductivity was calculated in the 10−9 to 10−8 S/cm range, where the participation of ions from the salts increases the transport of charges up to one order of magnitude. The highest conductivities belong to KR and NaCl + MgSO4 solutions, however, the conductivities in all polymers are very similar. 4. Conclusions Amine-functionalized polymers were synthesized by plasma in order to study the interaction of these materials with ionic solutions
in concentrations similar to the spinal cord. The results indicated that the chemical nature of the salts modifies the hydrocompatibility of the polymers. With water, the contact angles were higher than those obtained when salts were added to the solutions. The greatest affinity was obtained between PPy/I and the NaCl + MgSO4 solution. However, the tendencies indicate that the aliphatic and heterocyclic polymers of this work are good materials to interact with the ionic test solutions. The electrical response was measured as a function of the volume of solution added to the polymers and was very similar, composed by electronic and ionic mechanisms. The electronic conductivity was in the 10−11 to 10−8 S/cm range and the ionic conductivity increased up to three orders of magnitude. This will be the conductivity that the ionic pulses of neuronal cells would have to use to communicate among them through amine-functionalized polymers implanted in the spinal cord system. Acknowledgement The authors want to thank Conacyt for the partial financial support to this work under the project 47467. References [1] F.D. Egitto, Pure and Applied Chemistry 62 (9) (1990) 1699–1708. [2] P. Favia, R. d’Agostino, Surface and Coatings Technology 98 (1998) 1102–1106. [3] R. d’Agostino, P. Favia, C. Oehr, M.R. Wertheimer, Plasma Processes and Polymers 2 (2005) 7–15. [4] E. Poncin-Epaillard, G. Legeay, Journal of Biomaterials Science, Polymer Edition 14 (10) (2003) 1005–1028. [5] B.D. Ratner, Journal of Molecular Recognition 9 (1996) 617–625. [6] T. Otero, M.T. Cortez, Advanced Materials 15 (3) (2003) 279–282. [7] R. Olayo, C. Rios, H. Salgado, G.J. Cruz, J. Morales, M.G. Olayo, M. Alcaraz, A.L. Alvarez, R. Mondragon, A. Morales, A. Diaz, Journal of Materials Science: Materials in Medicine 19 (2008) 817. [8] M.G. Olayo, J. Morales, G.J. Cruz, S.R. Barocio, R. Olayo, Journal of Polymer Science, Part B: Polymer Physics 41 (2003) 1501–1508. [9] W. Ming, D. Wu, R. van Benthem, G. de With, Nano Letters 5 (1) (2005) 2298–2301. [10] R. Furstner, W. Barthlott, C. Neinhuis, P. Walzel, Langmuir 21 (2005) 956–961. [11] N.A. Patankar, Langmuir 20 (2004) 8209–8213. [12] W. Mertz, E.J. Underwood, Trace Elements in Human and Animal Nutrition, Academic Press, Inc., USA, 1987. [13] Y. Tamada, Y. Ikada, Journal of Biomedical Materials Research 28 (7) (1994) 783–789. [14] S.M. Mirabedini, H. Arabi, A. Salem, S. Asiaban, Progress in Organic Coatings 60 (2007) 105–111. [15] G.J. Cruz, J.C. Palacios, M.G. Olayo, J. Morales, R. Olayo, Journal of Applied Polymer Science 93 (2004) 1031–1036. [16] G. Shi, M. Rouabhia, Z. Wang, L.E. Dao, Z. Zhang, Biomaterials 25 (2004) 2477–2488. [17] J.H. Collier, J.P. Camp, T.H. Hudson, C.E. Schmidt, Biomedical Materials Research 50 (2000) 574–584.