Published: March 8th, 2019
This study describes synthetic routes for aminopropyl-terminated polydimethylsiloxanes and polydimethyl-methyl-phenyl-siloxane-block copolymers and for soft polysiloxane-based urea (PSU) elastomers. It presents the application of PSUs as accommodating an intraocular lens. An evaluation method for in vitro cytotoxicity is also described.
This study discusses a synthesis route for soft polysiloxane-based urea (PSU) elastomers for their applications as accommodating intraocular lenses (a-IOLs). Aminopropyl-terminated polydimethylsiloxanes (PDMS) were previously prepared via the ring-chain equilibration of the cyclic siloxane octamethylcyclotetrasiloxane (D4) and 1,3-bis(3-aminopropyl)-tetramethyldisiloxane (APTMDS). Phenyl groups were introduced into the siloxane backbone via the copolymerization of D4 and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenyl-cyclotetrasiloxane (D4Me,Ph). These polydimethyl-methyl-phenyl-siloxane-block copolymers were synthesized for increasing the refractive indices of polysiloxanes. For applications as an a-IOL, the refractive index of the polysiloxanes must be equivalent to that of a young human eye lens. The polysiloxane molecular weight is controlled by the ratio of the cyclic siloxane to the endblocker APTMDS. The transparency of the PSU elastomers is examined by the transmittance measurement of films between 200 and 750 nm, using a UV-Vis spectrophotometer. Transmittance values at 750 nm (upper end of the visible spectrum) are plotted against the PDMS molecular weight, and > 90% of the transmittance is observed until a molecular weight of 18,000 g·mol−1. Mechanical properties of the PSU elastomers are investigated using stress-strain tests on die-cut dog-bone-shaped specimens. For evaluating mechanical stability, mechanical hysteresis is measured by repeatedly stretching (10x) the specimens to 5% and 100% elongation. Hysteresis considerably decreases with the increase in the PDMS molecular weight. In vitro cytotoxicity of some selected PSU elastomers is evaluated using an MTS cell viability assay. The methods described herein permit the synthesis of a soft, transparent, and noncytotoxic PSU elastomer with a refractive index approximately equal to that of a young human eye lens.
Senile cataract, affecting the age group of ≥ 60 years, leads to the advanced opacification of the natural crystalline lens. This age-related condition is probably caused by oxidative changes that are accelerated by UV irradiation1,2,3. Conventional treatment for senile cataract involves the surgical extraction of the cataractous lens, followed by the implantation of an artificial intraocular lens (IOL) into an empty lens capsule via an injection system2. However, a majority of IOLs are manufactured from acrylic polymers (hydrophobic and hydrophilic acrylate or methacrylate polymers) with extremely rigid structures; hence, the eye loses its ability to accommodate to various distances2,4. Therefore, patients with monofocal IOL implants are dependent on spectacles for near vision (e.g., while reading a newspaper or a book)5.
Different approaches to restoring the accommodation ability after cataract surgery have been reported. Among these approaches, two principal strategies can be distinguished: refilling the empty lens capsule by injecting a liquid or gel-like polymers and developing soft, foldable a-IOLs6,7,8. The concept of "lens refilling" is promising because gels can be prepared with Young's moduli as low as those of the natural human eye lens (ca. 1 - 2 kPa)9; however, this approach is still experimental8, and studies are only conducted on animal eyes.
Lens capsules have been refilled by implanting inflatable silicone balloons10 filled with liquid silicone or by directly injecting silicone11,12 that was subsequently cured in the capsule via hydrosilylation. However, issues related to surface wrinkles on the balloons, a lower accommodation amplitude compared to the preoperative state, and the formation of severe secondary cataracts (anterior and posterior capsule opacification) have been noted7,8,12,13. In particular, long curing times (70 min - 12 h) cause an increased risk of leakage into the surrounding eye compartments, leading to postoperative inflammation10,14. Therefore, other materials for replacing the crystalline lens are recommended, including hydrogels based on polyethylene glycol diacrylate, acrylate-modified copolymers of vinyl alcohol (N-vinylpyrrolidone)15, methacrylate-modified polysiloxanes16,17, poloxamer18, and diisocyanate-crosslinked polyalcohols9. However, the monomer viscosity (i.e., gel swelling after the injection and crosslinking), extremely low or high refractive indices, mechanical stability and integrity, unpredictable postoperative refraction, low accommodation range, and after-cataract formation constitute the main issues6,7,8,9,15,18. Commercially, the accommodation ability is mainly restored by developing foldable a-IOLs. Such a-IOLs should provide accommodation by the movement of the IOL optic to the anterior site of the lens capsule via the contraction of the ciliary muscle. Several models have been introduced in the market in 1996, 2001, and 20027,8. However, during clinical studies, the estimated accommodation amplitudes for those implanted a-IOLs were extremely low (≤ 1.5 D) to permit unaided reading (3 - 4 D)6,7,8,19,20. Therefore, an a-IOL comprising two connected optics (dual-optic IOL) is being developed for increasing the accommodation range6,21. The design of only one lens has been examined for its accommodative performance in human eyes, albeit conflicting results have been reported22,23,24,25.
Typically, silicone elastomers are regarded to be biologically inert and nontoxic; therefore, silicone elastomers have a long history of being applied as biocompatible materials in medicine and medical engineering (e.g., in breast implants, craniofacial implants, joint prosthetics, wound dressings, catheters, drains, and shunts)26,27. Owing to their softness, transparency, and high oxygen permeability, silicone elastomers also find applications as contact lenses and IOLs2,28,29. However, silicones must be covalently crosslinked and often require reinforcing fillers to gain sufficient mechanical integrity. Crosslinking is disadvantageous as it prohibits the subsequent processing of elastomers either by thermoplastic methods (e.g., injection molding) or by processing from solutions (e.g., solvent casting). In contrast, thermoplastic polyurethanes exhibit mechanical stability but are susceptible to degradation within the biological environment, particularly if polyester- or polyether-based macrodiols are used. Therefore, efforts to combine flexibility and hydrolytic or oxidative stability with excellent mechanical properties are concentrated on the incorporation of hydroxyl- or amino-functional PDMS as soft segments into polyurethanes, polyurethane-ureas, and polyureas27. To enhance the compatibility of the polar urethane or urea hard segment with a highly nonpolar PDMS soft segment and to improve mechanical properties, different polyether-based macrodiols are incorporated along with PDMS30,31,32. Particularly, the Thilak Gunatillake group has systematically investigated the development of silicone polyurethanes with improved biostability and mechanical properties for long-term biomedical applications such as pacemaker insulation or artificial heart valves33. They synthesized aromatic polyurethanes with mixed soft segments comprising hydroxyl-terminated PDMS and different polyethers, as well as aliphatic polycarbonate diols. Among all the synthesized polyurethanes, the combination of polyhexamethylene oxide (PHMO) and PDMS exhibits the best mechanical properties with respect to hard segment compatibility30. In subsequent studies, they further examined the effect of the PDMS-to-PHMO ratio and the incorporation of a disiloxane-based chain extender on the mechanical properties of silicone polyurethanes34,35,36. The results revealed that a macrodiol composition of 80 wt% PDMS and 20 wt% PHMO, in addition to a co-chain extender, such as 1,3-bis(4-hydroxybutyl)-tetramethyldisiloxane (BHTD), yields softer polyurethanes with good mechanical properties and thermoplastic processability. Furthermore, these silicone-polyurethanes exhibit an enhanced biostability compared to a commonly applied soft polyether urethane37,38,39.
The biocompatibility and stability of similar materials and their use for cardiovascular applications have also been reported40,41,42. Based on these results, silicone-based polyurea elastomers (or PSUs) with a disiloxane-based chain extender are thought to yield high flexibilities and softness, albeit with sufficient mechanical strength, to retain their shape after the application of repeated stress. For instance, Hermans et al. have constructed an experimental polyurethane-based dual-optic a-IOL prototype because the design, which was previously used for a fabrication using silicone, was extremely soft to handle the applied loads within enucleated pig eyes43.
This article describes the synthesis of a soft siloxane-based PSU, which is optimized in terms of mechanical and optical properties for applications as an accommodating IOL. As the mechanical properties of the PSU elastomers can be altered by the siloxane molecular weight, the same procedure can be applied to developing siloxane-based PSUs, which may find applications in coatings and skin dressings. In addition, this procedure can be used to prepare siloxane-based polyurethane or polyurethane-urea elastomers if carbinol-terminated PDMS is used. Depending on the type of diisocyanate (i.e., aliphatic or aromatic) used for synthesis, reaction conditions (including time, temperature, and perhaps the solvent composition) may have to be altered. For the application of aliphatic diisocyanates such as 4,4-methylenebis(cyclohexylisocyanate) (H12MDI) or isophorone diisocyanate, the reaction has to be accelerated using an organotin catalyst, such as dibutyltin dilaurate or diacetoxytetrabutyl distannoxane. For example, the reaction between a hydroxypropyl-terminated PDMS and H12MDI proceeds in the presence of a catalyst. Furthermore, the reaction temperature needs to be increased to 50 - 60 °C.For the application of an aromatic diisocyanate such as 4,4-methylenebis(phenylisocyanate) (MDI), the reaction temperature must be moderately but sufficiently increased as aromatic diisocyanates are typically more reactive toward nucleophilic groups than aliphatic diisocyanates are. The reaction of MDI with carbinol-terminated PDMS can be promoted by using the solvent mixtures of anhydrous tetrahydrofuran (THF) and dimethylformamide (DMF) or dimethylacetamide (DMAc) as tertiary amines exhibit some catalytic activity.
CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Several chemicals used in the syntheses exhibit acute toxicity and strong irritation to the skin and eyes, as well as on inhalation. Please wear personal protective equipment (laboratory coats, safety glasses, hand gloves, full-length pants, and closed-toe shoes) and handle the chemicals, if possible, under a fume hood or in a well-ventilated place. Perform all syntheses under the fume hood. Tetramethylammonium hydroxide pentahydrate (TMAH): TMAH is a strong base, acutely toxic if swallowed, and upon skin contact, it causes severe chemical burns on skin and eyes. It is sensitive to air and is hygroscopic. Store it under refrigeration and nitrogen. Handle TMAH in a well-ventilated place because of its strong ammonia-like odor. APTMDS: APTMDS is sensitive to air and must be stored under nitrogen. It causes severe skin burns and eye damage. H12MDI: H12MDI is toxic upon inhalation and causes irritation to the skin and eyes. D4: D4 may impair fertility. THF: THF is harmful, causes irritation on inhalation, and presumably is carcinogenic. Chloroform (CHCl3): CHCl3 is harmful on inhalation, presumably carcinogenic, can cause possible damage to fertility and an unborn child, and its vapors may cause drowsiness.
1. Synthesis of the Catalyst and Amino-terminated Polysiloxane Macromonomers
2. Molecular Weight Determination of Polysiloxane
3. Synthesis of Polysiloxane-urea Elastomers
NOTE: This section describes the synthesis procedure for a PDMS-based urea elastomer of 10 w% hard-segment content (HS%) (PDMS: 15,500 g·mol-1).
4. Mechanical Testing Procedure
5. Cultivation Procedure for HaCaT Cells
6. Procedure for an MTS Cell Viability Assay Using HaCaT Cells
NOTE: In vitro cytotoxicity tests were performed according to Wenzelewski46, using cell medium extracts. PSU samples and biomedical-grade polyurethane samples were sterilized using ethylene oxide.
The ring-chain equilibration of D4 and D4Me,Ph with the endblocker APTMDS yielded aminopropyl-terminated polydimethylsiloxanes and polydimethyl-methyl-phenyl-siloxane-copolymers, respectively, which were synthesized with molecular weights between 3,000 and 33,000 g·mol-1 by adjusting the monomer ratio between D4 and APTMDS (Figure 6). Molecular weights of the prepared PDMS, which were determined from 1H-NMR spectra (Figure 5), were similar to the values obtained from titration. These values were in agreement with the calculated theoretical molecular weights of up to 15,000 g·mol-1. During the preparation of PDMS with higher molecular weights, the obtained molecular weights were slightly greater than those presumed by theoretical calculation.The copolymerization of the cyclic siloxane with pendant phenyl groups D4Me,Ph was deemed successful for slightly increasing the refractive index of polysiloxanes. The refractive index (determined using the Abbe refractometer at 37 °C) increased from 1.401 (unmodified PDMS) to 1.4356 (14 mol% methyl-phenyl-siloxane) (Figure 7).PSU elastomers were synthesized in two steps using the prepared aminopropyl-terminated PDMS, aliphatic diisocyanate H12MDI, and APTMDS, using THF as the solvent. This method permitted the construction of high-molecular-weight PSUs with a segmented structure of soft segments (PDMS) and hard segments (diisocyanate + urea). Inline FTIR spectroscopy confirmed the extremely rapid reaction of the isocyanate groups with the amino groups from the PDMS and the chain extender APTMDS (Figure 3 and Figure 8). Unlike the preparation of the polyurethane elastomers, which takes several hours, the preparation of the PSU elastomers was convenient.The transparency and mechanical properties of PSU elastomers were dependent on the PDMS molecular weight. Transparent PSU elastomer films exhibited a transmittance of >90% up to a PDMS molecular weight of 18,000 g·mol-1. At higher PDMS molecular weights, the PSU films became increasingly opaque (Figure 9).With the increase in the PDMS molecular weight, soft PSU elastomers could be prepared. The Young's modulus of PSU elastomers decreased from ~5.5 MPa (with a PDMS molecular weight of 3,000 g·mol-1) to 0.6 MPa (with a PDMS molecular weight of ≥26,000 g·mol-1) (Figure 10).Furthermore, mechanical hysteresis, which was used to evaluate the mechanical stability under repeated applied stress, was reduced for the PSU elastomers when they were prepared from high-molecular-weight PDMS. The hysteresis values for the first cycle at a 100% strain decreased from 54% (with a PDMS molecular weight of 3,000 g·mol-1) to 6% (with a PDMS molecular weight of 33,000 g·mol-1) (Figure 11).The applied synthesis method permitted the preparation of PSU elastomers that do not release cytotoxic residuals as examples shown in cell viability tests performed with extracts of some selected PSU elastomers on HaCaT cells (Figure 12).
Figure 1: Synthesis of the tetramethylammonium-3-aminopropyl-dimethylsilanolate catalyst.
Tetramethylammonium hydroxide pentahydrate (TMAH) and 1,3-Bis(3-aminopropyl)-tetramethyldisiloxane (APTMDS) were reacted 2 h in THF at 80 °C. The catalyst tetramethylammonium-3-aminopropyl-dimethylsilanolate is received as a white solid after washing the crude product with THF. Please click here to view a larger version of this figure.
Figure 2: Synthesis route for aminopropyl-terminated polydimethylsiloxanes (PDMS) and polydimethyl-methyl-phenyl-siloxane-copolymers. Cyclic monomers D4/D4Me,Ph are equilibrated using a disiloxane endblocker APTMDS at 80 °C for 24 h using the tetramethylammonium-3-aminopropyl-dimethylsilanolate catalyst. This figure has been modified from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 3: Two-step synthesis of segmented polysiloxane-based urea elastomers (PSU). In the first step, a prepolymer containing active isocyanate groups is formed after the reaction of H12MDI with aminopropyl-terminated polysiloxane (R = CH3: PDMS; R = Ph; copolymer). In the second step, the polymer molecular weight is increased via the reaction of the remaining active isocyanate groups with the chain extender APTMDS. The resulting elastomer is a segmented polymer comprising urea hard segments and silicone soft segments. This figure has been modified from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 4: Specification of the dog-bone-shaped test specimen for stress-strain tests. This figure has been modified from Keiper45. Please click here to view a larger version of this figure.
Figure 5: 1H-NMR spectrum of aminopropyl-terminated polydimethylsiloxane. For the molecular weight calculation, integral values of the methylene protons d (δ 2.69 ppm) and b (δ 0.56 ppm) and methyl protons a (δ ~ 0.07 ppm) were utilized. The peak c (δ ~1.5 ppm) is overlaid by the HDO peak49, corresponding to the proton exchange of water traces with solvent CDCl3; hence, this peak is not used to calculate the molecular weight. The PDMS molecular weight in this spectrum is ~16,365 g·mol-1. Please click here to view a larger version of this figure.
Figure 6: Linear correlation between the molecular weight of aminopropyl-terminated polydimethylsiloxanes and endblocker concentration. values were determined via 1H-NMR spectroscopy, the titration of amino end groups, and the theoretical calculation according to equation (1). This figure is reprinted with permission from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 7: Refractive indices of aminopropyl-terminated polydimethyl-methyl-phenyl-siloxane-copolymers. Refractive indices (RI) of polydimethyl-methyl-phenyl-siloxane-copolymers were determined at 20 °C (black squares) and 37 °C (red circles) using an Abbe refractometer. The RI values linearly increased with the amount of the incorporated methyl-phenyl-siloxane units. RI values at 0 mol% represent those from unmodified PDMS with a molecular weight comparable to the polydimethyl-methyl-phenyl-siloxane-copolymers. An optimal RI of 1.4346 (37 °C) was obtained for a copolymer with 14 mol% of methyl-phenyl-siloxane. This figure has been reprinted with permission from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 8: Isocyanate conversion during the synthesis of polydimethylsiloxane-urea (PSU). This figure shows a time-dependent plot of the NCO absorption band at 2,266 cm1 followed by inline FTIR-ATR spectroscopy during the synthesis of PSU.After the addition of aminopropyl-terminated polydimethylsiloxane, the height of the NCO band decreased, indicative of the formation of NCO-terminated prepolymer chains. After the addition of the chain extender APTMDS, the NCO band completely disappeared from the IR spectra. This figure has been reprinted with permission from Riehle et al.50. Please click here to view a larger version of this figure.
Figure 9: Dependence of the transmittance of PSU elastomer films at 750 nm and the molecular weight of polydimethylsiloxane. The transmittance of the PSU films was determined by UV-Vis spectroscopy. The transmittance of PSUs at 750 nm (the upper edge of the visible spectrum) was >90% if PSUs were synthesized using PDMS with molecular weights ranging between 3,000 and 18,000 g·mol-1. With an increasing molecular weight of PDMS, the opacity of films increased. This figure has been reprinted with permission from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 10: Young's modulus of PSU elastomers as a function of the molecular weight of polydimethylsiloxane. Young's moduli (YM) were determined from stress-strain measurements of the PSU films. The values are expressed as a mean value obtained from five repeated measurements. The error bars represent the standard deviation. The highest decrease of YM was observed for PSUs synthesized from PDMS ranging from 3,000 to 9,000 g·mol-1. At PDMS molecular weights between 12,000 and 18,000 g·mol-1, YM values were between 1.5 MPa and 1.0 MPa. At molecular weights greater than 26,000 g·mol-1, YM values were ~0.6 MPa. This figure has been reprinted with permission from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 11: 100% hysteresis curves of PSU elastomers. The first-cycle hysteresis curves of the PSU elastomers at 100% elongation are shown. The polymer notation refers to the PDMS molecular weight (e.g., PSU-3T is a polyurea elastomer prepared from PDMS with a molecular weight of 3,000 g·mol-1). The highest mechanical hysteresis (43% - 54%) was observed in PSU elastomers synthesized from low-molecular-weight PDMS, as indicated by the pronounced hysteresis curves. Hysteresis decreased with the increase in the PDMS molecular weight from 14% (15,000 g·mol-1) to 6% (33,000 g·mol-1). This figure has been reprinted with permission from Riehle et al.48. Please click here to view a larger version of this figure.
Figure 12: Results of in vitro cytotoxicity tests on HaCaT cells treated with PSU extracts. This figure shows the cell proliferation of the HaCaT cells treated with the cell medium extracts of PSU elastomers. The values are expressed as the mean value obtained from three tested extracts per sample, with six repeated measurements for each extract (18 replicates in total). The error bars represent the standard deviation from these measurements.The blank represents the cell medium DMEM (without the sample), which was treated analogous to the cell medium used for extraction. A medical-grade polyether urethane was selected as the reference material. Silicone-based polyurea elastomers (PSU-18T, PSU-16T, and PSU-14Ph) were selected as representative test samples, which were based on PDMS with molecular weights of 18,000 and 16,000 g·mol-1 (PSU-18T and PSU-16T), whereas PSU-14Ph was based on a polydimethyl-methyl-phenyl-siloxane-copolymer with 14 mol% of methyl-phenyl-siloxane and a molecular weight of ~16,600 g·mol-1. The mean proliferation of HaCaT cells, treated with the extracts of the PSU elastomers, and the reference polyurethane was 100% and higher. Therefore, the extracts of the PSU elastomers and reference polyurethane are not cytotoxic. Please click here to view a larger version of this figure.
To achieve high-molecular-weight aminopropyl-terminated PDMS via ring-chain equilibration, using an anhydrous, strongly basic catalyst is crucial. Other typically applied catalysts, such as tetramethylammonium hydroxide (TMAH) or potassium hydroxide (KOH), contain water residues, which promote side reactions; hence, a mixture of difunctional, monofunctional, and nonfunctional PDMS chains with similar molecular weights is obtained44. Furthermore, if TMAH is used, the reaction requires >48 h for completion and does not always proceed with complete monomer consumption44.
In particular, the weighing of the endblocker APTMDS is critical to obtain the desired molecular weight of PDMS. For example, instead of 0.9 g of APTMDS, if 0.85 g is used to synthesize PDMS, as described in section 2.1 of the protocol, this would lead to a theoretical molecular weight of approximately >900 g·mol-1. In addition, the theoretical molecular weight is dependent on the conversion. If the cyclic side products are not considerably removed via vacuum distillation, a high conversion value will be obtained. For example, to use the same synthesis procedure (as in section 2.1 of the protocol), a calculated conversion of 90% would lead to a theoretically calculated molecular weight; this value is 910 g·mol-1 greater than that if a conversion of 85% is presumed. Deviations in the polysiloxane molecular weight determination by titration are possibly related to the weighing of PDMS into the flasks, particularly if a 50 mL burette is used for titration. A deviation related to the weighing of 0.06 g of polysiloxane might lead to a calculated difference of ~650 g·mol-1. Hence, the use of a semiautomatic titrator is recommended.
The refractive index of PDMS can be increased by the incorporation of phenyl groups17,51, halogenated phenyl groups52, or sulfur-containing groups53. Attempts to incorporate phenyl groups into PDMS via the copolymerization of octaphenylcyclotetrasiloxane (D4Ph) as described by Yilgör, Riffle, and McGrath54 were unsuccessful under the applied reaction conditions, possibly because the bulky ring backbone made it impossible for the applied catalyst to break up the siloxane bonds at the selected reaction temperature. The D4Ph ring can be opened if KOH is used at a reaction temperature of 160 °C. However, polysiloxanes of extremely high molecular weight are obtained, which presumably contain high amounts of nonfunctional impurities. In addition, the removal of the catalyst KOH in those copolymers is not straightforward and requires a neutralization step using ethanolic HCl, followed by an aqueous extraction of the catalyst. Then, the PDMS has to be dissolved in an organic solvent, such as CH2Cl2, to separate the aqueous phase from the organic PDMS-containing phase. Finally, the organic phase must be dried over MgSO4, followed by filtration and vacuum distillation using a rotary evaporator54. In contrast, the method presented in this manuscript allows the catalyst to be removed immediately via thermal decomposition. Therefore, instead of using solid monomer D4Ph, phenyl groups are successfully introduced into the PDMS backbone by the copolymerization of the liquid monomer D4Me,Ph, as confirmed by 29Si-NMR spectroscopy50.
The synthesized PSU elastomers exhibited YM of 0.6 - 5.5 MPa and high elasticity with elongation values of up to 1,000%. Such high elongation values were related not only to the polymer segmented structure but also to the high molecular weights of the PSU elastomers ( > 100,000 g·mol-1)48. An instantaneous reaction occurs between the amino groups and aliphatic isocyanyate groups at room temperature, leading to rapidly increasing molecular weight. This result was further supported by conducting the reaction in a solvent, because a slight increase in viscosity did not appear to slow down the reaction speed significantly, which would otherwise dramatically affect the molecular weight for a nearly balanced stoichiometric ratio. In contrast, when a short chain diol, such as 1,4-butanediol, was used as the chain extender, the resulting polyurethane-urea elastomers were not only less elastic but also lost considerable mechanical stability, particularly if high-molecular-weight PDMS was used for synthesis. This result was presumably related to the considerably low molecular weights of the elastomers (results not published), corresponding to the incomplete conversion of all isocyanate groups at the last stage of polyaddition. In addition, differences in reactivity between the amino and hydroxyl groups toward aliphatic diisocyanates dramatically affected the results obtained from in vitro cytotoxicity tests. Extracts of the PSU elastomer prepared from the amino-chain extender APTMDS did not exhibit any cytotoxic effect on the HaCaT cells (Figure 12). However, if extracts of a siloxane-based polyurethane-urea elastomer were used, the cell viability was drastically reduced (results not published), which was possibly related to the low-molecular-weight leachables and residual unreacted isocyanate groups.
This protocol describes a convenient method for preparing amino-functional polysiloxanes, which can be subsequently used as macrodiamines for synthesizing high-molecular-weight, soft, and elastic polysiloxane-urea elastomers. As the mechanical properties of the PSUs can be varied according to the PDMS molecular weight, it is possible to use these polymers in other application fields. Furthermore, the procedure for preparing amino-functional polysiloxanes can be used for the introduction of side groups, such as vinyl groups, via the copolymerization of a cyclic siloxane with pendant vinyl groups (results not shown). This may open up new application fields, including the preparation of soft crosslinked polysiloxane gels (e.g., by Pt-catalyzed hydrosilylation with a hydride-functional silicone or by UV-activated thiol-ene addition of mercapto-functional PDMS) (results not shown).
The authors have nothing to declare.
The authors would like to thank the Federal Ministry of Education and Research (BMBF) for funding this work under grant number 13FH032I3. Financial support by the Deutsche Forschungsgemeinschaft (DFG, Gepris project 253160297) is gratefully acknowledged. The authors further like to express their thanks to Priska Kolb and Paul Schuler from the University of Tübingen for performing 1H-NMR and 29Si-NMR measurements. Thanks are also due to CSC Jäkle Chemie GmbH & Co. KG for their supply of H12MDI. The authors would like to thank Herbert Thelen and André Lemme from Biotronik for performing ethylene oxide sterilization of the PSU samples and Lada Kitaeva (Reutlingen University) for her support with stress-strain and hysteresis measurements.
|Octamethylcyclotetrasiloxane (D4), 97 %
|presumably impairs fertility, must be degassed before use
|sensitive to air, must be stored under nitrogen
|Tetramethylammonium hydroxide pentahydrate
|toxic if swallowed and upon skin contact, strong base, sensitive to air, hygroscopic, store under refrigeration and under nitrogen
|Covestro via CSC Jäkle Chemie GmbH & Co. KG
|toxic if inhaled, skin and eye irritant
|Tetrahydrofuran (anhydrous) 99.8 %
|stabilized with BHT
|Chloroform 99 %
|Grüssing GmbH Analytica
|stabilized with ethanol, presumably carcinogenic, can impair fertility and cause damage to an unborn child
|Chloroform-d, 99.8 %
|Dulbecco's modified Eagle's medium (DMEM) high glucose
|Thermo Fisher Scientific Life Technologies GmbH
|Fetal bovine serum (FBS)
|Thermo Fisher Scientific Life Technologies GmbH
|Trypsin/EDTA, 0.25 % phenol red
|Thermo Fisher Scientific Life Technologies GmbH
|Cell Titer Aqueous One Solution cell proliferation assay (MTS)
|CLS Cell Lines Service GmbH
|foldable accommodating intraocular lens
|Human Optics AG
|foldable accommodating intraocular lens
|Bausch and Lomb Inc.
|foldable accommodating intraocular lens
|dual-optic foldable accommodating intraocular lens
|AorTech International plc
|thermoplastic PDMS-PHMO-based polyurethane for medical applications
|Lubrizol Life Sciences
|thermoplastic polyether-based polyurethane for medical applications
|Zwick universal tensile testing machine model 81565 and software testXpert II
|Zwick GmbH & Co. KG
|tensile testing machine
|Roche Innovatis AG
|cell counting system
|Beckman Coulter Life Sciences
|cell counting system
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