In situ polymerizable polyester polyols for tissue sealant applications: effect of choice of acid and diol on sealant properties.
Polymer adhesives and sealants have been used for about fifty years in medical practice for joining and sealing tissues. During surgery or injury there are occasions where body fluids are lost and efficient procedures for controlling such damages are very important. For example during lung surgery if an in situ curable sealant is used in conjunction with sutures and staples, the air loss and patient trauma can be minimized. Different kinds of sealants were investigated before. Cyanoacrylates, fibrin glues and gelatin resorcinol formaldehyde glues, etc were extensively investigated (1). All these are chemically crosslinked on the target tissue. These begin to cure even while applying them to tissues and limit the ability of a surgeon to position and reposition them to his satisfaction. Light cured sealants, on the other hand, have advantage over them since they can be triggered at the will of the user. Prof. Hubbel's group has worked on such light curable sealants extensively using acrylated PEG block lactide and glycolide polymers (2-4). We recently started investigating PEG containing aliphatic polyester polyol acrylates for similar applications (5,6). In this paper we wish to discuss the effect of modification of hydrophobicity of these sealants on their properties and performance.
Sealant tissue interaction is a very important parameter, which affects not only the performance but also its overall bio-compatibility. It is well established in adhesive technology that a proper wetting of the target surface by the adhesive/ sealant is very important. Several physical and chemical factors of both the sealant as well as the target surface determine this interaction. Important among them being viscosity of the sealant, surface energy of the target, hydrophilic-hydrophobic interaction of the sealant and surface, presence or absence of any Van der Walls forces etc. In the context of tissue sealants the problem is even more complicated owing to the complex structure of tissue surface. Different tissues of the body have different surface properties depending on their function. The surface of any living tissue has a mosaic structure, characterized by alternating hydrophilic and hydrophobic microsections with high and low surface energy (7). Hence it is very unlikely that one sealant would serve more than one organ or surface. We wished to investigate the importance of wetting of a surface by a sealant on its ability to seal a leak. In the present communication we present sealing ability and other properties of a closely related series of sealants with increasing hydrophobicity.
Materials and Methods
Dicarboxylic acids, Polyethyleneglycol (600), 1,4-Butanediol and solvents bought from S. D. Fine Chem, Mumbai, India. Acryloyl chloride, benzophenone / hydroxycyclohexyl acetophenone were purchased from Aldrich Chemical Company, Milwaukee, WI, U S A.
Synthesis of polyester polyols
Polyesterpolyols were synthesized in this study using aliphatic dicarboxylic acids and polyethylene glycol- 600 (PEG) or a mixture of PEG and 1,4-Butane diol as shown in Table 1. Typical procedure used is as follows. Required amounts of acid and alcohols were taken in a three-necked round bottom flask fitted with an overhead mechanical stirrer, nitrogen bubbler and a distillation condenser. The contents were slowly heated to 180[degrees]C and kept there for four hours. Water formed in the reaction was collected in a round-bottomed flask. The products were characterized by [sup.1]H NMR, IR, Hydroxy and acid numbers.
[sup.1]H NMR of macromers synthesized in this study.
All NMRs were run in deuterated chloroform and peaks are expressed as [delta] ppm from reference TMS.
AAP Polyol: 1.6, m, OCC[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]CO; 2.35, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.2, t, O=C-O-C[H.sub.2].
GAP Polyol: 1.8, t, OCC[H.sub.2],C[H.sub.2]C[H.sub.2]CO; 2.4,q, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.1, t, O=C-O-C[H.sub.2].
SAP Polyol: 2.5, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.m, O=C-O-C[H.sub.2].
SAP25BD polyol: 1.7, m, O-C[H.sub.2],C[H.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.1, t, O=C- O-C[H.sub.2] (due to butane diol) and 4.25, t, O=C-O-C[H.sub.2] (due to PEG).
SAP50BD polyol: 1.65, m, O-C[H.sub.2]C[C.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.1, t, O=C-O-C[H.sub.2] (due to butane diol) and 4.25, t, O=C-O- C[H.sub.2] (due to PEG).
SAP75BD polyol: 1.65, m, O-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.55, m , O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.05, t, O=C-O-C[H.sub.2] (due to butane diol) and 4.25, t, O=C-O-C[H.sub.2] (due to PEG).
I.R spectra: All polyols had a 1740[cm.sup.-1] peak for carbonyl stretch.
Synthesis of polyesterpolyol acrylates
Polyester polyols made as above were acrylated using acryloyl chloride. Polyol (1 eq) was dissolved in dry dichloromethane at 10% solids concentration in a two-necked RB flask protected from moisture using a Ca[Cl.sub.2] guard tube. Triethyl amine (2 eq) was added and the contents cooled to 0[degrees]C followed by drop wise addition of acryloyl chloride (1.5eq). Stirred cold for two hours and allowed to attain room temperature and stirred for 24 hours. The reaction mixture was then washed with dilute HCl, and brine solution. Dried over [Na.sub.2]S[O.sub.4] and concentrated at reduced pressure using a rotary evaporator.
AAP acrylate: 1.6, m, OCC[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]CO; 2.3, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.1 to 4.3, m, O=C-O-C[H.sub.2], 5.8 to 6.5, m, -CH=C[H.sub.2].
GAP acrylate: 1.9, m, OCC[H.sub.2]C[H.sub.2]C[H.sub.2]C0; 2.4, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-0 from PEG; 4.1 to 4.4, m, 0=C-O-C[H.sub.2], 5.8 to 6.7, m, -CH=C[H.sub.2].
SAP.acrylate: 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.2, m, O=C-O-C[H.sub.2], 5.8 to 6.5, m, -CH=C[H.sub.2].
SAP25BD acrylate: 1.7, m, O-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.0 to 4.3, m, O=C-O-C[H.sub.2], 5.8 to 6.6, m, -CH=C[H.sub.2].
SAP50BD acrylate: 1.75, m, O-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.1 to 4.4, m, O=C-O-C[H.sub.2], 5.8 to 6.6, m, -CH=C[H.sub.2].
SAP75BD acrylate: 1.7, m, O-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]-O-; 2.6, m, O=C-C[H.sub.2]; 3.6, m, O-C[H.sub.2]C[H.sub.2]-O from PEG; 4.0 to 4.3, m, O=C-O-C[H.sub.2], 5.7 to 6.5, m, -CH=C[H.sub.2].
I.R.Spectra: All acrylates had a 1740[cm.sup.1] peak for carbonyl stretch and 1630[cm.sup.-1] peak for C=C stretch in their IR spectra.
Typically 1 g of above prepared macromer was mixed with 20 [micro]l of photo initiator viz., benzophenone / hydroxycyclohexyl acetophenone. This solution was taken in a glass mold of approximately 0.6 mm thickness and photopolymerized by exposing to long wave length U.V. light for about 15 seconds using a medium pressure mercury vapour lamp (2.66 W/[cm.sup.2]) in a lab cure unit (Wallace Knoght, U.K). Specimens used in burst test were cured with a spot-curing machine (0.8 to 1.0 W/[cm.sup.2], UVP, U.S.A).
Determination of gel content
Discs of 10 mm diameter were punched out of films obtained as above. Four such discs were weighed together ([W.sub.1]) and suspended in 50ml dichloromethane and extracted for 24 hours. The discs were then dried to constant weight and weighed again ([W.sub.2]). The gel content was calculated as [W.sub.2]/[W.sub.1] X 100. The gel content was determined in triplicate and average value is reported.
Water equilibrium swell
Four discs were taken and suspended in distilled water for 24 hours. The swollen discs were gently pressed between filter papers to remove adsorbed water and weighed ([W.sub.1]). The discs were then dried at 120[degrees]C until for 15 minutes and dry weight determined ([W.sub.2]). Equilibrium swell was calculated as [W.sub.1]-[W.sub.2]/[W.sub.2] X 100. Water equilibrium swell was determined in triplicate and average value is reported.
Contact angle measurement
Equilibrium water contact angles of cured discs was done using Contact Angle Measuring Instrument G10 of Kruss, Germany.
Disc shaped samples were suspended in 50 ml of 0.1 N NaOH in 100 ml polypropylene bottles and kept in a constantly shaking water bath at 37[degrees]C. The discs were observed from time to time and total time taken for complete digestion of discs in to soluble materials was determined. Four samples for each formulation were studied and average reported.
In vitro burst test
A gelatin film was used as a substrate to roughly simulate a biological surface. An in house made apparatus was used for this purpose [Figure 1]. This contained a small aluminum cup with wide lip and two inlets. One inlet was stoppered with a rubber septum and other was connected to a pressure gauge. A tin plate measuring same diameter as that of the mouth of the cup was fixed on to the cup using a rubber gasket. A hole measuring 3.8 mm in diameter was made in the middle of this plate. A gelatin film, also having a similar hole, was adhered to the plate using crazy glue. When polypropylene film was used as a substrate, it was attached to the tin plate with a double-sided adhesive tape. About 500[micro]l of test sealant was applied on and around the hole and cured with U.V. spot curing machine. The cup was pressurized with water using a syringe and pressures at which the sealant gave up and the mode of failure were noted. Five tests were done for each sealant and average reported.
[FIGURE 1 OMITTED]
In vitro sulfamethoxazole delivery
Acrylate macromers with 5-wt% of sulfamethoxazole and photo initiator were crosslinked in the form of concave discs in Kline Concavity slides. They were then suspended in pH 7.4 phosphate buffer and kept in a shaking water bath at 100 rpm and 37[degrees]C. Aliquots were collected from time to time and drug released was estimated using a standard curve generated by plotting absorption at 264 nm wavelength.
Results and Discussion
For any material to be useful for tissue sealant applications, it has to meet certain minimum characteristics such as a) it should be a viscous liquid so that it is convenient to place it at required target surface b) should very rapidly polymerize once triggered c) should wet and adhere to moist and bloody tissues under physiological conditions d) once polymerized should become solid rubbery material e) be biocompatible and degradable, in reasonable time, in to harm less water soluble materials that body can excrete easily (8). We believe that simple aliphatic polyester polyol acrylates meet these requirements and have been investigating them for tissue sealant applications. Sealants are generally used, not alone, but to augment other devices such as sutures and staples. For realizing a good seal it is absolutely essential that the used sealant should wet the target surface as well as the prosthetic optimally before it is cross-linked. This wetting is determined by structure of both the sealant as well as the tissue surface and also the prosthetic used along with the sealant. Wetting is a surface phenomenon and depends on several different parameters such as viscosity of the sealant, its surface tension, surface energy of the target etc. Surface of a living tissue is very complicated and is made of a mosaic of hydrophilic and hydrophobic zones of few angstroms width (7). In this context we wished to see if we can alter the performance of a sealant by carefully adjusting its overall hydrophilicity or hydrophobicity. Aliphatic polyester polyols lend themselves to such manipulation easily and it is possible to synthesize very closely resembling materials with different hydrophobicities. It is the subject matter of the present communication to discuss synthesis, characterization and in vitro properties of a range of polyester polyol acrylates especially with respect to their hydrophobicity.
Synthesis and characterization of Polyesterpolyols
Polyesters are usually prepared by melt polycondensation of a diacid and a diol in presence of an acidic catalyst. We have investigated this reaction with polyethylene glycol and succinic acid in our previous work (5, 6) and found that it is possible to make viscous acrylates, which rapidly polymerize in to coherent gel films upon exposure to long wave length UV light.
For the present work we needed polyols of graded hydrophobicity. It is possible to tune the hydrophobicity of the polyol by either changing the acid or by replacing PEG in part with more hydrophobic butane diol. We have investigated both the modifications and synthesized polyols from diacid homologues, succinic, glutaric and adipic acids and also by replacing 25, 50 and 75 mole% of PEG with butane diol [Fig 2]. The macromers synthesized and their physical properties are listed in table 1. It should be noted that though a 0.5 molar excess of diol was employed, at the end of the reaction the products still had finite acid numbers. It was not possible to reduce the acid numbers any further since PEG is heat sensitive. In case of butane diol containing formulations, prolonged heating and vaccum application resulted in removal of butane diol. Polyols with lower homologues of acid could not be made since the high temperatures required resulted in decarboxylation. Similarly we have not investigated a homologue with only butane diol as the diol component since it was a solid product.
[FIGURE 2 OMITTED]
From table 1 it is also evident that all the polyols except SAP75BD have useful viscosities. SAP75BD was a waxy solid at room temperature, but gentle heating at 40-45[degrees]C rendered it liquid and it was used as such.
Main characterization of all the polyols was done with the help of [sup.1]H NMR [Fig 3].
[FIGURE 3 OMITTED]
A near 1:1 integration for peaks at 2.4 and 4.2 in the polyol nmr suggested that the condensation is complete and no free acid existed in the polyol. In the case of polyols made with a mixture of PEG and butanediol, the incorporation ratio of respective diols could be easily determined from the integration of the ester methylene protons in the region 4 to 4.5 Fig. Methylene protons of butanediol resonated at a slightly lower field than those of PEG. Their ratios were same as the feed ratio suggesting that no butane diol escaped during the reaction.
[FIGURE 4 OMITTED]
Acrylates were prepared from these polyols using acylation reaction with acryloyl chloride in presence of triethylamine base. Introduction of terminal methylene protons at 5.8 to 6.5  confirmed the formation of acrylates [Fig 3].
Photopolymerization and gel content
All the macromers of the study were photopolymerized as thin films, approximately 0.9 to 1.2 mm thick, in glass molds by exposing to long wavelength UV radiation. About 10 seconds exposure was found to be sufficient to cross link all the formulations while using the high intensity Wallace Knight unit. In case of lower intensity, as obtained in spot curing instrument, longer times of exposure, of the order of 30 to 40 seconds, were necessary to obtain full cure. They all had nearly 90% gel contents suggesting very rapid and substantial cross-linking [Table 2].
Equilibrium water swelling data represents a trend depending on the expected hydrophobicity of the crosslinked gel [Table 2]. In the acid homologous series as the chain length of the acid decreased, the water absorption raised. Also as PEG was replaced with more and more of butane diol, the resulting gel absorbed less and less water. SAP75BD had substantially less water equilibrium swell as compared to SAP. Thus equilibrium swelling of the crosslinked matrices followed an expected trend based on their chemical structures.
Equilibrium contact angle with water
For a given surface, determination of contact angle with water is an easy and rapid method to estimate its hydrophilicty. Table 3 shows the equilibrium contact angle of water on the films of the present investigation.
As can be seen, as the chain length of the acid decreased in the acid homologues series, wetting improved and the contact angle decreased suggesting a trend to become less hydrophobic. In the series of butane diol modification as the amount of butane diol increased in the formulation, the contact angle increased excepting SAP75BD. Though we have prepared this particular formulation three times and also purified it by precipitation in water, the contact angle remained 30[degrees]. This obvious discrepancy may be due to it being a solid at room temperature and may be some crystallinity associated with it though we have not investigated it any further. But for this other members show the expected trend of increasing contact angles with increasing amount of butane diol. This shows that it is possible to tweak the hydrophobicity of these polyol acrylates by carefully choosing the stoichiometry of a mixture of diols.
In vitro Burst strength
Table 3 also shows the in vitro burst strengths and mode of failure of the sealants of the present study. The burst tests were done on two substrates a) a hydrophilic gelatin film and b) a hydrophobic polypropylene film. All the sealants failed only in adhesive mode and no failures in the cohesivity of the sealants were noticed. In case of the acid homologue series the modification of hydrophobicity of the sealant, changed the burst strength in a graded manner. As the acid chain length decreased in the polyol, its hydrophilicity increased and wetted the hydrophilic gelatin film better and hence resulted in greater burst strengths. But in case of butane diol modified sealants, such trend was not clearly visible possibly because they all provided very high adhesion values.
To check the above conclusions even more closely a hydrophobic surface was also tried. If the above wetting theory is as crucial as believed above, then the sealants should show a reversed trend. It indeed was the case. Burst strengths on PP films changed with both change in acid as well as butanediol content. With decrease in the chain length of acid, the burst strength values also decreased and they increased with increase in the content of butanediol. This shows that all other things being equal, adjusting the hydrophilicity / hydrophobicity can be used to optimize a sealant for a given surface.
Another important property the above modifications can result in is in their hydrolytic degradability. As the hydrophobicity of a matrix increases, it swells less in water and hence may take greater time for hydrolysis. To know this an expedited in vitro degradability study (9) was done on the cross-linked matrices of the study. Time taken by the crosslinked matrices of the present study to completely hydrolyze in to water-soluble product in 0.1 N sodium hydroxide at room temperature is presented in table 4.
As can be seen, the total times for degradation reflected the trend seen in their hydrophobicity. Sealants of the study degraded in as low as 32 hours to as high as 109 hours depending on the stoichiometry. Thus it is shown that it is possible to control the longevity of prosthesis in a body by properly adjusting its hydrophobicity.
[FIGURE 5 OMITTED]
In vitro delivery of sulfamethoxazole
These sealants, apart from serving the mechanical function they are intended for, may also work as reservoirs of suitable drugs for localized delivery. Again the delivery of the incorporated drug shall depend on the properties of the cross-linked matrix such as its rate of degradation. As a model drug sulfamethoxazole was incorporated in to these matrices and its delivery in vitro in to pH 7.4 buffer at 37[degrees]C was studied. Fig 5 & 6 represent the release of the drug over a twenty day period. As can be seen more hydrophobic matrices released the drug more slowly.
[FIGURE 6 OMITTED]
Polyesterpolyols and their acrylates intended for tissue sealant applications were prepared from aliphatic dicarboxylic acid homologues, succinic, glutaric and adipic acids and Polyethylene glycol.
Another series of polyesters were also prepared by replacing PEG by increasing amounts of 1,4-butane diol. Using either of the methods it was possible to make UV polymerizable macromers of graded hydrophobicity. They have shown expected trends in their properties such as water contact angles, water equilibrium swelling and hydrolytic degradation depending on their hydrophobicity.
In vitro burst tests were conducted with these sealants using gelatin film as a hydrophilic substrate and a polypropylene film as a hydrophobic substrate. This testing also suggests that it is possible to tweak sealant's efficiency by adjusting its structure.
Authors wish to thank DST, New Delhi for financial assistance in the form of a project. TTR thanks CSIR, New Delhi for senior research fellowship.
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Venkata Nivasu M., Thimma Reddy T. and Shekharam Tammishetti *
Organic Coatings and Polymers Division, Indian Institute of Chemical Technology, Hyderabad--500 007, Andhra Pradesh, INDIA
* Corresponding Author e-mail: email@example.com
Table 1: Macromers synthesized in this study PEG 600 Butane diol Acid No Code Acid(mol.eq) mol.eq (mol.eq) Initial AAP Adipic acid (1.0) 1.50 0 137 GAP Glutaric acid (1.0) 1.50 0 143 SAP Succinic acid (1.0) 1.50 0 184 SAP25BD Succinic acid (1.0) 1.125 0.375 127 SAP50BD Succinic acid (1.0) 0.75 0.75 156 SAP75BD Succinic acid (1 .0) 0.375 1.125 238 Acid No Hydroxy Code Final No. Viscosity cps AAP 57 90 700 GAP 54 87 775 SAP 52 70 825 SAP25BD 30 102 550 SAP50BD 29 140 700 SAP75BD 23 147 5800 Table 2: Gel content and water equilibrium swells of different cross-linked matrices of the study Gel Water eq. Sealant content,%(S.D) swell, %(S.D) AAP 90 (0.1) 145 (1.2) GAP 90 (1.0) 152 (1.4) SAP 89 (0.1) 156 (1.1) SAP25BD 94 (0.3) 136 (0.9) SAP50BD 94 (0.3) 102 (0.4) SAP75BD 93 (0.4) 51 (0.3) Table 3: Water contact angle and burst test results Burst strength, psi (S.D) Contact angle On gelatin On PP Sealant [degrees] film film AAP 58 (1.2) 29 (0.8) 29 (0.8) GAP 47 (1.2) 34 (1.5) 21 (1.2) SAP 41 (1.7) 45 (2.9) 18 (0.9) SAP25BD 57 (0.6) 42 (2.6) 25 (0.4) SAP50BD 63 (1.5) 44 (1.7) 35 (0.4) SAP75BD 30 (1.2) 40 (2.3) 45 (0.5) Table 4: Time taken for degradation of cross-linked matrices in 0.1N NaOH Sealant Hours AAP 47 (1.5) GAP 40 (0.8) SAP 32 (1.4) SAP25BD 57 (0.5) SAP50BD 62 (1.0) SAP75BD 109 (1.5)
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|Author:||Nivasu M., Venkata; Reddy T., Thimma; Tammishetti, Shekharam|
|Publication:||Trends in Biomaterials and Artificial Organs|
|Date:||Jul 1, 2004|
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