CHEN XiaoyingGUAN GuopingMA YanxueMENG FenyeLI Yuling

1 College of Textiles,Donghua University,Shanghai 201620,China 2 Key Laboratory of Textile Science and Technology of Ministry of Education,Shanghai 201620,China 3 Key Laboratory of Textile Industry for Biomedical Textile Materials and Technology,Shanghai 201620,China

Abstract:Textile vascular grafts are often used to treat the cardiovascular diseases. Scholars continue to search for new materials for the development of vascular grafts with excellent properties,polyimide (PI) fiber is a material suitable for making vascular grafts with high strength,radiation resistance and stable property,as well as non-cytotoxic and satisfying blood compatibility. This study investigated the tensile strength and hydrolytic degradation properties of PI,polyester (PET) and nylon(PA). The results suggested that the PI is suitable for preparing vascular grafts. And influences of different weaves and ratios of warp and weft density on the water permeability,thickness and porosity of PI vascular grafts were analyzed. Vascular grafts with six weaves and two ratios of warp and weft density were designed and prepared. The surface morphology,permeability and thickness were characterized to optimize the structure of the vascular grafts. The results showed that the wall thickness of all the samples is less than 100 μm except for the sample with the ratio 2∶3 and 1/3 twill pattern. Permeability is mainly determined by the weave and the ratio of warp and weft density. The samples in plain weave have the lowest water permeability compared with other samples.

Key words:polyimide(PI)；vascular grafts；permeability；thickness；porosity

Introduction

Developing and selecting novel materials for next generation vascular grafts with better performance has never been suspended over the globe,even though vascular grafts in large diameter succeed in clinical applications[1-8].Common materials for vascular graft preparation include silk,expanded polytetrafluoroethylene,polyurethane,and polyester (PET)[9-10].PET has favorable strength and biocompatibility,but the blood compatibility is not ideal.It is also easy to form embolism,so it is only used for large diameter vascular grafts.Polyimide (PI) is an inert material.Like PET,it has excellent mechanical properties,is non-toxic,and has nice biocompatibility.Moreover,its superiority to PET is the high blood compatibility[11-12].Water permeability and wall thickness are two important graft indicators.It is generally considered that vascular grafts having water permeability less than 300 mL/(cm2·min) do not require pre-clotting,and those samples having a thickness less than 100 μm are ultra-thin vascular grafts.Even if the theoretical tightness is the same,the actual tightness of different weaves will be different,which will affect the thickness and water permeability.At the same time,the ratio of warp and weft will affect them,too.Therefore,in the present work PI multifilament was used to weave vascular grafts,and the influences of the weaves and the ratio of warp and weft density on the permeability and the thickness were explored,aiming to achieve water permeability below 300 mL/(cm2·min) and the thickness less than 100 μm.Moreover,the pore size and the size distribution of each sample were evaluated,and the relationship between the pore size and the water permeability was discussed[13].

1 Materials and Methods

1.1 Materials

The yarn used for preparing samples was PI,and it was selected from Zhejiang Haorui New Material Technology Co.,Ltd.,Yiwu,China.The yarns for comparison in this study are shown in Table 1.

Table 1 Properties of yarns

1.1.1TensilestrengthtestofPI,PETandPAfilaments

The tensile strengths of PI,PA and PET yarns were tested according to the standard GB/T 3916-2013[14]on XL-1A yarn strength tester (Shanghai New Fiber Instrument Co.,Ltd.,China),the speed was set as 500 mm/min,and the clamping distance was set 500 mm.

1.1.2HydrolyticdegradationtestofPI,PETandPAfilaments

According to the environment required by the standard GB/T1688.13-2017[15],filaments of PI,PA and PET were placed in test tubes containing phosphate buffer saline (PBS) buffer,and the tubes were placed in a constant temperature air shaker,the temperature was maintained at 37 ℃ (this value is the normal human blood environment).One month later,the samples were taken out and washed with distilled water and then weighed.The change of the surface morphology of the filaments was observed under a TM 3000 scanning electron microscope(SEM,Hitachi,Japan).The pH of the buffer in the test tubes was measured by a pH meter.And the tensile strength of the sample was measured in the same way as section 1.1.1.

1.1.3Yarnpreparation

The synthesized fiber multifilament tends to generate static electricity and fluff during weaving process,so the sizing is needed.Silicone oil was thus used in the production process,so it was removed before the sizing process.The PI multifilament was dipped in the diluted emulsifier (emulsifier∶water is 1∶3).

The PI multifilament impregnated with the emulsifier was sized with sizing liquor.PVA1799 and PVA205 were used for blending.The general blending ratio in the literature is PVA1799∶PVA205=7∶3 or PVA1799∶PVA205=9∶1,the concentration was 10%.Since PVA205 has good solubility in water,PVA1799∶PVA205=7∶3 was selected for easy desizing,and added 1% glycerol to reduce the formation of serosal membrane in size liquor during sizing as well as increase the affinity of the slurry to the multifilament.

1.2 Preparation of vascular grafts

The sized multifilament was woven using a rapier loom following the designed structures(shown in Fig.1) and parameters(shown in Table 2).Considering that the average float has an effect on the actual tightness of the fabric,thereby affecting the thickness and water permeability,six weaves with four floats of 1,1.5,2,and 3 were designed.There were three types of woven structure with an average float of 2 to explore the effects of different woven structure on the thicknesses and the water permeability.And the ratios of warp and weft density were 1∶1 and 2∶3.Weft take-up was larger than warp take-up in fabric,so the set weft thread count was slightly larger,and the warp thread count was slightly smaller.

Fig.1 Weave diagrams for the vascular grafts:(a)1/3 twill;(b)2/2 twill;(c)4-shaft irregular satin;(d)plain;(e)1/2 twill;(f)6-shaft irregular satin

Table 2 Designed parameters

Then 12 kinds of vascular grafls were obtained after desizing and washing off the oil(shown in Table 3),and heat setting with a mould[10].The heat setting was at a temperature of 400℃(slightly above its glass transition temperature) for 15 min which was determined by pretests.

Table 3 Process for desizing and washing

1.3 Morphological observation of vascular grafts

Micro-structure of all the samples was observed under a PXS8-T stereo microscope(Shanghai Ciwei Photoelectric Technology Co.,Ltd.,Shanghai,China) and micro-photographs were obtained to evaluate the porosity.The pore size of pores with a side length >10 μm were mainly observed.The length and width of the pores were measured by the measurement tools provided by the microscope software,and then the area was calculated.The pores of each sample were observed for three points,and the average value was obtained.

1.4 Test of wall thickness

Wall thickness of each vascular graft was measured by YG141N(Changzhou Dahua Electronic Instrument Co.,Ltd.,Changzhou,China) digital sample thickness gauge (n=10).

1.5 Water permeability test

According to ISO 7198:1998[16],water permeability was tested on a self-developed apparatus under a pressure of 16 kPa for 8 min,and the amount of water permeability per minute was measured respectively.The amount of water leaked per square centimeter per minute was calculated[17].

1.6 Statistical analysis

The data in this paper were expressed as mean ± standard deviation,and one-way analysis of variance(ANOVA) was used to compare the significant differences between the groups.Whenp<0.05,the intra-group difference was considered to be significant,which was indicated by *;whenp<0.01,the intra-group difference was considered to be very significant,which was indicated by **;whenp<0.05,the difference between groups was considered to be significant,which was indicated by #.

2 Results and Discussion

The following results include the tensile strength,the degradation of three materials,and the thickness,the water permeability and the porosity of the 12 samples.

2.1 Test results

2.1.1TensilestrengthsofPI,PETandPAfilaments

Tensile strengths of PI,PET and PA filaments were shown in Fig.2,suggesting that PI has higher tensile strength at break than PET and PA,it can be concluded that PI can provide the tensile strength required for a vascular graft.However,the elongation of these samples showed no distinct differences,that is to say,PI vascular graft may possess the similar deformation ability with the PET and PA vascular grafts.

2.1.2HydrolyticdegradationpropertyofPI,PETandPAfilaments

The original and degraded yarns of PI,PET and PA filaments were evaluated through SEM shown in Fig.3.There is no changes on the surface of PI and PET,but the gully is deepened on the surface of PA.Furthermore,the weights of PI and PET were not basically changed,while the PA weight lost reached 0.49%.And the pH of the degradation solutions kept constant 7.4±0.03.And the tensile strength of PI and PET were not changed,but of PA reduced by 10.8%.So PI is as stable as PET in the environment of pH=7.4.

Fig.2 Tensile strengths of PI,PET and PA

Fig.3 SEM micrographs:(a) original PI yarn;(b) original PET yarn;(c) original PA yarn;(d) degraded PI for one month;(e) degraded PET for one month;(f) degraded PA for one month

2.1.3Morphologyofvasculargrafts

After weaving,desizing,washing and heat setting,the tubular vascular grafts as shown in Fig.4 is obtained.

Fig.4 Woven PI vascular grafts of 4-shaft irregular satin

And the porosity of the samples was shown in Fig.5.Many small pores can be found occasionally in plain weave samples whose ratio of warp and weft density is 1∶1.And the average area of the pores is 755 μm2,while when the ratio of warp and weft density is 2∶3,the pores can be barely found.Some pores can be seen in 4-shaft irregular satin weave sample both in 2∶3 and 1∶1,and the average areas of the pores are 1 086 and 900 μm2,respectively.And no pores were found on the samples with a side length (>10 μm) which can be observed by the stereo microscope in the other samples.

Fig.5 Micro-photographs:(a)plain 2∶3;(b)plain 1∶1;(c)1/2 twill 2∶3 ;(d)1/2 twill 1∶1 ;(e)1/3 twill 2∶3;(f)1/3 twill 1∶1;(g)2/2 twill 2∶3;(h)2/2 twill 1∶1;(i)4-shaft irregular satin 2∶3;(j)4-shaft irregular satin 1∶1;(k)6-shaft irregular satin 2∶3;(l)6-shaft irregular satin 1∶1

2.1.4Wallthickness

The wall thicknesses of the samples were shown in Fig.6.Overall,the 1∶1 samples are thinner than 2∶3 ones.Generally speaking,the walls of all the samples were thin and less than 100 μm except for the sample with the ratio 2∶3 and 1/3 weave.All the vascular grafts were belonging to thin-wall ones suitable for the grafts of stent-graft products.And the plain weave ones are generally thinner than others.It is known that the fabric made of the coarse yarn is thicker.The PI multifilament used in the study has a fineness of 40 dtex and is a very fine yarn,which determines that the wall thickness of the prepared vascular graft is very thin.And the samples prepared in this study were woven with multifilament.The multifilament is composed of many very fine monofilaments,and the surface is smooth.During the heat setting,the filaments tended to be evenly distributed and dispersed,which made the flat space utilization of the filaments larger.So the fabric was thinner and the pores were smaller.The plain weave as the closest weave has the most interlacing points,which makes the filaments have the lowest degree of buckling,so the samples are the thinnest.

Fig.6 Wall thickness of vascular grafts

2.1.5Waterpermeability

Due to the greater degree of distortion of the weft yarn in the fabric,this may result in a larger filling effect on the space.So the proportion of the weft yarn is appropriately increased in the text.Generally,the ratio of warp and weft density is 1∶1.When the proportion of weft yarns in the regular structure fabric(the weave in which the number of the weft/warp interlacing point on each warp/weft yarn in a weave repeat unit is the same.) is increased to a ratio of warp and weft density of 2∶3,the proportion of weft yarns reaches a limit.And thereafter,increasing the proportion of weft yarns causes a series of problems.The latitudinal tightness will be too large,and the consumption of raw materials could be too large and production could be difficult.Moreover,the rigidity of the fabric will be too large,which will affect the compliance of the vascular graft.What’s more,if the weft thread count is too small,the water permeability is likely to increase a lot.So the ratio of warp and weft density is 1∶1 and 2∶3 in this paper.

The water permeability of the samples was shown in Fig.7.When the ratio of warp and weft density is 2∶3,the permeability is relatively lower than that of 1∶1.Usually,the plain weave samples have the lowest permeability among all samples.Except for the 1/3 twill weave ones,the permeability of the others is below 300 mL/(cm2·min) which is normally considered as the threshold value for clinical pre-clotting.For regular fabrics,on the same conditions,the longer the average float,the looser the fabric,so the longer the average float is,the larger the water permeability is theoretically.The average float of 1/2 twill is 1.5,1/3 twill is 2,and 6-shaft irregular satin weave is 3.But the water permeability of 6-shaft irregular satin samples is not much higher than 1/2 twill samples,even much lower than 1/3 twill ones.Moreover,the water permeability of samples with 4-shaft irregular satin and 2/2 twill is much lower than that of 1/2 twill.So the water permeability and thickness of fabrics are different in different weaves even with the same average float.And twill weave fabrics may have higher water permeability.

Fig.7 Water permeability of vascular grafts

2.2 Experimental discussion

Tensile strengths test proves that PI has the similar mechanical properties for weaving vascular grafts.Hydrolytic degradation property test shows that PI is as stable as PET in the environment of pH=7.4.

Figure 6 can explain the results reflected in Fig.7 to some extent.For plain samples,there are pores only when the ratio of warp and weft density is 1∶1.For 4-shaft irregular satin weave,the pores of the sample with the ratio 2∶3 are generally bigger than 1∶1.And the water permeability follows the same law.No pores can be seen in the other samples whose water permeability is larger than the plain ones.Because the water permeability test is carried out under a high pressure,fabric will go concave due to pressure deformation.The yarns will move,and the pores appear between the yarns.The longer the average float is,the larger the pores appear,and the larger the water permeability is theoretically.However,the tested results are not exactly the same as this regulation.Twill weave fabrics may have higher water permeability.

The plain samples with the ratio 1∶1 and the 4-shaft irregular satin weave with both ratios have microscopically observable pores,so they may be more suitable for the endothelial cells ingrowth[18].

3 Conclusions

In the present work,twelve vascular grafts prototypes were designed and successfully prepared using PI.Among them,ten samples can meet the expected water permeability of less than 300 mL/(cm2·min) and the thickness of less than 100 μm without any other treatment after heat setting.By comparing the tensile strengths and hydrolytic degradation properties of PI,PET and PA filaments,it is found that PI is as suitable for the preparation of vascular grafts as PET in terms of mechanical property and stability.The water permeability and the thickness of most samples can meet the requirements for implantation.Overall,the influence of the ratios of warp and weft densities on the thickness is not significant.Due to differences in structure between weaves,except for the sample with the ratio 2∶3 and 1/3 twill weave ones,the wall thickness of all other vascular grafts meets the requirements (<100 μm).Except for the 1/3 twill weave ones,the permeability of the others meets the requirements of below 300 mL/(cm2·min),and when the ratio of warp and weft density is 2∶3,the permeability is relatively lower than that of 1∶1.Plain weave samples have the lowest permeability among all samples.And the plain samples with the ratio 1∶1 and the 4-shaft irregular satin weave with both ratios have observable pores.These vascular grafts may be satisfactory candidates for next generation vascular grafts if they further possess excellent both biological and mechanical properties.Therefore,those performances not tested in this study need to be tested and improved in the future.