Microwave-assisted functionalization of polyurethane surface for improving blood compatibility

Journal of Industrial and Engineering Chemistry 19 (2013) 1587–1592

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Microwave-assisted functionalization of polyurethane surface for improving blood compatibility Deqiang You a, Hao Liang a, Weizhong Mai a, Rong Zeng a,*, Mei Tu a,*, Jianhao Zhao a, Zhengang Zha b a b

Department of Materials Science and Engineering, College of Science and Engineering, Jinan University, Guangzhou 510632, PR China Guangzhou Overseas Chinese Hospital, the First Affiliated Hospital of Jinan University, Guangzhou 510630, PR China

A R T I C L E I N F O

Article history: Received 11 December 2012 Accepted 24 January 2013 Available online 31 January 2013 Keywords: Surface modification Microwave irradiation Polyurethane Blood compatibility Poly(ethylene glycol)

A B S T R A C T

In order to improve the hemocompatibility of polyurethane (PU), we report a rapid and efficient twostep approach to graft poly(ethylene glycol) (PEG) onto PU surface by a microwave-assisted method, involving diphenylmethane diisocyanate (MDI) – functionalization and subsequent PEG coupling. Compared with conventional heating, the effects of solvent, time and MDI concentrations on the microwave-assisted MDI-functionalization, and the effect of time on the microwave-assisted PEG coupling were studied. PEGs with different molecular weights were successfully grafted onto PU surface under the optimum microwave-assisted conditions within only 20 min, and characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and chemical titration. The hydrophilicity and in vitro blood compatibility of the surfaces were evaluated by water contact angle measurements, blood coagulation time (whole blood clotting time and prothrombin time) and platelet adhesion tests, respectively. All the PU-PEG surfaces had improved surface wettability and hemocompatibility. The results suggested that microwave-assisted functionalization may be a promising method for rapidly and effectively decorating polyurethane surfaces. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Blood compatibility plays a crucial role in blood-contacting biomaterials and biomedical devices, such as hemodialysis membranes, cardiovascular devices, artificial organs, drug delivery systems, and tissue engineering scaffolds [1]. Although millions of implants and devices have been successfully used in contact with blood for modern clinical diagnosis and therapies over the past decades, how to obtain truly blood compatible surfaces is still a serious problem unsolved ultimately [2]. For example, almost all the cardiovascular devices have thrombosis problems due to a multi-step and interlinked process induced by the surface of these devices, including protein adsorption, and platelet adhesion and activation [3]. Since the blood-material interactions affecting blood compatibility are significantly dependent on the surface characteristics of biomaterials, e.g. surface chemical composition and morphology, flexibility, wettability, surface free energy and surface charge, various approaches have been developed for modifying biomaterial surfaces to improve their hemocompatibility, such as grafting modification, physical adsorption, self-assembly

* Corresponding authors. Tel.: +86 20 85223271. E-mail addresses: [email protected] (R. Zeng), [email protected] (M. Tu).

technology, biomimetic modification, surface-modifying additives, and so on [4]. It is well known that polyurethane (PU) is widely used in bloodcontacting application due to its excellent physicochemical properties, high mechanical flexibility, and relatively superior biocompatibility. However, its blood compatibility has to be improved especially for long-term blood-contacting application. Several methods have been investigated for this aim by covalent immobilization of heparin [5], polyethylene glycol (PEG) [6], hyaluronic acid [7], and phosphorycholine polymers [8,9], which involved various surface functionalization techniques, e.g. wet chemical treatment [10], plasma treatment [11], and UV irradiation [12]. Among them, grafting PEG onto the PU surface has attracted great attention, because PEG can effectively reduce nonspecific protein adsorption as well as platelet adhesion mostly due to its large excluded volume and tightly bound water layer [6]. Moreover, PEG can be used as a spacer to immobilize the bioactive molecules, such as heparin, zwitterions for better improving biocompatibility [13]. For example, Huang and Xu reported a novel three-step procedure to graft zwitterion of sulfobetaine structure monomer to the PU surface through a PEG spacer for improving hemocompatibility [14]. Microwave irradiation as an effective and economical technology has been extensively investigated in the rapid synthesis of

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.01.027

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organic and inorganic materials, as well as surface functionalization of materials, due to its remarkable advantages such as shorter reaction times, higher yields, and limited generation of byproducts compared with conventional heating [15,16]. Recently, Park et al. reported a facile method to functionalize graphene oxide surface with polymer brush assisted by microwave irradiation [17]. In this work, we report a rapid two-step microwave-assisted approach for grafting PEG onto polyurethane surface, involving microwave-assisted diisocyanate-functionalization and subsequent microwave-assisted PEG grafting, which can be applied to polyurethane materials with any shape, even inner surface of tubes. Compared with conventional heating, the effects of solvent, reaction time and diisocyanate concentrations on the NCO surface density for microwave irradiation were studied, and the optimum microwave-assisted grafting PEG conditions were determined. The chemical structure of surfaces was characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and chemical titration. The hydrophilicity and in vitro blood compatibility of the original and modified PU surfaces were evaluated by water contact angle measurements, blood coagulation time and platelet adhesion tests, respectively. 2. Experimental 2.1. Materials Thermoplastic polyurethane, Desmophen1 385E (polyester resin) was obtained from Bayer Co. Ltd. 4,40 -Diphenylmethane diisocyanate (MDI, Alfa Aesar) and triethylamine (TEA, Damao Chemical Reagent Factory, Tianjin, China) were used without further purification. Polyethylene glycols with different numberaverage molar mass (Mn) of 400, 2000 and 10,000 (Alfa Aesar) were dried at 60 8C in vacuum over night. o-Dichlorobenzene (DCB, 98%, Jingchun Industrial Co. Ltd., Shanghai, China) was used as received. Toluene (Tol, Analytical reagent, Damao Chemical Reagent Factory, Tianjin, China) was dried over molecular sieves 4 A˚ for 48 h before using. Other solvents were all analytical grade. 2.2. Preparation of PU films The commercial PU pellets were extracted with methanol by Soxhlet extraction for 48 h to remove processing agents and lowmolecular weight components. About 200 mm thick PU films were cast from a 5% (w/v) solution in tetrahydrofuran in Petri dishes then dried at room temperature for at least 48 h followed by 24 h drying at 55 8C under vacuum, which were cut into 1 cm  1 cm pieces for the following experiments.

(w/v) solution (DCB/Tol = 5:5) at 55 8C for varied time (1–5 h) using conventional heating under nitrogen protection. 2.4. Microwave-assisted PEG grafting Microwave-assisted grafting PEG (Mn = 400, 2000, and 10,000) onto the PU surface was carried out by reacting the hydroxyl group of PEG with the isocyanate end group of MDI under microwave irradiation (100 W). To investigate the effect of reaction time on the PEG graft density of PU-PEG, the PU-PEG2000mw surface was prepared by incubating PU-MDImw in a stirred 2.5% (mol/L) PEG2000 solution (DCB/Tol = 5:5) at 80 8C under nitrogen protection with changing reaction time (2–10 min). The optimum grafting PEG conditions were also applied for coupling PEG400 and PEG10000. For comparison, the PU-PEG2000con surface was prepared by immersing PU-MDIcon in a stirred 2.5% (mol/L) PEG2000 solution at 55 8C with changing time (5–24 h) using conventional heating. 2.5. Determination of NCO content on the surfaces The NCO surface density on the modified PU films was determined by the standard dibutylamine titration method [6]. Briefly, the films were immersed in a toluene solution containing dibutylamine for 20 min, then added isopropanol. The resulting solution was titrated with hydrochloric acid using bromocresol green as the indicator. The NCO content on the surfaces was calculated from the volume of hydrochloric acid used. Each measurement was repeated in triplicate. 2.6. Attenuated total reflection Fourier transform infrared spectroscopy Attenuated total reflection Fourier transform infrared spectroscopy (ATR-IR) measurements were performed by a Bruker EQUINOX-70 spectrometer (Germany) at a resolution of 4 cm1 for 64 scans. 2.7. Water contact angle measurement Static water contact angle measurement was performed by the sessile drop method using a contact angle measuring system (Kru¨ss DSA100, Germany) at room temperature. A droplet (3 mL) of distill water was dropped on the surface using a micro syringe and the contact angle was calculated on the digital photos with the automatic software. Each sample was measured on at least three different locations and the results were recorded as an average of six measurements. 2.8. Blood coagulation time test

2.3. Microwave-assisted MDI-functionalization of PU surface In this step, the PU surface was functionalized with MDI assisted by microwave irradiation. In brief, the PU films were immersed into a DCB/Tol mixture solution containing MDI at different concentration, and 2.5% TEA (w/v) as a catalyst. After stirring at 80 8C under nitrogen amorphous and microwave irradiation (100W, CEM Discover1 microwave synthesis system, CEM Company, USA), the PU-MDImw surface was obtained and subsequently washed with toluene to remove unreacted reagents and dried at 55 8C for 24 h under vacuum. To obtain the effect of solvent, reaction time and MDI content on the NCO surface density of PU-MDI, the procedure was performed by changing DCB/Tol ratio (2:8–7:3), reaction time (1–10 min), and MDI content (2.5– 10% (w/v)). For comparison, the PU-MDIcon surface was prepared by incubating the PU films in a stirred 5% (w/v) MDI and 2.5% TEA

The whole blood clotting time (CT) and prothrombin time (PT) of the original PU and PU-PEG films were measured using fresh rabbit whole blood taken from New Zealand white rabbits (male, 2.5 kg), and the altex film was used as a positive control. 2.8.1. CT measurement CT was tested using the modified Lee–White method [18]. Briefly, fresh rabbit whole blood was added to 2 PP tubes each containing 6 cm2 sample film at a volume of 1 mL each other, which were previously incubated in a water bath at 37 8C. After incubation in a water bath at 37 8C for 3 min, one tube was picked up and inclined at 30 s intervals until clotting occurred. Then the blood coagulation was observed for the remaining tube in a similar manner. The blood clotting time was determined from the moment the fresh blood was withdrawn from a rabbit vein until it

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coagulated completely in the second tube. Measurements were done in triplicate.

surfaces, which was determined by a chemical titration method [6].

2.8.2. PT measurement PT was tested with a coagulation analyzer using mechanical end-point determination (STart4, Diagnostica Stago, France). In brief, platelet-poor plasma (PPP) was collected by centrifuging (2500  g, 10 min) citrate anticoagulated rabbit whole blood and utilized immediately. The sample films were contacted with 1 mL PPP in silanized glass tubes for 15 min at 37 8C. Then 0.1 mL of incubated PPP samples was mixed with 0.2 mL of PT reagent (Diagnostica Stago, France) for measurement. Each sample was tested for five times and the results were recorded as average value  SD.

3.1.1. Effect of solvent on the MDI-functionalization of PU surface We investigated the effect of solvent on the microwave-assisted MDI-functionalization of PU surface by varying the ratio of DCB to Tol between 2:8 and 7:3 at constant MDI concentration (5% (w/v)) and time (5 min) at 80 8C. As shown in Fig. 1, the NCO content of PU-MDI surfaces increased with increasing the ratio of DCB to Tol, which could be ascribed to the higher efficiency from microwave energy to the reactants for the larger ratio value of DCB to Tol, since DCB has a relatively higher loss factor value (tan d = 0.280 in 2.45 GHz) compared to tan d value of 0.040 for toluene [20]. However, if the ratio of DCB to Tol was above 6:4, the PU films would begin to swell and deform obviously after microwave heating. So we chose the DCB/Tol mixture at 5:5 as the solvent for enhancing the efficiency of microwave-assisted modification and keeping the shape stability of PU samples.

2.9. Platelet adhesion test Platelet adhesion studies were conducted on the original PU and PU-PEG films. Platelet-rich plasma (PRP) was prepared by centrifuging (150  g, 10 min) citrate anticoagulated rabbit whole blood. All the films were equilibrated with phosphate buffered saline (PBS, pH 7.4) for 24 h, and then immersed in PRP for 1 h at 37 8C. After gently rinsing with PBS for three times to remove non-adherent platelets, the platelets on the films were fixed using 2.5% glutaraldehyde solution for 2 h. The films were gently rinsed with PBS then dehydrated sequentially with 50%, 60%, 70%, 80%, 90%, and 100% gradient solution of ethanol/ water (v/v) for 30 min each, and subsequently allowed to dry at room temperature and finally sputter-coated with gold. Adhesion and deformation of platelets on surfaces were observed by scanning electron microscopy (SEM, Philips XL30 ESEM, The Netherlands). 3. Result and discussion 3.1. Microwave-assisted MDI-functionalization of PU surface Allophanate reaction between the N–H groups on the PU surface and the isocyanate groups has been regarded as an effective method to functionalize PU surface [10,19]. In this work, microwave irradiation was used to accelerate the allophanate reaction between MDI and N–H groups for activating PU surface using DCB/Tol mixture solution as solvent, and the factors affecting the MDI-functionalization of PU surface were investigated by evaluating the NCO surface density formed on the modified PU

3.1.2. Effect of MDI concentration on the MDI-functionalization of PU surface The effect of MDI concentration on the microwave-assisted MDI-functionalization of PU surface was investigated by varying its concentration between 2.5 and 10% (w/v) at 80 8C for 10 min using DCB/Tol mixture (5:5) as solvent, and the results were shown in Fig. 2. It could be seen that the NCO content of PU-MDI surfaces first increased with increasing MDI concentration, reached a maximum (107.4  108 mol/cm2) when MDI concentration was 5% (w/v), and then decreased. As the MDI concentration increased, the amount of MDI contacting the PU surfaces also increased, which resulted in an increased NCO surface density. However, at higher MDI concentration, the decrease of NCO surface density could be ascribed to the enhancement of self-polymerization of MDI. 3.1.3. Effect of reaction time on the MDI-functionalization of PU surface We also studied the effect of reaction time on the microwaveassisted MDI-functionalization of PU surface by varying the time from 1 to 10 min at constant MDI concentration (5% (w/v)) using DCB/Tol mixture (5:5) as solvent, and compared with those using conventional heating by changing the time from 1 to 5 h. The results were presented in Fig. 3. As can be seen, both PU-MDImw and PU-MDIcon surfaces exhibited a two-stage profile of the NCO surface density dependence on reaction time, but at different time

120 -2

100

10 mol cm

100

80

-8

-8

10 mol cm

-2

120

80

NCO content

NCO content

60 40 20 0

60

40

2:8

3:7

4:6

5:5

6:4

7:3

Solvent ratio (DCB:Tol) Fig. 1. Effect of solvent ratio (DCB:Tol) on the NCO content of PU-MDI surface ([MDI] = 5% (w/v), t = 5 min).

2.5%

5%

7.5%

10%

MDI content Fig. 2. Effect of [MDI] on the NCO content of PU-MDI surface (DCB:Tol = 5:5 (v/v), t = 10 min).

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3

4

5

100

-8

-2

NCO content ( × 10 mol· cm )

120

2

-2

120

Time (h) 1

10 mol cm

1590

80

60

40

Microwave irradiation Conventional heating

20

PEG surface density

-8

100

80 60 40 20 0

PU-PEG400

PU-PEG2000

PU-PEG10000

Fig. 5. PEG graft density on the PU-PEGmw surfaces.

0

2

4

6

8

10

Time (min) Fig. 3. Effect of reaction time on the NCO content of PU-MDI surface (DCB:Tol = 5:5 (v/v), [MDI] = 5% (w/v)).

scale. The NCO surface density of PU-MDImw films initially increased with the time till 9 min and then reached a saturation value about 107  108 mol/cm2, which was comparable with the saturation value for NCO surface density of PU-MDIcon films after about 4 h. The initial stage could be attributed to the increased reaction sites on the PU surface, and the following stage was owing to the saturation of the activated PU surface by MDI. The results also indicated that microwave irradiation can effectively accelerate the MDI-functionalization of PU surface. 3.2. Microwave-assisted PEG grafting onto surface Microwave irradiation can be also used for accelerating the following PEG coupling reaction. To investigate the effect of reaction time on the microwave-assisted PEG-grafting to PU-MDI surface by varying the reaction time from 2 to 10 min at constant PEG2000 concentration (2.5% (mol/L)) using DCB/Tol mixture (5:5) as solvent, and compared with those using conventional heating by changing the reaction time from 4 to 24 h. As shown in Fig. 4, the NCO content on both PU-PEGmw and PU-PEGcon surfaces decreased with increasing reaction time due to the reaction of the hydroxyl group of PEG with the isocyanate end group of MDI on the PU-MDI

Time (h) 6

NCO content

-8

10 mol cm

-2

40

12

3.3. ATR-FTIR analysis

18

35

24

Microwave irradiation Conventional heating

30 25 20 15 10 5 0 2

4

6

surfaces. After microwave-assisted grafting PEG for only 10 min, the NCO surface density was decreased from 107.4  108 mol/ cm2 for PU-MDImw to 4.6  108 mol/cm2 for PU-PEGmw, with a reduction of approximately 95.7%. While after grafting PEG for 24 h using conventional heating, the NCO surface density was decreased from 100.5  108 mol/cm2 for PU-MDIcon to 8.9  108 mol/cm2 for PU-PEGcon, with a reduction of approximately 91.1%. In addition, Chen et al. [6] reported that the percentage reduction of NCO on the PU surface was about 80% after grafting monobenzyloxy PEG to the PU-MDI surface using conventional heating. The above results indicated that microwave heating was more effective than conventional heating for PEG grafting reaction. PEG400 and PEG10000 were also grafted to PU-MDImw surface assisted by microwave irradiation under the optimized conditions ([PEG] = 2.5% (mol/L), DCB/Tol = 5:5, 80 8C, 10 min). The PEG graft density of PU-PEG surface was determined by the reduction of NCO content on the surface after grafting PEG and presented in Fig. 5. It could be found that the PU-PEG400mw and PU-PEG2000mw surfaces had a similar PEG graft density value slightly above 100  108 mol/cm2, but larger than PU-PEG10000mw surface (87.6  108 mol/cm2), suggesting that the grafting reactivity of PEG was related to their molecular weight. Since the microwaveassisted grafting of PEGs with different molecular weights was performed under the same conditions, the lower reactivity of PEG10000 was mainly due to its larger steric hindrance than PEG400 and PEG2000.

8

10

Time (min) Fig. 4. Effect of reaction time on the NCO content of PU-PEG2000 surface.

ATR-FTIR spectra of the original PU, PU-MDImw and PU-PEGmw with different PEG length surfaces were shown in Fig. 6. It was found that a new strong band ascribed to –NCO stretching vibration at about 2283 cm1 appeared for the PU-MDImw surface compared with that of PU surface, which almost disappeared for all the PU-PEGmw surfaces. Moreover, an increased shoulder peak at around 2870 cm1 for the PU-PEG400mw and PU-PEG2000mw surfaces, and a strong peak at 2880 cm1 for the PU-PEG10000mw surface could be observed, which was ascribed to the stretching vibration of –CH2 group of the PEG chains. Although the band of the formed urethane linkage between –NCO and PEG, and other bands of PEGs were overlapped with those of the PU substrate, a very intensive band assigned to the stretching vibration of C–O–C in the PEG chains at around 1107 cm1 was observed only for the PUPEG10000mw surface, which was in agreement with the observation of –CH2 stretching vibration, since the PU-PEG10000 surface

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100

Water Contact Angle (degree)

a

b

c d e

4000

3500

3000

2500

2000

1500

80

60

40

20

1000

-1

0

wavenumber/cm

-PU trol Con

Fig. 6. ATR-FTIR spectra of (a) PU, (b) PU-MDImw, (c) PU-PEG400mw, (d) PUPEG2000mw, and (e) PU-PEG10000mw surfaces.

P

EG U-P

400 PU

G - PE

200

0 PU

-

1 PEG

000

0

Fig. 7. Data of water contact angle on the original PU and PU-PEGmw surfaces.

contained much more C–O–C and –CH2 groups than both the PUPEG400 and PU-PEG2000 surfaces based on their molecular weights and graft density results. The above results confirmed the successful MDI-functionalization and the following covalent PEG grafting on PU surface assisted by microwave irradiation. 3.4. Water contact angle measurement Static water contact angles were measured on the original PU and PU-PEGmw with different PEG length surfaces obtained by microwave-assisted grafting. As shown in Fig. 7, the static contact angle value of water on the original PU film was 79.48  0.98, which was in consistence with the results in literatures of 788 [6] and 728  48 [11]. After grafting PEG, the water contact angles for all the PU-PEGmw films decreased significantly as expected, suggesting that their surface wettability was improved, which was mainly due to the strong hydrogen-bonding interaction between PEG chains and water. It was also found that the PU-PEGmw surface with longer PEG chain was more hydrophilic, although PU-PEG10000mw had a relatively lower graft density, because the –CH2CH2O– unit content on the surface was in the order: PU-PEG10000mw > PUPEG2000mw > PU-PEG400mw, determined by the measured PEG surface densities and their molecular weights, which can bound the water molecule by forming hydrogen bonding [21]. Similar experimental results were also reported by Han et al. [22] for the modified PU grafted with different molecular weights of PEO, the hydrophilicity increased with the increasing molecular weight of PEO. 3.5. In vitro blood compatibility The antithrombogenicity of PU-PEGmw films with different molecular weights of PEG, as well as the original PU films was evaluated by in vitro coagulation time tests, including CT and PT, which are commonly used for assessing in vitro blood compatibility of biomaterials. As shown in Table 1, the CT of PU-PEG400mw,

PU-PEG2000mw and PU-PEG10000mw was 1.23, 1.54 and 1.40 times longer than the original PU, respectively, and the PT of them was 1.34, 1.43 and 1.39 times longer than the original PU, respectively. All the CT and PT values of PU-PEGmw films became remarkably larger in comparison with the values of the unmodified PU films, revealing that the PEG-grafted PU surface exhibited improved anticoagulation activity. The possible mechanism was that the flexible hydrated PEG chains on the surfaces could prevent the microsurface blood proteins in stagnation, adhesion, and deformation and then suppress activation of the coagulation cascade and thrombin generation [23]. It was also worth noticing that the most hydrophilic PUPEG10000mw film did not show the best antithrombogenicity compared with other PU-PEGmw films with shorter PEG chains, since blood coagulation is a very complex process, which is affected by not only hydrophilicity but also other factors including surface morphology, flexibility, etc. For example, Lu et al. [24] reported the poly(MPC) brushes grafted PU with the lowest water contact angle did not show any improvement in blood compatibility compared to control PU, while Huang and Xu [14] reported that the PT duration almost kept unchanged but the activated partial thromboplastin time (APTT) and thrombin time (TT) were slightly prolonged by the introduction of PEG onto PU surface using conventional heating. Platelet adhesion test is also an important method for evaluating the blood compatibility of biomaterial, since platelet adhesion plays a key role in thrombus formation on the biomaterial surface [23,25]. The platelet adhesion results for the original PU and PEG-modified PU films observed by SEM were shown in Fig. 8. It could be seen that a number of platelets adhered on the original PU surface, and some of them showed obvious morphological changes from discoidal shape to pseudopodia and irregular shapes, indicating the platelet activation. While few platelets adhered on all the PU-PEG surfaces, suggesting that all the PEG-modified PU films exhibited a good platelet adhesion resistance. The results revealed that the PEG-modified PU films

Table 1 In vitro coagulation times (CT and PT) of the PU and PU-PEG films. Sample

Positive control altex

PU

PU-PEG400mw

PU-PEG2000mw

PU-PEG10000mw

CT (s) PT (s)

679  60 8.5  0.2

903  57 10.5  0.2

1114  70 14.1  0.2

1390  27 15.0  0.1

1262  51 14.6  0.2

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Fig. 8. SEM photographs of platelet adhesion on the (a) PU, (b) PU-PEG400mw, (c) PU-PEG2000mw, and (d) PU-PEG10000mw surfaces.

showed excellent blood compatibility, which was in good agreement with the blood coagulation time results. 4. Conclusion Microwave-assisted functionalization of PU surface has been successfully performed to improve its hydrophilicity and hemocompatibility. PU surface was firstly activated with MDI and then grafted with PEG assisted by microwave irradiation in comparison with conventional heating. The optimum microwaveassisted conditions were established for rapid and efficient MDI-functionalization and subsequent PEG-grafting of PU surface by following the change of the NCO surface density on PU surface. The whole reaction process was conducted within only 20 min. ATR-FTIR results confirmed the introduction of NCO groups and PEGs with different molecular weights onto the PU surface with assistance of microwave irradiation. Water contact angle measurements demonstrated that all the PU-PEGmw films had an improved surface wettability, and the hydrophilicity increased with the increasing molecular weight of PEG. The blood coagulation times and platelet adhesion test results indicated that all the PU-PEGmw films had significantly improved hemocompatibility. Microwave-assisted functionalization has a great potential usage for effectively decorating polyurethane surfaces.

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Acknowledgements

[21] [22]

The authors would like to acknowledge financial support from the National Natural Science Foundation of China (grant no. 31170911), Research Development and Innovation Fund of Jinan University (no. 21611410), and Open Fund of The First Affiliated Hospital, Jinan University.

[23] [24] [25]

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