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Brief communication

Poly(amidoamine) dendronized hollow fiber membranes: Synthesis,

characterization, and preliminary applications as drug delivery devices

Qian Zhang a, Na Wang a, Tongwen Xu a,⇑, Yiyun Cheng b,c,⇑

a Laboratory of Functional Membranes, Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of Chinab School of Life Sciences, East China Normal University, Shanghai 200062, People’s Republic of Chinac Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 7 September 2011

Received in revised form 15 November 2011

Accepted 21 November 2011

Available online 2 December 2011

Keywords:

Hyperbranched

Dendrimer

Poly(amidoamine)

Bromomethylated poly(2,6-dimethyl-1,4-

phenylene oxide)

Hollow fiber membrane

a b s t r a c t

Poly(amidoamine) (PAMAM) dendrons were prepared from hollow fiber membranes (HFM) consisting of

bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) in a stepwise manner. The prepared

HFM were characterized by Fourier transform infrared spectroscopy, elemental analysis, and scanning

electron microscopy. The drug loading efficiency and release behavior of the PAMAM dendronized

HFM were evaluated using sodium salicylate, sodium methotrexate, and Congo red as model drugs.

The results suggest that PAMAM dendronized HFM can be effectively loaded with a variety of drugs

and prolong the release of these drugs. The drug loading and release characteristics of the HFM depend

on the generation of PAMAM dendrons grafted on the membranes. The prepared PAMAM dendronized

BPPO HFM are promising scaffolds in drug delivery and tissue engineering.

2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Hollow fiber membranes (HFM) are one of the emerging mate-

rials which have been undergoing rapid growth during recent dec-

ades. Compared with other types of membrane, HFM are still in

their infancy, but have already exhibited several unique features:

(1) a large surface mass transfer area; (2) low operating costs;

(3) convenience of assembly; (4) flexible filtration modes (‘‘inside

out’’ or ‘‘outside in’’). Besides applications in separation technol-

ogy, HFM have been widely used in several biomedical fields, such

as blood purification, including hemofiltration, hemodialysis, plas-

ma separation, and blood oxygenation [1], protein separation and

purification [2], enzyme immobilization [3], bioreactors [4],

artificial organs [5], and drug delivery [6]. Generally, polymers

including polysulfone (PS) [7], polyethersulfone (PES) [8], polypro-pylene (PP) [9], and polyacrylonitrile (PAN) [10] have been used for

the fabrication of HFM. The scaffold materials of these HFM are

hydrophobic, thus there is a significant potential for the adsorption

of proteins onto the membrane surface, resulting in the activation

of platelets and leukocytes, and the clotting of hollow fibers [1]. To

solve this problem polymers such as polyvinylpyrrolidone (PVP)

[1], poly(ethylene glycol) (PEG) [11,12], poly(glycidyl meth-

acrylate) [13,14], poly(acrylonitrile-co-acrylic acid) [15], and

poly(ethylene glycol methyl ether methacrylate) [16] were either

blended with, or grafted onto scaffold materials of HFM to improve

their hydrophilicity and biocompatibility.

Dendrimers are hyperbranched, monodisperse, and three-

dimensional macromolecules with well-defined molecular

weights, sizes, and numbers of surface functionalities [17,18].

Poly(amidoamine) (PAMAM) dendrimers, which were first re-

ported by Tomalia in 1985, are the most investigated dendrimers

[19,20]. PAMAM dendrimers can be synthesized by Michael addi-

tion of amine groups to methyl acrylate, followed by aminolysis

of the resulting ester by ethylenediamine to create new reaction

sites for further Michael additions. These dendrimers have large

numbers of active functional groups, such as hydroxyl, amine,

and carboxyl groups, on the dendrimer surface, and thus have

excellent aqueous solubility and can be modified with a large num-ber of bioactive molecules [21]. Also, PAMAM dendrimers have

numerous relatively non-polar pockets in their interior, which

can encapsulate hydrophobic drugs within the dendrimers

[22–24]. Although the cytotoxicity of amine-terminated PAMAM

dendrimers is a problem, surface modification of these cationic

dendrimers by acetylation, PEGylation, or glycosylation can effec-

tively improve their biocompatibility [25]. Based on advances in

PAMAM dendrimers, we expected to be able to construct PAMAM

dendrimers on the surface of HFM, and that the prepared dendri-

mer-functionalized HFM would combine the characteristics of

HFM (scaffolding material applicability, easily recyclable, a large

surface area, and easy assembly into devices) and dendrimers

1742-7061/$ - see front matter 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.actbio.2011.11.027

⇑ Corresponding authors.

E-mail addresses: [email protected] (T. Xu), [email protected] (Y.

Cheng).

Acta Biomaterialia 8 (2012) 1316–1322

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(drug delivery systems with easy further modification). However,

non-perfect PAMAM structures were obtained because HFM are

insoluble in most organic solvents, and reaction between the solid

and liquid is incomplete during construction of the dendrimers.

Therefore, PAMAM dendronized HFM were prepared instead of

dendrimer-functionalized ones. Hyperbranched PAMAM have

common features with PAMAM dendrimers because of their simi-

larity in branching, molecular structures, interior cavities, and sur-

face functionalities [26–28]. The high density of amine groups on

the membrane surface are expected to load a variety of negatively

charged drug molecules, and interior pockets in these hyper-

branched PAMAM can encapsulate hydrophobic drugs.

In this study we use bromomethylated poly(2,6-dimethyl-1,4-

phenylene oxide) (BPPO) as the scaffold material of HFM. BPPO is

a hydrophobic material developed in our group and it has already

exhibited several advantages as a scaffold material in the fabrica-

tion of membranes, especially HFM [28–32]. BPPO HFM are conve-

nient for further chemical modification and were proved to be

biocompatible with several cell lines [6]. As shown in Scheme 1,

the bromide groups on BPPO were substituted with amine groups

under mild conditions, and the amines were further reacted with

methyl acrylate and ethylenediamine to produce PAMAM, as

described elsewhere [19,20]. In previous studies PAMAM hyper-

branched polymers or dendrimers were grafted on carbon nano-

tubes [33], mesoporous silica [34], and Fe3O4 nanoparticles [35],

however, to the best of our knowledge, this is the first report on

the synthesis of dendronized HFM. The drug loading and release

behavior of the PAMAM dendronized BPPO HFM were evaluated.

2. Materials and methods

2.1. Materials

Ethylenediamine, methyl acrylate, and methanol were

purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai,

China), and the chemicals were purified by atmospheric distillationbefore the synthesis of PAMAM dendrons. BPPO HFM were sup-

plied by Tianwei Membrane Co. Ltd (Shandong, China) as a gift.

BPPO has a 90% benzyl substitution ratio and 20% aryl substitution

ratio. Sodium salicylate (NaSA) was obtained from Tianjin Damao

Chemical Reagent Factory (Tianjin, China). Sodium methotrexate

(MTX) and Congo red (CR) were purchased from Sangon Biotech

Co. Ltd (Shanghai, China).

2.2. Synthesis and characterization of PAMAM dendronized BPPO HFM

BPPO HFM were immersed in ethylenediamine/methanol solu-tion in a three necked round-bottomed flask at 60 C for 7 days

and then washed three times with methanol to obtain the primary

amine-functionalized HFM (termed G0 HFM). The G0 HFM ob-

tained were immersed in methanol under a nitrogen atmosphere

at room temperature and an excess of methyl acrylate was slowly

added. The mixture was stirred for 7 days, followed by three

washes of the resulting ester-functionalized HFM (G0.5 HFM) with

methanol. The G0.5 HFM obtained were incubated with an excess

of ethylenediamine in methanol in an ice bath for 7 days, and the

resulting G1 HFM were washed three times with methanol. Higher

generation PAMAM dendronized HFM were prepared by repeated

Michael addition of amine with methyl acrylate and aminolysis

of the resulting ester by ethylenediamine as described above. For

G4 and G5 HFM the reaction periods were prolonged from 7 to

14 days. The ideal molecular structures of G0–G5 PAMAM dendr-

onized BPPO HFM are shown in Scheme 2. The products obtained

were lyophilized, characterized by Fourier transform infrared spec-

troscopy (FTIR), elemental analysis, and scanning electron micros-

copy (SEM), and stored in a dry place before further use.

2.3. Drug loading abilities of PAMAM dendronized BPPO HFM

The prepared PAMAM dendronized BPPO HFM, including G3,

G4, and G5 HFM, were loaded with three drugs, including NaSA,

MTX, and CR, through static adsorption. Lyophilized G3, G4, and

G5 HFM were incubated in the drug solutions in Eppendorf tubes

at room temperature for 24 h, after which the HFM were removed,

and the drug concentrations before and after the loading process

were determined using a PGENERAL (UT-1901) UV–vis spectro-photometer (Beijing, China). Wet materials were used for further

in vitro drug release studies. The concentrations of NaSA, MTX,

and CR were determined from the absorbances of the samples at

Scheme 1. Synthetic route for PAMAM dendronized BPPO HFM.

Q. Zhang et al./ Acta Biomaterialia 8 (2012) 1316–1322 1317



295, 303, and 498 nm, respectively. The drug loading efficiency

was calculated using the equation [6]:

S ¼V ðC 0 C Þ

mHFM

ð1Þ

where S (mg g-1) is the amount of drug absorbed per gram of HFM,

mHFM (g) is the weight of the dry membrane, C 0 (mg ml–1) and

C (mg ml-1) are the drug concentrations before and after the loading

experiment, respectively, V (ml) is the volume of the drug solution.Thevolume before andafterthe loading experiment scarcely changed.

2.4. Drug release behaviors of PAMAM dendronized BPPO HFM

The release behavior of drugs from PAMAM dendronized HFM

was evaluated by the following method. Drug loaded PAMAM

dendronized HFM were immersed in 50 ml of distilled water at

room temperature for 24 h. Samples (3 ml) were withdrawn at

specific intervals and the incubation solution was replenished with

3 ml of fresh water. Drug concentrations in the samples were

determined using a UV–vis spectrometer.

3. Results and discussion

3.1. Characterizations of the PAMAM dendronized HFM

FTIR is an effective tool in the analysis of the chemical compo-

nents in HFM and dendrimers [29]. As shown in Fig. 1, the absorp-

tion peak at 600 cm–1 corresponding to C–Br stretching in BPPO

HFM disappears in the FTIR spectrum of G0 HFM, suggesting reac-

tion of the C–Br groups on benzyl bromide with ethylenediamine.

The G0.5, G1.5, G2.5, G3.5, and G4.5 BPPO HFM showed strong

peaks around 1740 cm–1 in Fig. 1b, which is due to C@O stretching

of the ester groups. This is attributed to the Michael addition of

amine groups to G0 HFM with methyl acrylate. In addition, broad

peaks appear at 1650 cm–1 for the full generation PAMAM dendr-

onized HFM (Fig. 1a). These peaks are assigned to the free N–H

groups present on the periphery of the hyperbranched polymer,suggesting the aminolysis of esters with a mine groups. The FTIR

spectra of the PAMAM dendrons coated on the HFM surface are

similar to that of PAMAM dendrimers [36]. However, full genera-

tion PAMAM dendronized HFM also showed an ester peak at1740 cm–1, suggesting incomplete reaction of the esters with

Scheme 2. Ideal structures of the PAMAM dendronized BPPO HFM.

Fig. 1. FTIR spectra of (a) full generation and (b) half generation dendronized BPPO

HFM.

1318 Q. Zhang et al. / Acta Biomaterialia 8 (2012) 1316–1322



ethylenediamine due to the problems of solid–liquid reactions. The

ratio of absorbance at 1740 and 1650 cm–1 significantly decreases

in full generation PAMAM compared with the half generation ones.

FTIR gives a qualitative analysis of the PAMAM dendronized

BPPO HFM. To characterize the HFM in a quantitative manner,

the contents of C, H, and N in the prepared HFM were determined

by elemental analysis, and the results are shown in Table 1. The

nitrogen content in the HFM increase linearly with dendrimer gen-

eration (Fig. 2), which is consistent with the behavior of PAMAMdendrimers. Based on the elemental analysis data it is easy to cal-

culate the weight of PAMAM dendrons coated on HFM in each step.

Generally, the nitrogen content in the HFM can be calculated as:

N m ¼mN

mt

¼M D N t

1þM Dð2Þ

where N m (%) is the measured nitrogen content, mN (g) is the weight

of nitrogen in the HFM, mt (g) is the weight of the whole HFM, M D

(g g-1) is the weight of PAMAM dendron grafted per gram HFM, and

N t (%) is the theoretical nitrogen content in each generation of PA-

MAM dendrimer. M D can be described by the equation:

M D ¼ N m

N t N mð3Þ

The results for the weight of PAMAM dendron grafted per gram

HFM are shown in Table 1.

Besides the chemical components of the PAMAM dendronized

HFM, the surface and cross-sectional microstructures of the HFM

were characterized by SEM. As is shown in Fig. 3, BPPO HFM have

a smooth surface and finger-like macrovoids in the cross-sections

[6,29]. The macrovoids extend from the inner surface to the middle

of the cross-section, and the diameters of the macrovoids are in the

range 15–20 lm. No obvious changes were observed in the SEM

images of low generation PAMAM (G0–G3.5) dendronized HFM,

however, the surfaces of the high generation PAMAM (G4–G5)

dendronized HFM are much more rough and the partial surface

of the HFM even peeled off after functionalization with PAMAM

dendrons.

3.2. Drug loading behavior of PAMAM dendronized HFM

As shown in Fig. 4, unmodified BPPO HFM have a low drug load-

ing capacity for the three model drugs, while PAMAM dendronized

HFM can effectively load all of the drugs. G3–G5 HFM are able to

load NaSA at 90, 45 and 30 mg g–1 dry HFM, respectively. In the

case of MTX, the loading efficiencies of G3–G5 HFM are 70, 46

and 15 mg g–1, respectively. The results suggest that higher gener-

ation PAMAM dendronized BPPO HFM had lower drug loading effi-

ciencies. This is probably due to: (1) the lower generation PAMAM

more easily form electrostatic attachments with oppositely

charged drugs than higher generation ones, due to steric hindrance

on the PAMAM surface [37,38]; (2) as revealed by SEM imaging and

macroscopic observation, the partial surface of high generation PA-

MAM dendronized HFM were peeled off and the dendrimer con-

tents on high generation PAMAM dendronized HFM were not ashigh as expected. This speculation was confirmed by the abnormal

nitrogen content and calculated dendrimer amount in G5 PAMAM

dendronized BPPO HFM (Table 1 and Fig. 2a). Similarly, G3 HFM

had a higher loading capacity for CR (53 mg g–1) than G4 and G5

HFM (47 and 42 mg g–1, respectively).

3.3. Release of drugs from PAMAM dendronized HFM

As shown in Fig. 5, the prepared HFM exhibited sustained re-

lease behaviors when NaSA, MTX, and CR were used as the model

drugs. Accumulative amounts of 10%, 26%, and 23% of the en-

trapped NaSA were released from G3, G4, and G5 HFM after 24 h,

while 1.9%, 4.6%, and 3.4% of the loaded MTX was released from

G3, G4, and G5 HFM after 24 h. Surprisingly, extremely lowamounts of CR were released from the three PAMAM dendronized

HFM during the same period. The release rates of drugs from the

prepared HFM were slower than that from PAMAM dendrimers

[36,39,40], probably due to additional hydrophobic and p–p stack-

ing interactions between the BPPO scaffold and aromatic rings of

the drugs. Among the three model drugs, MTX and CR had two neg-

ative charged groups in their molecular structures, while NaSA has

only one carboxylate group. Drugs with two negatively charged

groups can be more readily retained by PAMAM dendrons than

those with only one charged group. Stable complexes were formed

between MTX/CR and HFM. MTX has two carboxylate groups while

CR has two sulfonic groups. Therefore CR exhibited slower release

from the HFM than MTX. It is worth noting that PAMAM dendri-

mers failed to load MTX in previous studies due to the formationof cross-linking structures between the cationic dendrimer surface

Table 1

The carbon, hydrogen, and nitrogen contents of the prepared HFM, the theoretical

nitrogen contents of the PAMAM dendrimers, and the calculated amount of PAMAM

dendrons per gram of BPPO HFM.

Material C (%) H (%) N m (%) N t (%) M D (g g1)

HFM 47.06 3.579 0.323 NA NA

G0 HFM 57.76 5.693 6.409 46.67 0.159

G1 HFM 69.80 11.03 7.915 29.17 0.372

G2 HFM 55.55 6.647 9.530 26.34 0.567G3 HFM 53.45 6.764 11.03 25.36 0.770

G4 HFM 50.99 7.252 12.15 24.94 0.950

G5 HFM 50.98 7.141 12.06 24.75 0.951

Fig. 2. Nitrogen contents of the prepared (a) full generation and (b) half generationPAMAM dendronized BPPO HFM.




and the two carboxylate groups of MTX [41]. However, MTX load-

ing and sustained release were achieved using PAMAM dendron-

ized BPPO HFM, suggesting that the prepared HFM serve as

delivery devices for a large families of drugs.

Interestingly, it was observed that G4 HFM shows the highest

release rate of the three drugs (G3 < G4 > G5). This is probablydue to the fact that lower generation PAMAM have a higher ability

to load drugs through ionic interactions [37,38], while high gener-

ation PAMAM have internal pockets and are able to encapsulate

drugs through hydrophobic interactions [39,40]. In combination,

for G4 HFM these two effects showed higher release rate of the

drugs. These results suggest that PAMAM dendronized HFM are

promising candidates as drug vehicles.

4. Conclusions

In the present study PAMAM dendronized HFM were synthe-

sized and characterized by FTIR, elemental analysis, and SEM.

The prepared HFM showed generation-dependent loading capaci-

ties and release characteristics for NaSA, MTX, and CR. Lower gen-

eration PAMAM dendronized HFM are able to load more drug than

higher generation ones, while G4 HFM showed a higher release

rate than G3 and G5 HFM. The PAMAM dendronized HFM showed

better performance in terms of sustained drug release than cationic

PAMAM dendrimers. Drugs with two oppositely charged groups

form stable complexes with the prepared HFM and can effectively

prolong the delivery of bound drugs. Although the cytotoxicity of

cationic PAMAM is a problem, further modification of the dendron

surface by acetylation, PEGylation, and glycosylation can effec-

tively solve this question. The biological evaluation and biocom-patibility testing of the prepared HFM are now under investigation.

Fig. 3. SEM images of G2–G5 PAMAM dendronized BPPO HFM.

Fig. 4. Drug loading efficiencies of G3, G4, and G5 PAMAM dendronized HFM using

NaSA, MTX, and CR as model drugs.




Acknowledgements

We thank the National Natural Science Foundation of China

(Grant No. 21025626), the Talent Program of East China Normal

University (Grant No. 77202201), and grant from the Science andTechnology Commission of Shanghai Municipality (Grant No.

11DZ2260300) for financial support of this project.

Appendix A. Figures with essential color discrimination

Certain figures in this article, particularly Figs. 1, 4, 5 and

Schemes 1 and 2, are difficult to interpret in black and white.

The full color images can be found in the on-line version, at

doi:10.1016/j.actbio.2011.11.027.

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