ARTICLEINFO

 

 

Article history:

Received 24 January 2011 Received in revised form 1 April 2011

Accepted 6 April 2011

Published online 19 April 2011

 

Keywords:

Polyvinyl alcohol (PVA) hydrogel Micro-structure

Deformation Mechanical properties Soft tissue

Liver

 

ABSTRACT

 

 

PVA has been proposed as a promising biomaterial suitable for tissue mimicking, vascular cell culturing and vascular implanting. In this research, a kind of transparent PVA hydrogel has been investigated in order to mimic the creatural soft tissue deformation during mini- invasive surgery with needle intervention, such as brachytherapy. Three kinds of samples with the same composition of 3 g PVA,  17 g de-ionized water, 80 g dimethyl-sulfoxide   but different freeze/thaw cycles have been prepared. In order to investigate the structure and properties of polyvinyl alcohol hydrogel, micro-structure, mechanical property and deformation measurement have been conducted. As the SEM image comparison results show, with the increase of freeze/thaw cycles, PVA hydrogel revealed the similar micro- structure to porcine liver tissue. With uniaxial tensile strength test, the above composition with a five freeze/thaw cycle sample resulted in Young’s modulus similar to that of porcine liver’s property. Through the comparison of needle insertion deformation experiment and the clinical experiment during brachytherapy,  results show that the PVA  hydrogel had  the same deformation property as prostate tissue. These transparent hydrogel phantom materials can be suitable soft tissue substitutes in needle intervention precision or pre-operation planning studies, particularly in the cases of mimicking creatural tissue deformation and analysing video camera images.

 

⃝c  2011 Elsevier Ltd. All rights reserved.

 

 

Introduction

 

 

Polyvinyl Alcohol (PVA), a hydrophilic, biodegradable and biocompatible synthetic polymer, has been widely used in different areas of the biomedical field (Paradossi et al., 2003). Recently, PVA hydrogels have become especially attractive to the field of ‘tissue engineering’ for repairing and regenerating a wide variety of tissues and organs (Woerly et al., 1996), (Hubbell, 1998), including arterial phantom (Chu and Rutt, 1997), heart valves (Jiang et al., 2004), corneal implants (Vijayasekaran et al., 1998), and cartilage tissue substitutes (Stammen et al., 2001).

 

In tissue engineering, PVA based scaffold has been studied to substitute the current available artificial grafts. Vrana focuses their investigation on the evaluation of the response of the vascular cells to the changes in the hydrogel structure by increasing freeze–thaw cycle number (Vrana et al., 2008). Hoffman has successfully used PVA hydrogels to cultivate living cells, because such hydrogels have large pores and are capable of degradation (Hoffman, 2002).

 

Hydrogels are water-swollen crosslinked polymer net- works, which often exhibit characteristics, such as tissue-like elasticity and mechanical strength, and the appearance and feel of PVA hydrogel are similar to those of native arterial( Corresponding address: School of Mechanical Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China.Tel.: +86 22 27890307; fax: +86 22 27890307.E-mail address: shanjiang@tju.edu.cn (S. Jiang).1751-6161/$ - see front matter ⃝c  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmbbm.2011.04.005)tissue (Nuttelman et al., 2001). The mechanical properties of PVA arterial vessels developed by Chu and Rutt are similar to those of porcine aortas (Chu and Rutt, 1997).PVA can be used not only for bioartificial materials but also for phantom materials commonly used in medical research. Forecasting soft tissue deformation by analysing therapeutic interventions and performing minimally invasive surgery simulations may greatly improve the proposed treatment, as well as the accuracy of surgical procedures (Chanthasopeephan et al., 2007). Simulation experiments require the use of tissue-simulating objects that mimic the properties of human or animal tissues. These phantom materials should have similar deformation rates of elasticity as compared with the target tissue, as well as exhibit long- term structural stability, high water content and excellent transparency.

 

Using an appropriate ratio of PVA and water, a gel can easily be formed that possesses tissue-mimicking properties. Such a phantom material intended for medical image processing has been introduced by Mano et al. (1986). Likewise, Surry prepared a PVA-containing material with well-described ultrasound and magnetic resonance (MR) characteristics. He found that the velocity of sound in the PVA material was 1520–1540 m/s, which is within the typical range for tissue and is useful for biopsy precision studies (Surry et al., 2002, 2004). T1 values of PVA obtained through MR studies seemed to be similar to values for grey and white matter and muscle. Thus, it was concluded that this material may be appropriate for T1 imaging studies (Bushberg et al., 1994). Gobbi and Peters developed a PVA brain phantom that has been useful in multi-modality research (Gobbi and Peters, 2003).

 

In order to investigate the biopsy precision, PVA materials are adopted in this  research  to  substitute  for  real  tissues  in vitro. This paper presents the preparation of transparent PVA hydrogel material and the evaluation of the effect of freeze/thaw cycle number on the hydrogel structure and mechanical property.

 

 

 Materials and methods

 

 

Preparation of hydrogels

 

 

Transparent PVA hydrogel materials were developed in order to quantify PVA phantom deformation by analysis with a video camera in lab. This type of analysis equipment is more readily available and user friendly than CT or MR equipment. During a needle intervention proceeding, a biopsy needle is inserted into the phantom, and a marked target will move during the deformation. To measure the  target  trajectory, the phantom should be transparent, as well as maintain mechanical properties similar to those of creatural soft tissue. Here, rather than focusing on details of the phantom deformation experiment, we describe the preparation of the transparent PVA materials and the corresponding mechanical results.

 

It is known that the incorporation of DMSO into water    as a solvent will improve the transparency of  a  material  and maintain certain mechanical properties (Hyon et al.,1989). Here, the mixing ratio of the  PVA  solution  to  DMSO was maintained at 20/80 by weight, resulting in  a  PVA hydrogel transparency greater  than  90%  (Hyon  et  al.,  1989). A homogeneous PVA solution was obtained  by  heating  the PVA mixture, 3 g PVA, 17 g de-ionized water, and the 80 g DMSO solvent at 140 ◦C for 2 h. To  form the PVA  solution into a solid phantom, the solution was allowed to rest for approximately 20 h in a  humid  environment,  to  allow  the  rise and dissolution of air bubbles at the solution surface. All  the  solutions  were  cast  into  perspex  moulds  for  solidifying.

 

Each sample was frozen from room temperature to −20 ◦C  and  maintained  at  this  temperature  for  10  h.  At  the  end  ofthe freezing stage, the phantom was allowed to thaw to room temperature over 5–9 h, completing one freeze–thaw cycle. Same procedure was repeated for another set of hydrogels 3 times and 5 times respectively. The number of freeze/thaw cycles was varied to evaluate its effect on the micro-structure and mechanical properties of the corresponding PVA sample. The PVA materials were then stored in clean de-ionized water, and the water was replaced regularly to help extend the application life of the phantom material.

 

Micro-structure comparison

 

 

The cross-sectional structure of the PVA hydrogels were examined with a scanning electron microscope-SEM (Hitachi X-650).   Before   SEM   examination,   the   hydrated hydrogel samples which were 10 mm in thickness and 50 mm in diameter  were  frozen  overnight  at  −20◦  and  then  freeze- dried in dry freezone system (Labconco FreeZone 2.5 L) for 20 h. The dehydrated hydrogels were placed in liquid nitrogen for brittle rupture. Therefore, the micro-structure of cross section can be scanned more distinctly. The porcine liver tissue sample of 15 × 15 × 15 mm3  was frozen-dried at the same condition so that its structure could be compared with the PVA hydrogels.

 

 

Uniaxial tensile strength test

 

 

To determine the relationship between the freeze–thaw cycle number and the mechanical properties of the PVA hydrogels and compared them with porcine liver tissue, uniaxial tests were performed. The shape of the sample was cylindrical with a fixed diameter of 10 mm and a height ranging from   10 mm to 12 mm. It was assumed that there was no change  in weight during the experiments, the PVA samples were isotropic and the materials were incompressible. Force and displacement were measured during the loading test by a tensile  strength  tester (Laizhou  LLY-06B).   This  instrument has a resolution of ±1% and supports loading rates ranging from  0.5  to  60  mm/min.  The  load  cell  is  able  to measure a force of up to 5 N. The temperature was approximately 20 ◦C, and the humidity was maintained between 60%–70% to prevent drying during the procedure. The loading rate was 10 mm/min. The test setup is shown in Fig. 1(a)–(c).

 

 

Needle insertion deformation test

 

 

Our investigation focuses on the developing of a  kind  of PVA hydrogel phantom material which can be used in the(

Fig. 1 – PVA hydrogel uniaxial tensile test setup. (a) Clamping state. (b) Sample on the tensile strength tester. (c) Porcine liver property test setup.)

 

 

soft tissue deformation research during needle intervention. Brachytherapy, an effective treatment for cancer, is a treatment that needs to implant radioactive seeds into people’s organ or around the tumour. Precisely implant of seeds with planned radioactive dose, to irradiate surrounding tissue over several months, will minimize the damage of healthy tissue while maximize the destruction of cancerous cells. However, the force loaded on the tip of insertion needle will cause the tissue deformation during the process and result in the misplaced seeds. This will lead to underdosed regions and complications. As the human tissue is usually unobtainable, with the help of PVA phantom, researcher can  investigate  the  tissue  and needle  deformation  during brachytherapy. This will guide the surgeons to implant the seeds more accurately without touching the important organ or tissue. The insertion  experiment  setup  is  shown as Fig. 2(a)–(c). The setup is built up with two high speed cameras  (Basler  Scount,  SCA640-74FM,  Sensor  pixel is  659 ×494,  74  frame/s)  for  receiving  images  during  the  needle insertion, and with one transparent PVA phantom (edge  of the cube is 60 mm), so that the camera can catch the target movement in the phantom. The target is sphere-shape object implanted in advance. The camera caught its original position in the phantom at the beginning of the test. We restricted the hydrogel cube by five planes to complete the boundary conditions. The needle insertion mechanism is an ultrasound motor driving mechanism. The insertion speed is 1 mm/s. Force sensor impedance head (Bruel & Kjaer 8001), charge amplifier (Bruel & Kjaer 2635), and the data acquisition card (NI USB9233) are used to receive the force data during insertion.

 

1. Conclusions

 

This preparation of PVA material may be a suitable substi- tute for creatural soft tissue in investigations of soft tissue deformation for biopsy precision research. Notably, for ex- periments that use video cameras to observe PVA deforma- tion, transparent PVA materials are required. By varying the freeze/thaw cycles of PVA hydrogel preparations, transpar- ent PVA phantom is developed, which exhibits similar me- chanical properties and morphological characteristics to that of porcine liver,  a reference material for human soft tissue.   In addition, the number of freeze/thaw cycles of PVA had a marked effect on the stress–strain relationship of PVA hydro- gel and its morphological structure. Moreover, the PVA defor- mation during the video based deformation experiment has the same status as measured during clinical surgery. By main- taining appropriate mechanical properties and transparency, such PVA hydrogels may be suitable substitutions for creatu- ral soft tissues in biopsy precision research.

 

Acknowledgments

 

The fund for this project was provided by the National Natural Science Foundation of People’s Republic of China,grant No. 60703045 and also the project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

 

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