Stable and Flexible Materials to Mimic the Dielectric ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

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Stable and Flexible Materials to Mimic the Dielectric Properties of Human Soft Tissues John Garrett, Student Member, IEEE, and Elise Fear, Senior Member, IEEE

Abstract—Emerging biomedical applications require realistic phantoms for validation and testing of prototype systems. These phantoms require stable and flexible tissue-mimicking materials with realistic dielectric properties in order to properly model human tissues. To create a tissue-mimicking material to fulfill these needs, carbon powder and urethane rubber mixtures were created, and the dielectric properties were measured using a dielectric probe. Both graphite and carbon black were tested. Mixtures of graphite and urethane (0% to 50% by weight) provided relatively low permittivity and conductivity, suitable for mimicking fatty tissues. Mixtures of carbon black and urethane (0% to 15% by weight) provided a broad range of suitable properties. Samples with 15% carbon black had permittivity and conductivity similar to higher-water-content tissues, however the cured samples were not mechanically suitable for moulding into complex shapes. Finally, mixtures of graphite, carbon black, and urethane were created. These exhibited a range of dielectric properties and can be used to mimic a variety of soft tissues. The mechanical properties of these samples were tested and presented properties that exceed typical phantom requirements. This tissue-mimicking material will be useful when creating thin, flexible, and robust structures such as skin layers. Index Terms—Biomedical engineering, biomedical imaging, dielectric materials, dielectric measurement.

I. INTRODUCTION

T

ISSUE-MIMICKING materials (TMMs) are required in order to test and validate a variety of emerging biomedical applications, such as microwave imaging [1]–[7]. Biological tissues exhibit a large range of dielectric properties. Lowerwater-content tissues have lower permittivity and conductivity than higher-water-content tissues [8]. The properties of tissues also change with frequency. Since the dielectric properties of human tissues are largely related to the water content of the tissue, many of the first TMMs were water-based. Guy created an initial TMM from a mixture of saline solution, polyethylene powder, and a gelling agent [9]. This TMM mimics muscle tissue from approximately 200 to 2000 MHz with a dielectric constant of 49–58 and a loss tangent of 0.33–1.7. Many other groups have applied similar materials to test a range of applications. For example, Lazebnik et al.

Manuscript received March 07, 2014; accepted March 18, 2014. Date of publication March 20, 2014; date of current version April 09, 2014. This work was supported by Alberta Innovates Health Solutions, Alberta Innovates Technology Futures, and Biovantage. The authors are with the Department of Electrical and Computer Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: garrettj@ucalgary.ca; fear@ucalgary.ca). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2312925

created an ultrawideband TMM using a gelatin-in-oil dispersion [10]. By varying the ratio of oil to water, this TMM mimics the dispersive dielectric properties of a variety of human tissues from 500 MHz to 20 GHz. Specific recipes were identified for various soft tissues by comparing dielectric property results to 4-pole Cole–Cole model results reported in a dielectric property study of various tissues [8]. Water-based TMMs similar to [9] and [10] have been used to create complex breast phantoms [4], [11], [12], which include a skin layer, a fat layer, and fibroglandular masses. The water-based TMMs used to create these phantoms, however, are not mechanically robust, and air exposure leads to evaporation and changes in properties [12]. To avoid the limitations associated with water-based phantoms, materials suitable for use as TMMs have also been made from mixtures of carbon and an insulating matrix. Carbon is a semiconductor. However, it can act as a dielectric when placed in an insulating matrix where the carbon aggregates create a space charge polarization at the interface of the two materials [13], [14]. Materials such as ceramic powder [15], [16], plastic resin [17], and silicone rubber [18], [19] have been used as the insulating matrix. The advantage of silicone rubber is its flexibility for applications that involve deformation of the phantom. Gabriel used a graphite-silicone mixture to create a hand phantom [19]. In this study, the graphite concentration was varied from 30% to 60% by weight (wt%) and was measured from 600 MHz to 6 GHz. A TMM composed of 50-wt% of the target value graphite was found to be within representing the hand. The carbon-rubber materials mentioned in [18] and [19] were able to mimic a broad range of tissues. However, the exact materials and methods were not revealed in sufficient detail to recreate these materials. To create stable, flexible, and realistic phantoms of human structures, a TMM that mimics a wide range of tissues is required. Additionally, a material that can be created in standard research labs (i.e., without expensive machinery) would be very advantageous. The procedure, the materials, and the concentrations used to create the TMMs are outlined in Section II. The measured dielectric and mechanical properties are presented in Section III along with additional improvements that were made to the TMMs. II. METHODOLOGY Since a flexible and mechanically strong TMM was desired, the TMM was chosen to be created from a carbon-based conductive filler and a rubber matrix. For the rubber matrix, silicone and urethane rubber were tested. For the conductive filler, two varieties of carbon powder were tested. The specific

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CARBON POWDERS

TABLE I RUBBER MATERIALS USED TMM DEVELOPMENT AND

IN THE

carbon powders and rubber materials used during the TMM development are listed on Table I. The carbon powders are similar to the conductive fillers used in previous carbon-based TMM studies [15]–[17], [19]. To create the phantom materials, rubber solution (prior to curing) was weighed and mixed in a container by hand. The carbon powder was then weighed, added to the rubber mixture, and the materials were mixed with a metal stirrer for several minutes until a homogeneous appearance was achieved. Due to the carbon powder, this entire process was completed in a fume hood. For samples with high carbon content, the mixture was very thick and required kneading by hand. Only visual uniformity was assessed during mixing stage; a more comprehensive assessment was completed after curing by measuring the dielectric properties in multiple locations. For each combination of materials tested, cylindrical samples were prepared with diameters of 5 cm and heights of 3 cm . This permitted the collection of multiple measurements with a dielectric probe without overlapping the sensed volumes. To create materials with a range of properties, different concentrations of carbon powder were tested. First, graphite and rubber samples were prepared. The concentration of graphite was varied from 10 to 50 wt% with either silicone or urethane used as the rubber matrix. Beyond 30-wt% graphite, the graphite-silicone mixtures remained very sticky even after setting for several days. Concentrations up to 50 wt% were successfully created with urethane. Concentrations beyond 50 wt% became very difficult to mix as the mixture became granulated and the cured samples exhibited inhomogeneities. Samples composed out of carbon black and rubber were also prepared. The highest concentration of carbon black successfully utilized with urethane was 15 wt%. Beyond this concentration, the mixture became granulated, and the carbon black was not completely incorporated into the rubber matrix. As a result, the cured samples contained weak layers and cracks. The samples described above were allowed to set overnight, after which a dielectric probe [20] was used to measure the dielectric properties. Time-gating was used to isolate the reflection from the aperture of the probe, de-embedding was used to remove the effects of the probe, and a rational function model was used to obtain an estimate of permittivity

and conductivity [20]. Measurements were acquired at three locations and over the range from 1 to 10 GHz with an Agilent N5230A PNA-L vector network analyzer ( MHz, kHz, and dBm). Three sensing volumes were measured at least 1.5 cm apart, which allowed for the majority of the measurements to be collected without the sensing volumes overlapping [21]. From these measurements, an average was found and used for plotting the results shown in Section III. Similar to previous TMM studies, experimental measurements of tissues are also included for comparison. Dry skin, cancellous bone, and noninfiltrated fat [8] were chosen to provide a range of water concentrations and therefore dielectric properties. During the assessment of the carbon-rubber TMMs, the mechanical properties were also assessed. Samples were deemed inadequate if the sample did not fully cure overnight (i.e., it remained tacky or viscous), if the sample remained brittle or grainy after curing, or if the sample tore easily during examination. A tensile strength test was performed to determine the elasticity and the tensile breaking point. A uniaxial testing machine (H1KT, Tinius Olsen, Rock Hill, SC, USA) was used to perform this test. III. RESULTS AND DISCUSSION During testing, it was found that the choice of rubber did not have a noticeable effect on the dielectric properties. The urethane-based samples cured into more flexible and mechanically strong samples than the silicone-based samples. Furthermore, the mixing process was found to be easier with urethane rubber, and urethane was therefore used for the remainder of the testing. Through testing the dielectric properties of the graphite/urethane mixtures, it was found that increasing the graphite concentration resulted in higher dielectric properties. The mixture consisting of 10-wt% graphite provided dielectric properties that are suitable for mimicking fat. Over the 1–10-GHz frequency range, the average absolute error between this mixture and infiltrated fat (the dielectric properties of fat are reported in [8]) was 2.46 for relative permittivity and 0.15 S/m for conductivity. It was found that graphite does not provide the conductivity required to mimic high-water-concentration tissues (e.g., skin or bone). The highest concentration that was successfully created consisted of 50-wt% graphite. At 5 GHz, this sample had a relative permittivity of 23.88 and a conductivity of 0.70 S/m, which is far too low to mimic high-water concentration tissues. These properties were much lower than the properties found by Gabriel [19], where a sample composed of 50-wt% graphite provided properties very similar to skin tissue. This could be due to degassing techniques that would increase the dielectric properties, or due to different varieties of rubber or graphite powder. Details on the exact materials and methods were not provided in [19]. Additionally, a criterion for the TMM described in this letter was to be easy to manufacture in a standard lab, so no additional machinery was tested. The dielectric properties of carbon black/urethane mixtures are shown in Fig. 1. Compared to graphite, carbon black provides greater conductivity and creates mixtures with much higher dielectric properties, however has a much lower mixing limit. It was found that 15-wt% carbon black was able to

GARRETT AND FEAR: MATERIALS TO MIMIC DIELECTRIC PROPERTIES OF HUMAN SOFT TISSUES

Fig. 1. Dielectric properties of carbon black/urethane mixtures with skin, bone, and fat properties included for comparison [8].

TABLE II AVERAGE RELATIVE ERROR IN DIELECTRIC PROPERTIES BETWEEN CARBON/URETHANE MIXTURES AND TISSUE PROPERTY MEASUREMENTS FROM [8] FROM 2 TO 10 GHZ

approximate the permittivity and conductivity required to mimic lossy tissues in the 2–10-GHz frequency range. The average relative error in dielectric properties between the 10and 15-wt% materials and two tissues (bone and skin) is shown in Table II. The exact concentrations could be further tuned to approximate the necessary tissue type. However, it was obvious that the conductivity of the carbon black/urethane mixture was too high for bone. The 15-wt% carbon black mixture was also very thick and difficult to mould into complex shapes. In thin layers, the 15-wt% carbon black mixture tore easily, so it was not suitable to represent skin. Carbon black/graphite/rubber mixtures were therefore created to provide simultaneously high permittivity and lower conductivity and to improve upon the mechanical properties of the carbon black/urethane mixtures. The results are shown in Fig. 2. All of the concentrations listed on this figure were relatively easy to create by hand and, qualitatively, possessed adequate mechanical properties (as defined in Section II). The 30-wt% graphite and 7-wt% carbon black sample was much easier to mix than the 15-wt% carbon black sample, which makes it easier to mould into complex shapes. It also provided a better match to skin, as shown in Table II. This average relative error further reduced to 12% for permittivity and 15%

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Fig. 2. Dielectric properties of graphite-carbon black/urethane mixtures with skin, bone, and fat properties included for comparison [8]. TABLE III STANDARD DEVIATION OF DIELECTRIC MEASUREMENTS TAKEN AT 6 GHZ FROM CARBON/RUBBER MIXTURES

for conductivity for the 4–7-GHz frequency range. This sample showed that better approximations of permittivity and conductivity and better mechanical properties could be achieved with carbon black/graphite/urethane mixtures. Further adjustments to concentrations are expected to improve the agreement between the dielectric properties of the TMM and skin. For some of the carbon black/graphite/rubber samples, mixing was still quite difficult due to the high concentration of carbon powder and kneading was required. To assess the homogeneity of the mixtures, the properties were measured in three nonoverlapping sensing volumes, and the standard deviation was found. Example measurements that are representative of properties seen in other materials are shown in Table III. The consistency seen in Table III suggests that good mixing was achieved for these samples. The carbon black/graphite/rubber mixtures also exhibited good stability over time. For example, a 37-wt% graphite and 2-wt% carbon black sample was prepared, and the dielectric properties were measured 8 months apart. During the second measurement, the permittivity and conductivity were both found to be within 5% of the original value. To test the mechanical properties, a tensile strength test was performed. Three samples (“tendons”) were created with 5 10 mm cross-sectional areas from a 30-wt% graphite and 4-wt% carbon black mixture. During testing, all of the tendons

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Fig. 3. Tensile properties of a urethane sample and a 30-wt% graphite and 4-wt% carbon black sample.

tore in the center (i.e., they did not tear at the vice clamps of the testing machine). The results are shown in Fig. 3. Relative to urethane, the carbon black/graphite/rubber sample required much more force to elongate the sample, which is beneficial because the plain urethane rubber is exceedingly soft. At the breakdown point, the sample had an ultimate stress of 1.62 MPa (i.e., 81.0 N over 50 mm ) and an ultimate elongation of 118%. The original urethane has an ultimate breakdown stress of 2.76 MPa; therefore, the addition of carbon to the urethane for this sample resulted in a 41% reduction in the ultimate tensile strength. The strength of this carbon/urethane mixture, however, still exceeds the requirements of most phantoms. For example, a skin layer with a 2-mm 10-cm cross-sectional area can withstand a force of 324 N along the direction perpendicular to the cross section. With this carbon/rubber mixture, 2-mm skin layers are flexible (i.e., they can bend into different shapes), they are stable in that the structure does not change over time, and when used in large phantoms, they maintain their structure under the weight of the phantom materials. IV. CONCLUSION A TMM composed of graphite, carbon black, and urethane has been presented. By changing the concentration of carbon powder, this TMM exhibits a wide range of dielectric properties and is able to mimic a variety of human soft tissues from 1 to 10 GHz. This TMM is also flexible, electrically stable, and mechanically strong, which makes it ideal for creating complex phantoms for a variety of biomedical applications. Unlike previous studies, these TMMs were created using simple equipment found in most labs. A breast phantom that incorporates these materials is currently under development. ACKNOWLEDGMENT The authors would also like to thank J. Bourqui. REFERENCES [1] E. C. Fear, J. Bourqui, C. Curtis, D. Mew, B. Docktor, and C. Romano, “Microwave breast imaging with a monostatic radar-based system: A study of application to patients,” IEEE Trans. Microw. Theory Tech., vol. 61, no. 5, pp. 2119–2128, May 2013.

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