SOFT GELS WITH HIGH ELECTRIC, ULTRASOUND

CONDUCTIVITY AND STABLE THREE-DIMENSIONAL

CONFIGURATION AS ENERGY TRANSMISSIBLE MEDIA

Yasuo Shikinami, Kazuhiro Yasukawa

Takiron Co.,Ltd. Medical Institute.7-1-19, Minatojima, Minamimachi, Chuo-ku, Kobe, 650-0047, Japan

Kaoru Tsuta

Takiron Co.,Ltd. Medical Institute.7-1-19, Minatojima, Minamimachi, Chuo-ku, Kobe, 650-0047, Japan

Keywords: Soft gel, Energy transmissible media, Electric conductivity, Ultrasound transmission, 3D configuration.

Abstract: We had synthesized single component permanent gels, segmented polyurethane gels (SPUGs), essentially

consisting of gelatinizing component-only, of which almost of segments and dangling chains are liquid state

at ambient temperatures. SPUGs transmit energy well such as electricity, light and ultrasound. In this article,

SPUGs were improved by adding dispersive media into swollen SPUGs (S-SPUGs), which acquired higher

electric conductivity (1.2×10

-3

S/cm) at room temperature and lower ultrasound attenuation (0.13 dB/ MHz

cm) than SPUGs and readily responded to very low mechanical stress (Young modulus 3.7×10

Pa) due to

its high flexibility like soft tissues of living body. The S-SPUGs, which have three-dimensionally casting

moldability and stable configuration, are potentially applicable to soft biomaterials with energy

transmissible, transducing faculty.

1 INTRODUCTION

This research relates to potential application of novel

soft gels with stable three-dimensional configuration

and high electric, ultrasound conductivity as energy

transmissible media for biomaterials.

Gels generally consist of the both components of

a material to be gelatinized and dispersion media,

and are classified from various viewpoints. In a

viewpoint, gels usually form a collection of mobile

molecular chains including dispersion media, of

which reticular structures are three-dimensionally

developing through aggregation, intertwining,

molecular orientation, and covalent bonds. The

swollen gel that a polymer has absorbed the

dispersion medium generally shows molecular

aggregation of morphology (A) or (B) in Figure 1.

Both of them are two-component systems, and non-

ionic gels belonging to (A) are the commonest. The

morphology (B) shows polyelectrolyte as hydrogels,

which repeatedly shrink and swell in response to

external stimulations such as pH, salt concentration,

composition of the solvent, ionic composition,

temperature, electricity, and light, and the

applications as mechanochemical actuators, which

directly convert chemical energy into mechanical

energy, is being evaluated (Tatara, 1989). Hydrogels

categorized in (A) that gelatinizing materials

absorbed water as the medium are popularly used as

various foods and sanitary goods as well as

biomaterials regarded as substitutes for biological

gels. However, the medium, water evaporates with

time in an open system. Therefore hydrogels are

unstable and cannot be used excepting in the closed

circumstance where water is always existing.

Similarly, organic solvents in organogels, lipid

solvents in lipogels and alcohol in alcoogels easily

release from the gels with time because of poor

interaction between gelatinizing materials and

voluminous dispersion media. In view of the

disadvantages of these swollen gels, more than 20

years ago, we had synthesized stable single

component permanent gels, segmented polyurethane

gels (SPUGs) (Shikinami, 1991, Shikinami, 1992)

essentially consisting of a single gelatinizing

component, of which almost all of segments in a

240

Shikinami Y., Yasukawa K. and Tsuta K. (2008).

SOFT GELS WITH HIGH ELECTRIC, ULTRASOUND CONDUCTIVITY AND STABLE THREE-DIMENSIONAL CONFIGURATION AS ENERGY

TRANSMISSIBLE MEDIA.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 240-244

DOI: 10.5220/0001056002400244

 SciTePress

three-dimensionally cross -linked polymer and its

dangling chains are in liquid state at ambient

temperatures. Figure 1 (C) shows the molecular

aggregation morphology of single component

SPUGs, of which segments and dangling chains

consist of low weight molecule, oligomers.

2 PURPOSE OF THIS STUDY

In SPUGs, liquid segments three-dimensionally

entangles at junctions move fluidly and dangling

chains with free terminals are not involved in the

rigidity but increase the viscosity by serving as

dispersion media. Therefore, SPUGs with much

dangling chains are extremely flexible like highly

viscous liquids and have an adhesive surface. A

SPUG doped by LiClO

and a SPUG-alone have

been respectively used for functional pressure-

sensitive adhesives as skin contact media such as

bio-electrodes (disposable electrodes, therapeutic

electrodes for low-frequency electric stimulation)

and acoustic coupler gels for ultrasonographic

examination. In this research, we improved SPUGs

into swollen SPUGs (S-SPUGs) without releasing

excessive dispersive media in order to enhance the

electric conductivity and ultrasound acoustic

characteristics, which are flexible and remaining

stable three-dimensional (3D) configuration as

energy transmissible media for soft biomaterials

(Figure 2).

Figure2: S-SPUG as an ultrasonographic coupler gel.

3 METHODS AND RESULTS

Methods, apparatuses and conditions used in this

study summarized in Table 1 and each result of S-

SPUGs with SPUGs was shown in Table 2.

Table 1: Methods.

Method, Apparatus and Condition

Density

Density determination device with

balance (METTLER TOLEDO Co.,

Ltd) in hexane at 25°C

Elasticity

(Young

modulus)

Venustron Biosensor (AXIOM Co.,

Ltd) at 23°C

Transparency

Transmission

rate at 600nm

[%T]

UV-VIS spectrometry (SHIMADZU

Co., Ltd) Wavelength; 600nm

Sample thickness; 2mm / 10mm

Ultrasound

velocity and

attenuation

Sing-around method at 20°C and a

reference

Attenuation

Sing-around method by means of UVM-

2 (Ultrasonic Industry Co.)

Acoustic

impedance

Sound velocity x density

Electric

conductivity

Impedance analyzer Li/O = 5 / 100

Frequency(υ); 1kHz at 24°C

Fungus

resistance

Test for Fungus Resistance (Japanese

Industrial Standards Z-2911)

(B)

(A)

Figure 1: Morphologies of molecular aggregation o

swollen gels. SPU consists of PAO (EO/PO) segments

(liquid state at ambient temperatures). Swollen SPUG (D)

incorporates liquid dispersion media among the PAO

network.

liquid segment

molecular chain

dispersion medium (water, organic solvent, oil)

4. 5.

unction ion group

air ion

6. 7. dangling chain

dispersive media

SOFT GELS WITH HIGH ELECTRIC, ULTRASOUND CONDUCTIVITY AND STABLE THREE-DIMENSIONAL

CONFIGURATION AS ENERGY TRANSMISSIBLE MEDIA

241

3.1 Molecular Design of SPUGs and

Swollen SPUGs

One method to obtain single component gels, of

which category has never been found until we had

developed (Shikinami, 1998), is to extend low-

molecular weight polymers, which are liquid at

room temperature, and to loosely crosslink them.

SPUGs have liquid segments and dangling chains,

which chemically compose of polyalkylene oxides

(PAOs) (EO/PO; ethylene oxide and propylene

oxide copolymer chains, preferably random

copolymers) that function as a medium incorporated

into cross-linking segmented polyurethane chains.

We have practically used them so far in the field of

medical engineering (ME) as skin contact media for

energy transmission. Polyethylene glycol (PEG) and

its mono methyl ether having molecular weights

(Mw) of less than 600 and polypropylene glycol

(PPG) having Mw of less than tens of thousands are

liquid and flow at room temperature. Also

copolymers of ethylene

oxide and propylene oxide (EO/PO) are viscous,

fluid liquids at room temperature depending on the

ratio of EO in EO/PO and the molecular weight. The

aggregation energy of components of these PAOs

(Bunn, 1955) and surface tension of the terminal

groups and units of repetition are relatively low

(0.8~1.77 Kcal/mol). The aggregation energy of the

urethane bond (-NHOCO-) obtained as the OH

group at the terminal of PAO reacts with the NCO

group of poly isocyanate including liquid PAO

(these groups are used as multi-functional groups for

chain extension or formation of reticulation) is high

at 8.74, and makes a highly restrained junction.

Therefore, if a segmented polyether urethane

(SPEU) consisting of almost all of liquid segments

and having many dangling segments of M-PEG can

be synthesized, it is expected to be a single

component gel-state molecular aggregation. Figure 3

Table 2: Comparison of S-SPUGs with SPUGs.

Sample

Young

modulus

[Pa]

Transmission

rate at 600nm

[%T]

Ultrasound

velocity

[m/s]

Attenuation

[dB/MHz cm]

Electric

conductivity

(Li/O=5/100

)

[S/cm]

SPUG OH/NCO=2.10 4.1×10

t=2mm 90.0

t=10mm 88.8

1506 1.1 —

S-

SPU

-I

OH/NCO=2.10

PC 50wt%

3.7×10

t=2mm 90.7

t=10mm 89.8

1468

(1450-1550)

0.13 1.2×10

-3

S-

SPUG

-II

OH/NCO=1.45

TG 50wt%

1.18×10

t=2mm 90.0

t=10mm 89.3

(1477-1563)

0.09 3.7×10

-4

Huma

n body

(Soft

tissues

)

—

Side abdomen

6.7×10

Stomach

1.06×10

Arm (outside)

1.45×10

(inside)

2.01×10

— 1540 0.5 —

Density; SPUG=1.03, Swollen SPUG-I=1.13, Swollen SPUG-II=1.02

PC; Propylene carbonate, TG; Tetraglyme

PC 25wt%; E.C. = 5.0×10

-4

S/cm, PC 60wt%; E.C. = 2.1×10

-3

S/cm, TG 25wt%; E.C. = 1.1×10

-4

S/cm

Li/O; ratio of Li ion versus ether oxygen in poly alkylene oxide as segments and dangling chains

OH/NCO; end group of PAO, OH versus end group of Isocyanate, NCO

* Depending upon the concentration of poly acrylic powder contained in the gel

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

242

shows the structure of the basic molecule of SPUG.

This selection was adequate, and many kinds of

SPUGs could be obtained.

Figure 3: Structures of liquid molecular chains.

3.2 Electric Characteristics of SPUGs

(Shikinami, 1991)

The mechanism by which complexes of PAOs, PEG

or EO / PO copolymers and LiCIO

enhance electric

conductivity due to ionic conduction has already

been clarified (Ccallum, 1987). Solid solution of

LiClO

with PAO as a medium is produced by

interaction between dipoles (ligands) in PAO and

ions. Ionic conduction is originated when Li

moves

to the other ether oxygen in the physical space with

surfboard riding-like motion resulting from

molecular movement of PAO (Wright, 1975)

segments. As the glass transition temperature (Tg) of

the PAO segments is lower, molecular movement

becomes greater, and ionic conduction becomes

higher, hence a greater specific electric conductivity.

From this fact, SPUGs and LiClO

are also

considered to form complexes with large specific

electric conductivity because of their low Tg. This

specific electric conductivity is obviously dependent

on the molecular configuration of segments of PAO.

In the ion-conductive SPUGs that we synthesized as

skin contact pressure sensitive adhesives, the ionic

conduction rate σ (S/cm) becomes highest when the

segments of P-OH and P-NCO components are

random copolymers of EO/PO (molar ratio: 50/50)

was 10

-4

~10

-5

at RT~100°C, and 10

-6

at 0°C when

the salt concentration [Li] / [-0-] was 1/100 ~10/100.

These values of ionic conductivity are considered to

be one of the highest among various ion conductive

PAO materials studied to date. This SPUG is

presently used as the contact medium of disposable

leads for electrocardiography and of stimulation

electrodes of various low frequency therapeutic

instruments. The SPUG conducts accurately various

low-frequency waveforms through the electrode to

the skin. However, SPUGs would be utilized in

furthermore various ways if electric resistance were

lower than these values by one figure, 10

-3

(S/cm).

S-SPUGs were developed for the purpose.

3.3 Ultrasound Acoustic

Characteristics of SPUGs

Ultrasound characteristics, conduction velocity,

acoustic impedance and absorption coefficient of

body tissues have already been examined. Despite

slight variations according to the water contents at

different sites of the body, i.e. blood, brain, adipose

tissue, kidney, liver, muscle, excepting bone, the

velocity of ultrasound conduction is around

1,500m/s, and the ultrasound impedance (ρC), which

is the product of the density and the sound

conduction velocity, is about 1.4 to 1.7(10

kg/m

s),

approximately equal with water and the body fluid.

Attenuation (α; dB/MHz cm) is about 0.2 to 3.3,

while that of water is very low at 0.0022. These

values of SPUGs are insufficient for use in the

phantom gels for ultrasonograph and improved by S-

SPUGs (α; 0.13) as shown in Table 2.

4 CONCLUSIONS

S-SPUGs increased electric conductivity and

decreased ultrasound attenuation (impedance) of

SPUGs remaining moldability with 3D stable

configuration, so that these soft gels serve the

potentialities of electroactive polymers for artificial

muscles as the electric conductive media and of the

stable phantom gel for ultrasound transmission

according to the international quality standards that

examine ultrasonographic faculty with good fungus

resistance unlike unstable hydrogels. The casting

moldability into stable 3D configuration and high

flexibility give the artificial soft body and muscle for

androids with the energy transmissible, transducing

faculty (Minato, 2005), in addition to applications as

skin contact media of existing SPUGs for disposable

electrodes, therapeutic electrodes and

ultrasonographic coupler gels.

O ( CH

O ) H

HO ( AO

)

M－PEG

P－OH

1,2

; Alkylene Oxide

(

)

( CH

)

NCO

CHO

(

)

( CH

)

NCO

O ( AO

) CN

( CH

) NCO

P－NCO

SOFT GELS WITH HIGH ELECTRIC, ULTRASOUND CONDUCTIVITY AND STABLE THREE-DIMENSIONAL

CONFIGURATION AS ENERGY TRANSMISSIBLE MEDIA

243

REFERENCES

Bunn, C.W., 1955. J. Polym. Sci., 16, pp323-343.

Ccallum, J.R.M., 1987 and 1989. Polymer Electrolyte Rev.

1 and 2. Elsevier Applied Science, London and New

York.

Minato, T., Shimada, M., Itakura, S., Lee, K., and

Ishiguro, H., 2005. Proc. of the 4th IEEE

International Conference on Development and

Learning, pp.106-111.

Shikinami, Y., 1991. Jpn. J. Adh. Soc., 27, pp526-533.

Shikinami, Y., Hata, K., Morita, S., and Kawarada, Y.,

1991. Jpn. J. Adh. Soc., 27, pp266-274.

Shikinami, Y., Hata, K., Morita, S., and Kawarada, Y.,

1991. Jpn. J. Adh. Soc. 27, pp313-319.

Shikinami, Y., Morita, S., Tsuta, K., and Taniguchi, M.,

1992. Jpn. J. Polym. Sci. and Tech., 49, pp19-28.

Shikinami, Y., Morita, S., Tsuta, K., and Taniguchi, M.,

1992. Jpn. J. Adh. Soc. 49, pp29-36.

Shikinami, Y., Tsuta, K., and Taniguchi, M., 1998. Jpn.

Soc. Polym. Sci. 47, pp356.

Tatara, Y., 1989. Functional & Materials. 9, pp53-57.

Wright, P.V., 1975. Br. Polym. J., 7, 319.

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

244