230 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 1, FEBRUARY 2008

Analysis of Surface Sterilization and Properties ofMedical Poly(tetrafluoroethylene) in

Remote Argon PlasmaHong-Xia Liu and Jie-Rong Chen

Abstract—We have used a remote-argon-plasma reactor inwhich the active species can be separated at a special plasmafield. Argon gas is used as the work gas. In this remote argonplasma, the distribution of electrons, ions, and radicals andgermicidal effect (GE) of Escherichia coli on the surface of medicalpoly(tetrafluoroethylene) (PTFE) film are studied. Then surfaceproperties of sterilized PTFE are investigated by water-contact-angle and mass-loss measurements, as well as scanning electronmicroscopy and X-ray photoelectron spectroscopy, respectively.The results show that the concentration of electrons and ionsdecrease rapidly with increasing the sample position, which ap-proximate to 0 at 30 cm, whereas the concentration of radicalschange little within 40 cm. Relatively high-purity radicals areobtained in afterglow zone. GE values reach 4.633 in the activedischarge zone (0 cm) and exceed 3.769 within 40 cm under theconditions of plasma RF-power of 100 W, exposure time of 120 s,and argon flux of 20 cm3/min. But remote-argon-plasma ster-ilization (40 cm) makes the PTFE surface higher hydrophilicitythan the direct-argon-plasma sterilization (0 cm) and does notgive remarkable degradation and damages in terms of mass-lossand surface morphology. Moreover, the remote argon plasmacontributes more effectively to the defluorination (F/C = 2.24)from the PTFE than the direct argon plasma (F/C = 2.48),and thus, more oxygen functionalities (e.g., C = O) are formedon remote-plasma-sterilized PTFE surface. The essential reasonis that remote argon plasma can enhance interactions betweenradicals and PTFE surface, which are inhibited in the activedischarge zone. These experimental results show that the remoteargon plasma is more appropriate to surface sterilization ofmedical PTFE in obtaining superior surface properties.

Index Terms—Poly(tetrafluoroethylene) (PTFE), remote argonplasma, sterilization, surface property.

I. INTRODUCTION

M EDICAL-STERILIZATION technology pays attentionto higher stability and security than other ones. Plasma-

based sterilization can overcome limitations of traditionalmethods by its many merits, such as low temperature, fast,dry and environmentally sound, consequently becomes a greentechnique, and is regarded as one of the most promising steril-

Manuscript received July 20, 2007; revised September 21, 2007. This workwas supported in part by the National Natural and Science Foundation of Chinaunder Grant 30571636, by the Specialized Research Fund for the DoctoralProgram of Higher Education under Grant 20060698002, by the Key ScientificTechnique Item of Shaanxi Province under Grant 2003K10-G61, and by theKey Scientific Technique Item of Xi’an City under Grant GG06049.

The authors are with the Department of Environmental Science and En-gineering, Xi’an Jiaotong University, Xi’an 710049, China (e-mail: jrchen@mail.xjtu.edu.cn; hxliu72@mail.xjtu.edu.cn).

Digital Object Identifier 10.1109/TPS.2007.913925

ization techniques [1]–[3]. But when plasma acts on microor-ganisms, the surface of substrate is impacted consequentially,particularly to polymers, which are more and more used inmodern medical devices and transplantable materials. Whena polymer surface is exposed to plasma containing electrons,ions, and radicals, two main reactions occur simultaneouslyon the polymer surface: One is the introduction of functionalgroups such as carbonyl and hydroxyl. Radicals in plasmacontribute mainly to the formation of functional groups. Theother reaction is the degradation of polymer chains to productswith low molecular weight. Ions and electrons in plasma mainlyinitiate the degradation reactions [4]. The former reaction con-tributes to surface modification, but the latter reaction nevercontributes to surface modification. As long as plasma is used todevelop reactive species for modification of polymer surfaces,the degradation process is unavoidable during the modificationreactions [5], [6]. Therefore, to perform effective modification,a key is how to accelerate the introduction reaction withoutthe degradation reaction. During the remote-plasma treatment,reactions of radicals with the polymer surfaces will occurpredominantly, but electron and ion bombardment will scarcelyoccur [7]. The polymer sample by the remote-plasma treatmentis positioned away from the plasma region, and the polymersample by the direct-plasma treatment is positioned just inthe plasma region. In our previous works [8]–[10], the remoteargon plasma is distinguished from the direct argon plasmaby the degradation occurring on the poly(tetrafluoroethylene)(PTFE) and PVC film surface. The remote plasma is an ade-quate procedure for surface modification. It causes less polymerdegradation on the film surface.

The direct-plasma sterilization can effectively destroy ther-mal matter, such as bacteria, virus, and metabolic product[11]–[14]. But based on the traits of remote-plasma modifica-tion on polymer surface, we expect to use remote plasma tosterilize. If remote plasma can also effectively inactivate bacte-ria, both the feasible sterilization and surface modifications ofpolymer surface can synchronously be obtained.

In this paper, remote argon plasma was applied to performthe sterilization of Escherichia coli, which was on the surface ofmedical PTFE film. The influence of the remote-plasma treat-ment conditions on the germicidal efficiency were discussedfirst, and then, the water contact angle, mass loss, scanning elec-tron microscopy (SEM), and X-ray photoelectron spectroscopy(XPS) of sterilized PTFE surface were measured. Moreover, toclearly explain the changes of surface properties of sterilized

0093-3813/$25.00 © 2008 IEEE

LIU AND CHEN: ANALYSIS OF SURFACE STERILIZATION AND PROPERTIES OF MEDICAL PTFE 231

Fig. 1. Schematic structure of remote-plasma reactor. (1) Gas bottle. (2) Valve. (3) Mass-flow meter. (4) Inductance coil. (5) Matching system. (6) RF generator.(7) Sample. (8) Reaction chamber. (9) Vacuum gauge. (10) Electromagnetism valve. (11) Vacuum pump. (12) Grounding protection.

PTFE at different position, double Langmuir electron probe andelectron spin resonance (ESR) were applied to verify the distri-bution of electrons, ions, and radicals in remote argon plasma.

II. EXPERIMENTS

A. Gas-Plasma Generator

Fig. 1 shows the experimental apparatus schematically.The remote-plasma generator, which was homemade, consistsof gas inlet, reaction chamber, gas exhaust, power supply(SY-500W 13.56 MHz), and matching network (SP-IImatcher).The reaction chamber is Pyrex glass tube (length of 1000 mm,diameter of 45 mm), where inductively coupled RF dischargeis initiated.

B. Double Langmuir Electron Probe and ESR Diagnosis

The location of the double Langmuir electron probe in theplasma reactor can be regulated. The measured volt–amperecharacteristics of the electrical probe were used to determineof the electrons and ions concentrations [15].

ESR is the most efficacious measuring method of radicals[16], [17]. In this paper, clean and inartificial wool was usedas catcher of radicals [18]. These wool were positioned ondifferent distance from the induction coil to be treated. Thetreated wool were conserved for 24 h at room temperature, thendetermined the concentration of radicals using ESR spectrome-ter (made in BRUKER company, ESP-500). The measurementconditions adopted were the following: room temperature, mi-crowave frequency of 9.8 GHz, microwave power of 3.177 mW,modulation amplitude of 0.2 mT, modulation frequency of100 kHz, time constant of 163.84 ms, and scan time of163.84 ms.

C. Sterility Testing

The samples to be sterilized were made of 50 mm × 25 mmsheets of medical PTFE contaminated by Escherichia coli(E. coli) 8099. During the experiment, sheets were positionedon the carrier and exposed to the remote argon plasma atdifferent position. The sterilization conditions were at plasmaRF power of 20–100 W, treatment time of 10–120 s, and argonflux of 20–100 cm3/min.

In order to count the surviving bacilli, several process stepsmust be applied to the test sheets: First, the bacilli have to beremoved from the sheets into phosphate buffer solution, thenthey were transferred to a Petri dish which contained nutritionalagar and incubated at 37 ◦C for 48 h prior to the determinationof the resulting number of colony-forming units. Germicidaleffect (GE) was determined as follows [19], [20]:

GE = log N0 − log Nt (1)

where N0 and Nt are the number of colony-forming units ofcontrol and sterilized, respectively.

The plasma-sterilized PTFE sheets were rinsed with distilledwater, dried naturally at room temperature, and then, used formeasuring surface structure and performance.

D. Contact-Angle Measurement

The static contact angle was measured immediately afterplasma sterilization by a contact-angle meter (JY-82) made inChengde, China. To lessen the effect of gravity, the volume ofeach drop was regulated to about 0.2 cc by a microsyringe.The measurement was carried out at 20 ◦C and humidity of45% R.H. The average value of the angles of the both sides ofeach drop was counted as one measurement. Each contact anglewas determined from an average of ten measurements with astandard deviation of 1◦.

E. Changes in Mass

Masses of PTFE sheets before and after sterilization wereobtained by electronic analytical balance (Mettler AE240).Mass loss can be calculated as follows [21]:

Mass loss =(M0 − Mt)

M0100% (2)

where M0 and Mt are the sample mass before and aftersterilization (in µg), respectively.

F. SEM

The surface morphology of the PTFE sheets before and aftersterilization were examined using SEM in a JEOL instrument

232 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 1, FEBRUARY 2008

Fig. 2. Distribution of electrons, ions, and radicals in remote argon plasma(power: 90 W; treatment time: 3 min; argon flux: 20 cm3/min).

(Model JSM–5800, Japan) after vacuum coating the specimenswith gold.

G. XPS

The XPS of the surface of remote-argon-plasma-sterilizedPTFE sheets were obtained on a ESCALAB MKII using anonmonochromatic MgKα photon source. The anode voltagewas 10 kV, the anode current is 30 mA, and the backgroundpressure in the analytical chamber is 5 × 10−8 Pa. The size ofthe X-ray spot was 2-mm diameter, and the take-off angle ofphotoelectrons was 15◦ with respect to the sample surface. O/Cand F/C atomic ratios at the PTFE sheet surface were estimatedfrom the relative intensity of the O1s core level against the C1s

core within an experimental error of ±0.01.

III. RESULTS AND DISCUSSION

A. Distribution of Reactive Species in the RemoteArgon Plasma

Fig. 2 shows the distribution of electrons, ions, and radicalsin the remote argon plasma. The concentration of electrons andions reduce rapidly with the increase of the sample position atthe same RF power, as shown in Fig. 2; they approximate tozero at 30 cm, whereas the concentration of radicals changeslittle within 40 cm. This occurs due to different life-spans ofvarious reactive species, namely, the rate constants of electron-positive-ion recombination, and radical recombination are 10−7

and 10−33 cm3/s, respectively [22]. Thus, electrons, ions, andradicals can be separated at a special plasma field, and relativelyhigh-purity radicals can be obtained. This result indicates that,in remote argon plasma, the causes of surface modification atdifferent position can be explored even more clearly.

B. GE at Different Plasma Treatment Conditions

In order to investigate the germicidal efficiency of remoteargon plasma, the GE was used as a function of the plasmaRF power, the treatment time, the argon flux, and the sampleposition. The GE showed a strong dependence on these con-ditions. Figs. 3–5 show the effects of the plasma RF power,the treatment time, and the argon flux on the GE at a constantdistance of 0, 20, 40, 60, and 80 cm from the center of the

Fig. 3. Effect of RF power on germicidal efficiency at different sampleposition (treatment time: 120 s; argon flux: 20 cm3/min).

Fig. 4. Effect of plasma treatment time on germicidal efficiency at differentsample position (power: 100 W; argon flux: 20 cm3/min).

Fig. 5. Effect of argon flux on germicidal efficiency at different sampleposition (power: 100 W; treatment time: 120 s).

induction coil, respectively. Fig. 6 shows the typical effect ofthe sample position on the GE.

The GE fluctuantly changes with the increase of the RFpower up to 80 W, as shown in Fig. 3; the changes are small,and subsequently, a large increase in GE shows between 80 and100 W regardless of the sample position; the peak GE valuereaches 4.633 at the sample position of 0 cm. The reason is that,

LIU AND CHEN: ANALYSIS OF SURFACE STERILIZATION AND PROPERTIES OF MEDICAL PTFE 233

Fig. 6. Effect of sample position on germicidal efficiency (power: 100 W;treatment time: 120 s; argon flux: 20 cm3/min).

above 80 W of RF power, the ionization degree of argon gasand average energy of reactive species are augmented rapidly;consequently, the probability of action on bacilli is enhanced.

In Fig. 4, the effects of plasma treatment time are also ob-served after 30 s, particularly at sample positions of 0, 20, and40 cm: GE values are 1.517, 1.676, and 1.328 at 30 s and 4.633,4.192, and 3.769 at 120 s, respectively. At sample positionsof 60 and 80 cm, the GE values increase little. However, thereduction of GE before 30 s is not well understood, but it couldbe as follows.

In RF argon plasma, Ar+ ions and quick atoms which havehigh mass and low velocity of flow need undergoing 25–30 RFperiods to achieve stable states, and electrons with lesser massvary fluctuantly during the course. Therefore, at the start ofdischarge, all particles are in unstable states [23]. At thesame time, some higher energy electrons may be ionized tometastable states, consequently, causing a reduction of electrontemperature and slow down the rate of diffusion [24]. Thesereasons lead to stochastic interaction between particles andcells. The attack to cells by these particles is low efficiency.

Fig. 5 shows the effect of argon flux on GE of PTFEsurface. When treatment power and time are fixed, the energyof system received is a constant. At low flux, the number ofparticles colliding with the bacilli is less than that at high flux.However, the average energy of particles is higher than that withhigh flux. At low argon flux, the probability of each particlecolliding with the bacilli increases, so GE enhances. At highargon flux, however, the number of particles is large but theaverage energy and residence time is lower, so the action onbacilli is comparatively small. Consequently, when argon fluxchanges while other conditions are fixed, action on bacilli isthe combining effect of the energy and the amount of reactivespecies and residence time. From values shown in Figs. 3–5,we find that the optimum argon-plasma-sterilization conditionsare 100-W power, 120-s treatment time, and 20-cm3/min argonflux for the maximum GE of medical PTFE surface.

Fig. 6 shows the GE as a function of the sample positionunder the optimum sterilization conditions. The GE values are4.633, 4.192, 3.769, 2.579, and 2.042 at sample position of0, 20, 40, 60, and 80 cm, respectively. These values meanthat argon plasma can inactivate E. coli effectively at sample

Fig. 7. Effect of sample position on the contact angle of PTFE sheet (power:100 W; treatment time: 120 s; argon flux: 20 cm3/min).

Fig. 8. Effect of sample position on mass loss of PTFE sheet (power: 100 W;treatment time: 120 s; argon flux: 20 cm3/min).

position of 0–40 cm. Here, we call the argon-plasma steriliza-tion at a sample position of 40 cm as the remote-argon-plasmasterilization to distinguish it from conventional or direct-argon-plasma sterilization, which is done at a sample position of 0 cm.From these results, we have the following conclusions.

1) Germicidal efficiency of E. coli using argon plasma de-pends on the sample position as well as the plasma RFpower, the treatment time, and the argon flux.

2) Both direct argon plasma (at a sample position of 0 cm)and remote argon plasma (at a sample position of 40 cm)can inactivate E. coli which is on the surface of medicalPTFE sheet effectively under the optimum sterilizationconditions.

C. Changes in Surface Properties of Medical PTFE SheetAfter Argon-Plasma Sterilization

1) Hydrophilicity Studies: The contact-angle measurementswere carried out immediately after finishing the plasma-sterilization experiments to minimize changes in the surfaceproperties. The hydrophilicity is evaluated by the contact angleof water. Fig. 7 shows that the plasma sterilization leads to aconsiderable decrease in the contact angle of the PTFE sheets.The contact angle of unsterilized sheet is 108◦, whereas it isreduced to 70.5◦ and 58.5◦, respectively, after the direct- and

234 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 1, FEBRUARY 2008

Fig. 9. Scanning electron micrograph (×8000) of (a) unsterilized PTFE sheet; (b) direct-plasma-sterilized PTFE sheet; and (c) remote-plasma-sterilized PTFEsheet (power: 100 W; treatment time: 120 s; argon flux: 20 cm3/min).

remote-plasma sterilization under 100 W, 120 s, and20 cm3/min. But contact angle at sample positions of morethan 40 cm rises up with increasing the sample position. Theseobservations indicate that the surface hydrophilicity of PTFEsheet is best after remote-plasma sterilization. Compared withthe curve in Fig. 2, we find that the introduced reaction on PTFEsurface by relatively high-purity radicals in the afterglow zonemay be the essential reason for better hydrophilicity during theremote-argon-plasma sterilization.2) Degradation Studies: As shown in Fig. 2, both charged

species and radicals in the active discharge zone are in highconcentration, so great etching action on PTFE surface initiatedby heavy collision of the charged species during direct-plasmasterilization is unavoidable. These etching actions can causelarge molecular-chain rupture and removal, thereby leading toPTFE mass loss. However, this matter would be inhibited inthe afterglow zone for the concentration of charged speciesclosing to zero. Therefore, we expect that sterilization byremote argon plasma can escape from these injures to PTFE sur-face. In confirming this matter, we investigate plasma-sterilizedPTFE through measuring mass loss and examining surfacemorphology with SEM, respectively. Fig. 8 shows effect ofthe sample position on the mass loss of the plasma-sterilizedPTFE sheet under the optimum sterilization conditions. FromFig. 8, the mass loss decreases rapidly from 37% to 5.9% withincreasing the sample position. But in view of GE are worseat sample position of 60 and 80 cm, as shown in Fig. 6, theargon-plasma sterilization at a sample position of 40 cm, i.e.,remote-argon-plasma sterilization is the best way for minimaldegradation of PTFE surface. Fig. 9 shows the SEM picturesof plasma-sterilized PTFE surface. From Fig. 9, we find thatthe remote-argon-plasma-sterilized PTFE surface is as smoothas the unsterilized PTFE sheet, but the direct-argon-plasma-sterilized PTFE sheet shows a rougher surface, and thereare numerous lumps all over it. This result also indicatesthat remote-argon-plasma sterilization can obtain better surfaceproperties. Conclusively, from Figs. 6–9, we conclude thatgoing with effective sterilization, the remote argon plasma doesnot initiate remarkable degradation and damages on the PTFEsurface with hydrophilic modification, whereas the direct argonplasma initiates heavy degradation and damages on the PTFEsurface with hydrophilic modification.3) XPS Studies: The XPS spectra for various samples are

shown in Fig. 10. The C1s spectra is decomposed into fivecomponents: CHFCH2 and C = O groups at 287.6–287.9 eV,CH(OR)CHF groups at 289.6–289.8 eV, CH2CHF groups at285.9–296.0 eV, CF2CF2 groups at 292.5 eV, and CF3CF2

and/or CF(OR)2CF2 groups at 293.7–294.1 eV [25]. The

Fig. 10. C1s spectra of XPS analysis for (a) unsterilized PTFE sheet;(b) direct-plasma-sterilized PTFE sheet; and (c) remote-plasma-sterilized PTFEsheet (power: 100 W; treatment time: 120 s; argon flux: 20 cm3/min).

relative concentrations of these components, which can beestimated from the relative peak areas of decompositions, showconspicuous changes.

LIU AND CHEN: ANALYSIS OF SURFACE STERILIZATION AND PROPERTIES OF MEDICAL PTFE 235

TABLE ISURFACE COMPOSITION OF THE REMOTE AND DIRECT-ARGON-PLASMA-STERILIZED PTFE SHEET BY XPS

The surface composition of plasma-sterilized PTFE sheets,which were determined from XPS analysis, are listed in Table I.From Table I, it can be seen that the F/C atomic ratios afterdirect- and remote-argon-plasma sterilizations decrease from3.27 to 2.48 and 2.24, respectively, while O/C atomic ratiosincrease from 0.02 to 0.04 and 0.09, respectively. It is obviousthat both the remote- and direct-argon-plasma sterilizationsand subsequent exposure to air lead to not only the surfaceoxidation of PTFE but also the surface defluorination. Thegeneration of the oxygen moieties may be due to postreactionsof carbon radicals with oxygen in air after the plasma steril-ization. Taking into account that the decrease in the F/C ratioexceeds the increase in the O/C ratio, it can be concluded thatthe C–F bonds on the sheet surface are broken by the argon-plasma sterilization, and the C–O and C = O are introducedeffectively [25].

From Table I, we also find that the remote-argon-plasma ster-ilization gives a lower F/C atomic ratio and higher O/C atomicratio than the direct-argon-plasma sterilization distinctly. Thismeans that the remote-argon-plasma sterilization contributesmore effectively to the defluorination from the PTFE thanthe direct-argon-plasma sterilization, and thus, introduces moreoxygen into PTFE. The reason may be the enhanced inter-actions between radicals and PTFE surface in the afterglowzone, where etching actions initiated by electrons and ions areinhibited.

From the XPS survey spectra, it can be seen that the de-fluorination from PTFE is the key of surface modification byargon plasma. Remote argon plasma has a higher capabilityfor defluorination than the direct argon plasma. Thus, thesurface polarity and hydrophilicity of remote-argon-plasma-sterilized PTFE increase more than those of direct-argon-plasma-sterilized PTFE [25].

IV. CONCLUSION

Surface sterilization of the PTFE film by the remote argonplasma has been investigated from a viewpoint of compari-son with the direct argon plasma. Results are summarized asfollows.

1) GE strongly depends on the plasma RF power, the treat-ment time, the argon flux, and the sample position. Underthe conditions of 100-W power, 120-s treatment time, and20-cm3/min argon flux, we are able to prepare excel-lent sterile PTFE sheet by both remote and direct argonplasma.

2) In argon plasma, the concentration of electrons and ionsis close to zero in the afterglow zone (sample positionfrom the center of induction coil further than 30 cm),while the concentration of radicals changes little within40 cm. Relatively high-radicals concentration is obtainedin the afterglow zone. This characteristic makes remote-argon-plasma sterilization has the capacity in optimizingthe surface properties of medical PTFE.

3) Compared with the direct-argon-plasma sterilization, theremote argon plasma leads to higher hydrophilicity ofPTFE surface, at the same time, the degradation anddamages are least in terms of mass loss and surface mor-phology. The main reaction of the PTFE surface initiatedby both remote- and direct-argon-plasmas sterilizationis the defluorination. But the remote argon plasma hasa higher capability for defluorination from the PTFEsheet and, thereby, introduces more oxygen-containinggroups into it than the direct argon plasma. This hap-pens because the radicals reactions are strengthened, andetching actions by electrons and ions are inhibited in theafterglow zone.

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Hong-Xia Liu was born in Xi’an, China, onJanuary 5, 1972. She received the Master’s degree inenvironmental engineering from Xi’an Architectureand Technology University, Xi’an, in 2000. She iscurrently working toward the Ph.D. degree in bio-chemistry and molecular biology in the Group ofLife Sciences, Xi’an Jiaotong University, Xi’an.

Jie-Rong Chen was born in Shanghai, China, onFebruary 18, 1949. She received the Ph.D. degreefrom Kyoto Institute of Technology, Kyoto, Japan,in 1991.

She is currently a Professor of plasma chemistryand the Head of the Environmental Science andEngineering Department, Xi’an Jiaotong University,Xi’an, China, where since 1999, she has been doingresearch in low-temperature plasma chemistry andplasma application in the medical field. For the pastfive years, she has been studying the main chemical

features of complex gas media excited by a different kind of gas discharge.Dr. Chen is a syndic of the polymer and a member of the American

Association for the Advancement of Science.