Permanent-use haemofilter based on porous glass capillaries

Journal of Membrane Science, 11 (1982) 275-288

275

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PERMANENT-USE HAEMOFILTER BASED ON POROUS GLASS CAPILLARIES *

H . v. BAEYER ** , F . KOCHINKE, G . KLOPP, R . MOHNHAUPT, H .W . REINHARDT, M. KESSEL

Fachbereich 3 (Klinikum Charlottenburg), Freie Uniuersitdt Berlin, Berlin (F.R.G.) and R . SCHNABEL **

Schott Glaswerke Mainz, Mainz (F.R,G.) (Received September 16, 1981 ; accepted in revised form March 19, 1982)

Summary Modules of surface-modified porous glass capillaries with a hydraulic conductivity of 0 .25-0 .70 ml min' mmHg - ' m -2 were tested for suitability for clinical haemofiltration by ex vivo dog experiments and by in vitro perfusion, The results show that (1) the construction of reusable haemofilters based on porous glass capillaries is possible ; (2) operational data of testmodules are comparable with hollow-fiber high-flux haemofilters based on asymmetric cellulose acetate membranes ; and (3) blood-surface interaction of porous glass capillaries is characterized by protein deposition which entails very low protein cutoff.

Introduction Twenty years after the introduction of dialysis therapy, the number of patients dependent on artificial kidneys is still growing throughout the world . At the same time, the cost of treatment is steadily increasing, which makes the search for cheaper alternatives more and more urgent . Two alternatives are theoretically feasible as means of limiting costs : 1 . The prevention of uraemia by timely diagnosis and successful treatment of renal disease . However, the present situation in this field does not support the hope for striking change in the near future . 2 . The application of less expensive forms of treatment . Here, kidney transplantation ranks first but is limited by selection criteria for recipients as well as by donor shortages . Lowering of costs for the artificial kidney has to be taken into consideration . Further development of available technology offers possibilities in this regard . * Paper presented at the 3rd Symposium on Synthetic Membranes in Science and Industry, September 7-9, 1981, Tubingen, West Germany . ** To whom correspondence should be addressed .

0376-7388/82/0000-0000/$02 .50 © 1982 Elsevier Scientific Publishing Company

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The work presented here deals with the application of capillaries consisting of surface modified porous glass for haemofiltration, a version of the artificial kidney developed in the last decade . The initial idea of this work was the design of an implantable artificial glomerulum [2], and the immediate aim is the construction of a reusable haemofiltration module which would set a milestone towards the final goal . At present test modules with effective surface areas of 0 .2-0 .3 m2 are being studied in dogs . In this article we present the technical specifications and the filtration dynamics of these test modules . Methods and materials Beagle dogs weighing about 10 kg were used for the ex vivo tests . Vascular access was accomplished by puncturing a superficialized carotid artery and a leg vein . The animals were anaesthetized by intubation narcosis during the test . Blood was heparinized by continuous infusion of 1 .000 I .U . heparin (Liquemin Roche) per hour after a priming dose of 1 .000 LU. . The test modules were arranged in series with a peristaltic pump (Masterflex Serva E 25) which was capable of maintaining a blood flow of between 15 and 100 ml per min . All tests were performed in the laminar flow range . The transmembrane pressure difference was set by a clamp at the outlet blood line . In some tests, evacuation of the pericapillary space by means of a second peristaltic pump attached to the outlet line of the ultrafiltrate was carried out to forther increase transmembrane pressure difference (P 3 ) . Pressures were registered by pressure transducers (Bentley Trantec) which were connected with the inlet and outlet blood lines close to the test module (P 1 , P2 ) . Transmembrane pressure difference was calculated as (P 1 + P2 )/2-P3 . The pressure transducers were calibrated by means of a mercury tonometer adapted by Gauer (Kannenberg Instruments, Berlin) . The flow rates were determined by outflow measurements into volumetric vessels . The ultrafiltration rate was measured by weighing the ultrafiltrate using a laboratory balance with automatic tare deduction . The ultrafiltration volume was substituted by infusion of lactic acid buffered Ringer's solution (Braun Melsungen) at the same rate . Total protein concentration in ultrafiltrate was determined . Before the tests, the arterial haemotocrit was 31 .6% ± 5 .1%, afterwards it became 35 .4% ± 5 .1% . Calculation and documentation of the test data were accomplished by a microcomputer (TRS 80) with online input via an interface of our own design . The following chemical analyses in blood and ultrafiltrate were performed : (1) Arterial haematocrit (centrifugation method) . (2) Free plasma haemoglobin concentration (Cyan method [1]) . (3) Plasma fibrinogen concentration (Clauss' method [6]) . (4) Total protein concentration (modified Biuret method [10]) . The porous glass capillary membranes were manufactured as described elsewhere [121 . Membranes with pore radii between 5 and 25 nm were



277

produced as capillaries, and the latter were bundled and cast in a module tube . The modules had uniform channel length of 22 cm, and the capillaries -2 cm and 2 .9 X 10 -2 cm . The efan internal diameter of between 2 .3 X 10 fective surface area was between 0 .2 and 0 .3 m2 . This area was chosen in order to match the haemodynamic situation of a 10 kg dog . The wall shear rates at average operating conditions were around 100-200 sec - ' . L p (NaCI sol . 0 .9%) amounted to 0 .25-0 .70 ml min - ' mmHg - ' m`2 . Results and discussion

1 . Filtration dynamics Figure 1 depicts a comparison of filtration dynamics with dog blood ex vivo and saline solution (aqueous NaCl solution 0 .9%) investigated by flow and pressure variation . The ultrafiltration rate and the filtration fraction were plotted as functions of transmembrane pressure difference at constant flow, or as functions of flow at constant transmembrane pressure difference . In the figure, each point represents the average of 5-15 single measurements of 1 minute duration . Curves were obtained for saline solution by means of regression analysis of approximately 50 measurements . An increase in transmembrane pressure difference is not associated with a proportional increase in the blood ultrafiltration rate (fig .1A), a result which contrasts with that found with saline solution . Apparently, the ultrafiltration rate approaches an asymptote . The extreme range of transmembrane pressure difference was established by evacuating the pericapillary space with an additional peristaltic pump . This procedure resulted in a maximum pressure difference of 1 .200 mmHg . The experiments show that the ultrafiltration rate is flowdependent in the case of blood, in contrast to saline solution (Fig .1B) . The filtration fraction UFR/Q B is one critical factor in clinical application . The results demonstrate that filtration fraction becomes independent of its determinant pressure difference when flow is held constant (fig .1C) . The flow dependence of the filtration fraction is equally important (fig .1D) . The blood curve exhibits a hyperbolic shape but does not correspond exactly to a hyperbola, in contrast to the saline solution curve . Blood ultrafiltration dynamics are currently best described by the concentration polarisation hypothesis as part of the general description of marcomolecular filtration dynamics according to Blatt et al . and Kozinski and Lightfoot [4, 7] . This theory applies to the fully polarized region of filtration dynamics where fluxes become independent of TPM but not of Q B , or, more exactly, of y W . The concentration polarization theory demands that flux limitation be given quantitatively for any high flux membrane, on the basis of the following expression : Jv=ksln CG CB

(1)





Fig .l .

0

0

200

25

50

75

100

Flow rate Iml min

NaCl

)

B

Transmembrane pressure (mmHg) r //-1-T+ r -300 400 500 600 800 850

T----T---

Ultrafiltration rate (ml min I)

100

NaCl

Ultrafiltration rate (ml min -1 )

100

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0

Filtration fraction

0

25

I

200

r

50

I

75

I

100

Flow rate (ml min' 1 )

NaCl

-

0

Transmembrane pressure (mmHg) r I I I f/- T --T-500 600800850 300 400

C



279

were k s is given by :

ks = 0 .816 (yl D2,1/3

(2)

/

Figure 2 presents our data obtained in the fully polarized region . Three modules with the same membrane modifications but with different surface areas have been employed . Linear regression analysis of the logarithmic values results in a curve with a slope of 0 .45 . For comparison experimental and theoretical curves are depicted . Curve no . 1 represents a linear regression line according to experimental data reported by Colton et al . [6] . Curve no . 2 corresponds to the equation in Fig . 30 taken from Blatt et al . [4] . Curve no . 3 depicts the theoretical treatment given by Porter [9] . Our own results agree with the cited literature fairly well . Detailed discussion of the curve characteristics, however, is not intended in this paper . Clinical haemofiltration operates in the quasi polarized region of the filtration characteristic, where both determinants yw and TMP are influential . Quantification of this region is desirable, since microprocessor control could improve efficacy of the filtration procedure, and since comparison of data collected with variation of the determinants yw and TMP in multiple applications is of interest . Such quantification is possible if double reciprocal transformation of the measured values is performed . This graphic procedure is a mere empirical approach to ease the problems associated with a curvilinear characteristic . The major weakness is that large values are transformed into small magnitudes and vice versa . Formally, this procedure corresponds to the Lineweaver-Burke treatment of enzyme kinetics [8] . Linear regression analysis of 1/x and 1/y data yields a linear equation with an y intercept equal to 1 /(maximum ultrafiltration rate) and a negative x intercept equal to the reciprocal value of the determinant at which the ultrafiltration rate is at half of its maximum . Such plots are illustrated in Fig .3 .

2 . Dimensional considerations for clinical applications The optimal range of operational parameters in clinical use is known : wall shear rate jy w should have a magnitude that minimizes concentration polarization effects . The membrane area should not exceed 1 m 2 because of the Fig .1 . Filtration dynamics of porous glass capillaries . Module 720 ; ex vivo dog tests and saline solution permeation . Specifications ; effective length 16 .8 cm, internal diameter 2 .27 x 10 -2 cm, number of capillaries 1560 . 1A . Ultrafiltration rate as function of transmembrane pressure at constant flow ; QB= 65 ml min - ' . 1B . Ultrafiltration rate as function of flow rate at constant transmembrane pressure ; TMP = 200 ± 5 mmHg . 1C . Filtration fraction as function of transmembrane pressure at constant flow rate ; Qa = 65 ml min' . 1D . Filtration fractions as function of flow rate at constant transmembrane pressure ; TMP = 205 ± 5 mmHg .



280

Jv

A

2

n_1

Ultrafiltrate flux (ml m

cm-

)

4-

2 -

4 -

2y„„/i

3 1 1

-1

Average wallshear rate/channel length I

I

(cm sec)

2

1

I

1

1 1 11

4 6 8 10

I

20

I

I

40

T

T

r

l 11100-

60 80

Fig .2 . Power dependence of blood ultrafiltration flux upon average wall shear rate/channel length . Ex vivo dog tests . Module No 787 ∎ Module No 787 nn Module No 789 • Module No 790 • Module No 7900 For comparison curves from the literature are depicted . Curve (1) : According to Colton et al . [ 6 ],

JV = 1 .28

Y yv

° •s

x 10-3

CC,

In _-

l-

I

CB

Curve (2) : According to Blatt et al . [4], .33

Jv

W

= 49 l Y

DZ

/ 0

CG

In CB

Curve (3) : According to Porter [9 J, l y W

Jv

= 0 .816' - D2 \

0 .33

CG In -

CB

Curve (1) and (2) fit experimental data ; curve (3) is a theoretical curve .



f

200

t

250

i

300

350

B

0 .2-

(min ml - 1

)

1 Transmembrane pressure (mmHg t r I *0 .2 0 .6 0 .8 0 .4

Ultrafiltration rate

1

1)

D

Fig .3 . Effects of determinant variation on ultrafiltration rate, linear and reciprocal plots ; ex vivo dog tests . Module 747 : effective length 17 .3 cm, internal diameter 2 .94 x 10 "2 cm, number of capillaries 1800 . Ultrafiltration rate as function of transmembrane pressure at flow rate variation ; 2A, linear plot ; 2C, reciprocal plot ; QB = 103 ml min - ' QB = 28 ml min' A, Ultrafiltration rate as function of flow rate at transmembrane pressure variations ; 2B, linear plot ; 2D, reciprocal plot ; TMP = 365 mmHg • TMP = 295 mmHg ∎ TMP = 204 mmHg A TMP = 137 mmHg o

t

150

t

100

Transmembrane pressure (mmHg)

Ultrafiltration rate (ml min - 1 )

0- !/

2-

A 14-

A



28 2 amount of fibrin and platelet deposition which may take place . Transmembrane pressure should not require a blood traumatizing axial pressure drop . Finally, evaluation of hemofilter operation in clinical use must be performed for comparison with hemofilters already on the market . Current treatment parameters for the gambro hemofilter FH 102 used in our hospital are given in Table 1 . TABLE 1 Clinical haemofiltration ; technical data (x, S .D.) Ultrafiltration volume (1)

Blood flow (ml min')

Treatment time (min)

Filtration fraction

Arterial haematocrit (% )

19 .8 ± 4 .2

278-310

214 ± 25 .3

0 .337 ± 0 .028

24 .4 - 4 .2

These considerations provide a set of definitions which allow calculation of a full size hemofilter when r, l and 5i are given, or are assumed to remain constant in both the test and the clinical hemofilter . 1 -1 UFR = 20 4 hr = 83 .3 ml min QB = 300 ml min - ' Fmax = < 1m2

=2nrln +

TMP = 300 mmHg 5

E {200 sec -1 , 300 sec -1 }

Definitions (1) and FF =

(2) results in

0 .28

(6)

and from (3) follows :

n

Fmax

(7)

2Trrl

If a test module fulfills eqn . (6) at operation conditions according to eqns (4) and (5), then the number of capillaries, N, is given by N=

QB(max) n

(8)

QB which is required in the clinical haemofilter . If N < n+ the haemofilter will meet established criteria for applicability .

(9)



28 3

3. Reuse In animal experiments, sterilisation procedure consisted of rinsing the module immediately before use with tap water in direct and back flushing ; 500 ml of sterile NaCl 0 .9% solution was perfused afterwards . This perfused solution was tested for bacterial contamination but the number of micro-organisms found could not be determined quantitatively . No septic reactions were observed in more than 160 animal experiments . Figure 4 depicts a comparison of the filtration dynamics of 2 test units at the beginning and the end of a reuse series . It is obvious that no significant deterioration of ultrafiltration occurred throughout the experiments . Ultrafiltration rate (ml min -1 } 16-

A 14 12 -

°

10-

°

6-

. 4 2Transmembrane pressure (mmHg)

I

I

I

I

I

50

100

150

200

250

i 300

~'

350

Fig .4. Blood ultrafiltration rate as function of TMP at constant flow rate obtained in two reuse series . Module No 711 initial experiment o Module No 711 final experiment (n = 18) Module No 721 initial experiment ° Module No 721 final experiment (n = 9) y u,/l Module No 711 32 .4 (cm sec) - ' y w /l Module No 721 11 .3 (cm sec) - '

Figure 5 shows the number of reuses obtained in the available test units . In all cases, mechanical damage to the sealing part of the module determined the application .

4. Biocompatibility Introduction of a new extracorporeal device in human medical treatment requires demonstration of harmless blood-surface interaction . Haemolysis and fibrinogen consumption have so far been studied .



284 Module No . 747 721 711 678 593 539 I

5

0

10

I

I

20 25 Number of applications

15

Fig.5 . Reuse numbers of porous glass capillary haemofiltration test modules . Ex vivo dog tests. Figure 6 shows free plasma haemoglobin concentrations in seven applications of a single module ; the concentration remained within original limits . This excludes significant haemolysis under operational conditions, similar to those in haemofiltration in humans . However, blood-surface interaction is evident in the case of fibrinogen . Figure 7 illustrates that plasma fibrinogen drops slightly but significantly to 88 .7 ± 7 .0% of the initial value during the first 15 minutes and remains at this level thereafter . At 15 minutes, a volume equal to the total blood volume of the dog has passed through the module . The fibrinogen decrease may be explained by hypothesizing that Plasma haemoglobin concentration (mg dl -1 l

-10

0

+10

30

50

70

90

110

130

150

Fig.6 . Plasma free haemoglobin concentration as function of application time .



2 85 Plasma fibrinogen concentration change (%) 120-

100-

80-

60-

40-

20-

0-

i

I

0

20

r

I

40

I

I

60

I

I

80

I

I

100

I

I

120

I

I

140

I

I

160

I

180 Time (min)

Fig .7 . Fibrinogen concentration change as function of application time .

the protein layer built up by concentration polarisation is enriched with fibrinogen . Another possibility is that fibrinogen decay is reflected in the appearance of fibrin precipitated immediately upon the glass surface . This hypothesis is supported by scanning electron microscopy of the inner capillary surface which shows a fibrin deposit (fig .8B) . Figure 8A shows the native glass capillary for comparison . A biological surface certainly offers better biocompatibility which may be advantageous in terms of the ultimate goal, implantation of a filtering device . The protein layer has the advantage of reducing plasma protein loss during haemofiltration . Our own recent in vitro studies [2] with polymer solutions of defined molecular weight have shown that the sieving coefficient for human albumin is in the range of 0 .4--0 .6, which leads to a predicted protein loss of 1 .200 mg dl - ' in the ultrafiltrate, since a dog has approximately 3 .000 mg dl - ' albumin in its plasma . However, the measurements shown in Table 2 reveal that only 10-20 mg dl - ' are found in the ultrafiltrate . Protein deposition of whatever kind must influence ex vivo protein cut-off to a considerable degree . Generally, if one considers the SEM pictures as representive of ex vivo experimental conditions, the operating glass membrane may be considered as part of a hybrid membrane of the sandwich type, with three different layers : the glass matrix, the fibrin deposit, and the cake of soluble plasma proteins . These layers play different functional roles ; the glass matrix defines hydraulic permeability of saline, the fibrin deposit influences

to w rn



287 TABLE 2 Plasma protein cut-off of porous glass capillaries, ex vivo dog tests Module No . Total protein concentration in ultrafiltrate (mg dl - ')

711 x S .D. n

720

11 .75

17 .7

± 3 .80 57

± 5 .14 6

721 12 .1 ± 5 .7 9

the plasma protein cut-off, and the protein cake leads to a saturation phenomenon of blood ultrafiltration . This hypothesis needs further substantiation by more experimental work . Acknowledgements This work was supported by a grant of the Bundesministerium Air Forschung and Technologie, Bonn (FRG) . The authors gratefully acknowledge the excellent technical assistance of Mrs . W . Glaser and the skillfull secretarial help of Mrs . H . Lessenthin . List of symbols UFR QB TMP FF JV CG CB l Fmax n{ N n t yw

r D Lp

Ultrafiltration rate (ml min - ') . Blood flow rate or saline solution flow rate (ml min - ') . Transmembrane pressure difference (mmHg) . Filtration fraction, UFR/Q B . Ultrafiltration flux (ml min - ' cm -2 ) . Protein gel concentration (g dl - ') . Average bulk protein concentration (g dl - ') . Channel length (cm) . Maximum membrane area in clinical application (cm 2 ) . Calculated number of capillaries for clinical application . Number of capillaries. Number of experiments . Time (min) . Wall shear rate, (secQB /15 n 1T r 3 Radius of capillary (cm) . Diffusion coefficient (cm' sec - ') Hydraulic conductivity (ml - ' min - ' mmHg - ' m -2 ) .

Fig .8 . Scanning electron microscopy picture of the internal surface of a module before (8A) and after (8B) application in ex vivo dog tests . (Courtesy Prof . Herbst, FB 3 WE 6 Klinikum Charlottenburg, Berlin) . F = Fibrin layer, IS = Inner glass surface, M = glass membrane matrix . Magnification x 2 .400 .

288

References 1 C . Bachmann, Modifizierte Cyan-Methode zur Bestimmung des freien Hamoglobins, in : R . Richterich and J .P . Colombo (eds .), Klinische Chemie, S . Karge, Basel, 1978, pp. 457-459 . 2 H . v. Baeyer, R . Schnabel, W . Vaulont and R . Mohnhaupt, Wiederverwendbare Hamoprozessoren auf der Basis einer oberfl£chenmodifizierten porosen Glasmembran, Biomedizinische Technik, 25 (1980) 292-294 . 3 H. v. Baeyer, R . Schnabel, W . Vaulong and G . Kaczmarczyk, Properties of porous glass membranes with respect to application in blood purification, Trans . Am . Soc . Artif. Intern. Org ., 26 (1980) 309-313 . 4 W.F . Blatt, A . Dravid, A.S. Michaels and L . Nelson, Solute polarization and cake formation in membrane ultrafiltration causes, consequences and control techniques, in : J .E. Flinn, (ed .), Membrane Science and Technology, Plenum Press New YorkLondon, 1970, p . 47-97 . 5 A . Clauss, Gerinnungsphysiologische Schnellmethode zur Bestimmung des Fibrinogens, Acta Haemotol ., 17 (1957) 237 . 6 C .K. Colton and L .W . Henderson, Kinetics of hemodiafiltration, I, In vitro transport characteristics of a hollow-fiber blood ultrafilter, J . Lab . Clin . Med ., 3 (1975) 355371 . 7 A .A. Kozinski and E .N . Lightfoot, Protein ultrafiltration : A general example of boundary layer filtration, AIChE J ., 5 (1972) 1030-1040 . 8 K.J . Laidler, The Chemical Kinetics of Enzyme Action, Oxford at the Claredon press, Oxford, 1958, Chap . 3 . 9 M .C . Porter, Concentration polarization with membrane ultrafiltration, Ind . Eng . Chem . Prod . Res . Develop ., 3 (1972) 234-248 . 10 J . Savory, P .H. Pin and W . Sunderman Jr ., A biuret method for determination of protein in normal urine, Clin . Chem ., 14 (1968) 1160-1171 . 11 R . Schnabel, H . v . Baeyer and H . W . Reinhardt, Glass membrane capillaries as basic elements for an artificial glomerulum, ASAIO Abstracts, 7 (1978) 51 . 12 R . Schnabel and W . Vaulont, High-pressure techniques with porous-glass membranes, Desalination, 24 (1-3) (1978) 249 .