Artificial.lung design: sheet-membrane units* John D. S. Gaylort

Lyle F.Mockros

Technological Institute, Northwestern University, Evanston, III. 60201, USA

Abstract--An overall design analysis is presented for oxygenators with sheet-membrane blood channels. Those types considered are clear channels without and with transverse mixing and woven-mesh filled channels with thick and thin membranes. The analysis accounts for oxygen-transfer rates and increased flow resistance produced by the screens and for such fabrication variables as blood header length, header-thickness-to-channel-height ratio, and blood-screen void volume. For reasonable priming volumes, practical channel lengths and total widths, oxygenators having screen-filled channels with thick membranes or clear channels without mixing are only suitable for partial adult or neonatal support. Adult total bypass is feasible with screen-filled channel designs using thin or highly permeable membranes. Since the configuration of existing clear channel devices with secondary mixing (e.g. Couette or Taylor-vortex types) usually involves a single and relatively thick channel, they are restricted to neonatal or partial bypass applications despite their inherent high transfer efficiencies. Keywords--Artificial-lung design, Membrane oxygenator, Blood screens

List of symbols A = surface area of membranes and header compartments b = height of blood-header chamber D = either D, or De, whichever is appropriate De = effective diffusivity of Oz in whole blood (for screen-filled channels) D,, = normal diffusivity of 02 in whole blood (for clear channels) g = height (gap) of blood channel L* = dimensionlesslength for oxygenation, D L,/gq La = active (gas exchange) length of channel Lh = axial length of blood-header chamber n = number of channels in parallel q = blood flow rate per unit width of channel, Q/wn Q = total flow rate of blood through oxygenator V = total priming volume of oxygenator w = width of blood channel ~t = packing factor of header chamber, big ,8 = void volume factor of blood screen r / = apparent viscosity of blood Ap = pressure drop across oxygenator

that results with induced secondary flows has been delineated for many devices, explicit design considerations are lacking. The many designs in use may be divided into two classes: tubular units and sheet units. Design aspects of tubular units were presented in a previous paper (MocKROS and GAYLOR, 1975), and the present paper considers sheet-membrane units. In most cases, sheet-membrane oxygenators possess a number of parallel-connected and usually vertically stacked blood-containing channels or layers. The channels are often formed by the opposition of two sheets of silicone rubber or silicone copolymer membrane. Passages that also function as membrane supports convey ventilating gas to the membrane surfaces not in contact with the blood. Sheet-membrane units may be subdivided into three types: those with rectilinear blood flow in clear

g

Introduction THE INCREASINGuse of membrane auxiliary lungs in prolonged cardiopulmonary bypass (LEFRAK et al., 1973) attests to their minimal blood trauma characteristics. A variety of designs have been and are being developed. Although gas-transfer performance is well known when the blood flow is parallel to the membrane surfaces and the improved performance "First received 5th November 1973 and in final form 20th February 1974 tPresent address: Bioengineering Unit, Strathclyde University, 106 Rottenrow, Glasgow 64 ONW, Scotland

Medical and Biological Engineering

May 1975

Fig. 1 Sketch indicating notations used in th~ design analysis of a typical blood channel. Most units would employ a number of such Channels

425

channels, those with induced secondary flow in clear channels, and those with screen- or cone-filled channels. The gas-transfer performance in clear channels without secondary mixing is the most easily analysed and the most thoroughly understood. The early sheet-membrane units (CLOWES et al., 1956) were of this type. Because of its poor transfer efficiency, however, this type is rarely used today. The clear channels with induced secondary mixing include those of the type discussed by KELLER (1969), CANTEKIN and WE]SSMAN (1970), WEISSMAN and HUNG (1971), CHAN6 and MOCKROS (1971) and GAYLOR et al. (1973). The most common type given serious commercial development is the screen-filled channel. In the Travenol Modulung rM ( B o v D e t al., 1972; TRUDELL et al., 1972) and Bramson-Cutter (BRAMSON et al., 1972) lungs, the blood channels contain a woven-nylon-mesh screen that maintains a uniformity in the gap between the membranes, reduces the blood volume, and provides some degree of secondary mixing in the laminar blood flow. The unit described by KoLoaow and BOWMAN (1963) utilises the wavyness of Dacron-reinforced membranes and pulsations of the gas phase to induce secondary flows. The General Electric-Pierce artificial lung, on the other hand, has vacuum-formed conical projections on the membrane surface that stabilise the channel gap and also promote a secondary mixing in the blood. The Lande-Edwards oxygenator has not been considered for the present analysis since it is a hybrid of the parallel plate and tubular membrane cases. In this unit, small elliptic cross-section capillaries are formed by the deformation of silicone membrane sheets into grooved plastic plates. In each blood layer the capillaries are interrupted at five equidistant positions to provide an entry or exit for the blood. The unit, although using sheet membranes, is a staged tubular unit.

Design considerations Much of the following analysis is based on the assumption that the oxygenator is composed of a number n of vertically stacked and parallel-connr blood channels or layers, one of which is shown in Fig. 1. Two sheet membranes of width w and length L , form the rectangular blood conduit of gap g. In the Travenol and Bramson-Cutter units, a woven mesh screen of thickness g is in the channel between the two membranes. Deoxygenated blood enters the upstream headers and passes along the length of the channels, where 02 uptake and CO2 release occurs. Arterialised blood is drained from the downstream headers. Rectangular header compartments of length Lh, width w and height b are representative of various geometries that could be used. A useful parameter, the packing factor ~, is defined as the ratio of the header height to channel thickness, i.e. ~t = big. Since headers are nonfunctional in terms of gas exchange, their volume should be minimised and ~ and L~ should be small. The 426

smallness of ~, however, is somewhat constrained by manufacturing technology, and too small an L~ creates a large pressure loss along the header and impairs blood distribution to the channel entrances. Selection of a spacing factor that gives an even distribution is largely empirical since it is influenced by the pressure loss across the blood channels, the channel width, and the blood flow rate. In the

'

so,. . . .

,,.'.u,.,roo.

.o:o.,.

Ap E 200

-

o Iool-

Normal-odult

.....

...o0,,o., ....

.....

Neonatol

~

t

mmHg 150

~

/ /

'

~l 30

oo I-

,op 20

Artificial-lung design: sheet-membrane units.

Artificial.lung design: sheet-membrane units* John D. S. Gaylort Lyle F.Mockros Technological Institute, Northwestern University, Evanston, III. 602...
1MB Sizes 0 Downloads 0 Views