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Review TRENDS in Biotechnology Vol.21 No.2 February 2003 89

Sonobioreactors: using ultrasound for

enhanced microbial productivity

Yusuf Chisti

Institute of Technology and Engineering, PN456, Massey University, Private Bag 11 222, Palmerston North, New Zealand

Enhanced metabolic productivity of microbial, plant and animal cells in bioreactors can greatly improve the economics of biotechnology processes. Ultrasound is one method of intensifying the performance of live bio- catalysts. Ultrasonication is generally associated with damage to cells but evidence is emerging for beneficial effects of controlled sonication on conversions cata- lyzed by live cells. This review focuses on the pro- ductivity enhancing effects of ultrasound on live biological systems and the design considerations for sonobioreactors required for ultrasound-enhanced biocatalysis.

Many methods have been applied to enhancing bioreactor productivity [1]. Use of ultrasound in specifically designed sonobioreactors can potentially substantially increase the productivity of a biological process. However, little work has been reported on the effects of ultrasound on live microbial and other cellular systems in bioreactors and design and operation of ultrasound enhanced sono- bioreactors have been investigated even less. This article is concerned primarily with ultrasonic enhancement of the performance of live microbial and other cells in sonobioreactors.

Ultrasound, or sound of frequency .20 kHz (Box 1), is inaudible to the human ear. Irradiation with ultrasound is widely used in medical imaging, sonochemical processing [2], ultrasonic cleaning of surfaces and as the basis for underwater sonar ranging. Ultrasound is potentially useful in many food-processing applications [3–6]. Medical imaging applications use megahertz range (1–10 MHz), low power, diagnostic ultrasound. High energy or ‘power ultrasound’ in the 20–100 kHz frequency range is used in many sonochemical processes. High power ultrasound treatment in aqueous media has been used to reduce hatch times of fish eggs and germination times of seeds [3] (which can be achieved under dry conditions). There is now ultrasound equipment for processing large quantities (e.g. 600 kg h21) of dry seed at 20 kHz and a vibrational ampli- tude (Box 1) of between 1 mm and 40 mm [3].

Enhanced membrane permeation (a phonophoretic effect) of ultrasound on cells has been widely reported [7]. Ultrasound has been used successfully to induce transfer of genetic material into live animal [8] and plant cells [9]. At sufficiently high acoustic power inputs, ultra- sound is known to rupture cells and ultrasonication is a

well-established laboratory technique of cell disruption [10]. A cell can be inactivated by ultrasound at intensities less than those needed to cause disruption [11]. Intense ultrasound is known to damage macromolecules such as enzymes [3,10,12], probably from unfolding and scrambl- ing the native protein and breaking the chain into radicals or small peptides.

High-power ultrasound induces cavitation, generation of free radicals and other mechanical and chemical effects. During cavitation, microbubbles form at various nuclea- tion sites in the fluid and grow during the rarefaction phase of the sound wave. Then, in the compression phase, the bubbles implode and collapsing bubbles release a violent shock wave that propagates through the medium [10,13]. Cavitation is associated only with power ultra- sound and is used to explain the performance enhancing effects of ultrasound in nonbiological sonochemical sys- tems. Cavitation causes intense local heating with tem- perature rising to .4 0008C and pressure in a collapsing cavitation bubble can reach ,1 000 atm. Local tempera- ture in the vicinity of a forming or collapsing bubble can change extremely rapidly at .1108C s21. Cavitation, often accompanied by emission of light, can break apart rela- tively robust small molecules and bioactive macromol- ecules, and thus life does not survive cavitation for long. Intermittent, power ultrasound of short duration can cause a productivity enhancing effect in live systems. Cavitation generates microstreaming and other actions in the fluid. Just as chemical additives can be used to dampen hydrodynamic turbulence in animal cell culture [14,15] similar additives can be used to modulate ultrasound effects such as extracellular microstreaming. In addition, a high viscosity suppresses cavitation.

The mechanism of cell disruption by ultrasound is probably linked with cavitation phenomena and the resulting shock wave, and not ultrasound-induced micro- eddies. Ultrasonic cell disruption generally results in very fine cell debris that is morphologically different from the coarser debris produced during other fluid shear-based processes [10] of disrupting cells.

In sonochemical processing, cavitation is desired. Effec- tive insonation in a sonochemical process requires the energy input to exceed the cavitation threshold through- out the working volume of the fluid. By contrast, in a sonobioreactor the cavitational threshold energy is not exceeded in most of the reactor volume. Cavitation thresh- old values can vary widely depending on the fluid being

Corresponding author: Yusuf Chisti ( 0167-7799/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0167-7799(02)00033-1

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