LETTER TO THE EDITOR
Year : 2010 | Volume
: 21 | Issue : 4 | Page : 748--749
Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems
Khaled M Talaat
Internal Medicine Department, Zagazig University, Zagazig, Egypt
Khaled M Talaat
Internal Medicine Department, Zagazig University, Zagazig
|How to cite this article:|
Talaat KM. Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems.Saudi J Kidney Dis Transpl 2010;21:748-749
|How to cite this URL:|
Talaat KM. Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems. Saudi J Kidney Dis Transpl [serial online] 2010 [cited 2020 Jul 7 ];21:748-749
Available from: http://www.sjkdt.org/text.asp?2010/21/4/748/64669
To the Editor,
In forward osmosis (FO), water is drawn from the feed solution to dilute a more concentrated draw solution across a water permeable, but solute impermeable membrane. Recently, there has been a great interest in the possible applications of FO including osmosis pumps for drug delivery, water desalination and the hydration bags made by Hydration Technologies, Inc., Oregon. ,,
A similar concept may be adopted for dialysis fluid regeneration, whereby a concentrated draw solution consisting mainly of sodium chloride is used to draw water from the spent dialysis fluid across a FO membrane. After FO treatment, the fluid obtained on the draw solution side of the membrane increases in volume and its composition becomes more similar to the fresh dialysis fluid while the spent dialysis fluid looses a substantial portion of its water content and the uremic toxins become more concentrated in the remaining fluid, which is then discarded. Theoretically, if a highly concentrated draw solution containing physiologically balanced electrolytes is used, up to 50% of the spent dialysis fluid water may be retrieved by a FO treatment.
An efficient method for dialysis fluid regeneration is a pre-requisite for developing a feasible ambulatory dialysis system. The small bulk of the necessary equipment, mainly a FO membrane and a low pressure pump as well as the very low external energy needed, make FO a very attractive option for dialysis fluid regeneration. The CTA forward osmosis membrane made by Hydration Technologies, Oregon outperformed other membranes in FO applications. The United States Patent Application 20060226067 disclosed an asymmetric FO membrane having a salt rejection ratio of more than 99%, and comprising cellulose fibers. A disadvantage of the cellulose membrane is its suboptimal urea rejection ratio.  However, the clinical toxicity of urea is minimal,  and a decreased urea clearance may not adversely affect the clinical usefulness of the process. In another approach to more efficient FO membranes, Kumar et al developed a highly permeable polymeric membrane into which the bacterial water channel protein, aquaporin Z was incorporated. This membrane showed excellent water permeability and solute rejection while its urea rejection ratio was almost complete. 
The source of the energy that drives FO is the thermodynamic energy generated by the concentration difference between the draw solution and the feed solution.  Consequently, increasing the concentration of the draw solution increases the intrinsic potential energy of the system and the amount of water that can be retrieved from the spent dialysis fluid. A more concentrated draw solution will decrease the bulk of the fluid carried by the patient and increase the efficiency of the FO system.
In a system using FO for dialysis fluid regeneration, sodium chloride must be the main solute in the draw solution. Other physiologically important electrolytes such as calcium, potassium, magnesium and a bicarbonate generating base must also be added in physiologically balanced proportions to obtain a reusable regenerated solution on the draw solution side of the FO membrane. Given that the amount of salt in the draw solution is equal to the amount of salt in the spent dialysis fluid, the water retrieval ratio will always be less than 50% of the water content of the spent dialysis fluid and the reclaimed fluid will always be hyperosmolar in comparison to the spent dialysis fluid. These obvious drawbacks must be corrected by a complimentary form of treatment to make FO a viable option for dialysis fluid regeneration. Adding free water from a refillable water reservoir attached to the system is a very feasible option. However, the water volume needed will be 50% of the volume of reclaimed dialysis fluid. Adding nutrients like glucose to the draw solution will increase the water fraction that can be retrieved by approximately 50 %, while the final glucose concentration in the regenerated dialysis fluid will be approximately 5%. Consequently, in a ten hour/day dialysis prescription, the total energy gain from the dialysis fluid will be unacceptably high (2000 calories/day).
Because of the limitations and peculiarities of the proposed concept, modified dialysis therapy prescription is necessary when FO is used for dialysis fluid regeneration. In essence, a prolonged daily therapy will be acceptable to the patients because they remain ambulatory during the therapy. An extended time dialysis prescription may also result in a better removal of phosphate and protein-bound uremic toxins, if a more permeable hemodialysis membrane is used. Also, slow extended time ultrafiltration may offer better volume control and cardiovascular stability during the dialysis treatment. It may be possible to combine a short daily conventional home hemodialysis therapy with a prolonged ambulatory dialysis therapy using FO for dialysis fluid regeneration. The proposed dual modality therapy may allow the use of two hemodialyzer membranes, each optimized for a particular prescription. Urea and small solutes are satisfactorily removed during a short treatment time if high dialysate flow rate and a dialyzer with high urea clearance are used. Phosphate, middle weight uremic molecules and protein-bound uremic substances are better removed during a prolonged dialysis therapy using a highly permeable membrane even with a low dialysate flow rate. Small priming volume, low thrombogenicity, high permeability for the larger uremic toxins and better biocompatibility are the major prerequisites of the membrane used for the prolonged dialysis prescription in which the dialysis fluid is regenerated by FO.
To test the proposed concept, a multilayer FO device e.g. a 6-8 layer parallel plate and frame module should be specifically built for the purpose of dialysis fluid regeneration. The water retrieval rate of the proposed module and its rejection ratio of the large number of the well recognized uremic toxins should be determined in vitro. If satisfactory in vitro results are obtained, animal studies should be conducted.
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