Biomacromolecules 2005, 6, 2204-2212
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Engineering Temperature-Sensitive Hydrogel Nanoparticles Entrapping Hemoglobin as a Novel Type of Oxygen Carrier Jaqunda N. Patton and Andre F. Palmer* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556 Received February 28, 2005; Revised Manuscript Received April 29, 2005
Temperature-sensitive oxygen carriers that are responsive to changes in temperature while maintaining colloidal stability would benefit physiological conditions characterized by tissue hypoxia due to decreased body temperature. These conditions are often accompanied with reduced blood flow and vasoconstriction. Temperature-sensitive oxygen carriers should ideally possess increased oxygen affinity when the body temperature is reduced, to selectively target tissues that are hypoxic as a result of temperature drops. This study expands on previous work, which introduced hydrogel based oxygen carriers as a new class of oxygen carrier that can be synthesized within liposomal reactors via photoinitiated free radical polymerization [Patton, J. N.; Palmer, A. F. Biomacromolecules 2005, 6, 414-24]. In addition to the ability of poly(Nisopropylacrylamide) hydrogel nanoparticles encapsulating bovine hemoglobin to swell and shrink in response to physiological changes in temperature, the effect of temperature changes on zeta potential, oxygen affinity, and cooperativity are also examined. The methemoglobin level and hemoglobin encapsulation efficiency of hydrogel-based oxygen carriers are also presented. It was observed that nanoscale hydrogel particles swelled as the temperature decreased from 40 to 29 °C, which suggests expansion of the hydrogel matrix and reduced resistance to oxygen transport. 1. Introduction Hydrogel-based oxygen carriers were previously introduced as a new class of oxygen carrier that was sensitive to environmental stimuli.1 These particles were shown to be spherically shaped with narrow size distributions and oxygen affinities, methemoglobin levels, and colloidal osmotic pressures comparable to that of cellular and acellular oxygen carriers currently being developed.1-6 In addition, nanoscale hydrogel particles (NHPs) were found to have hemoglobin encapsulation efficiencies comparable to that of polymer capsules and higher than that of liposome encapsulated hemoglobin dispersions.1,2,7 Despite the presence of various designs of oxygen carriers in the literature, the concept of an oxygen carrier that has the ability to swell or shrink in response to environmental stimuli such as temperature or pH changes has not been examined to date. Temperature-sensitive oxygen carriers would provide enhanced oxygen delivery only when the body temperature falls below normal, and revert to a lessened degree of oxygen delivery once conditions returned to normal. The oxygen affinity of human red blood cells (RBCs) naturally increases when the body temperature decreases. This is due to an increase in oxygen affinity of hemoglobin contained in RBCs elicited by a decrease in body temperature.8 Hemoglobin cross-linked within the hydrogel matrix of nanoscale hydrogel particles (NHPs) will embody an oxygen carrier that conserves this natural response of hemoglobin toward temperature changes. Expansion of the hydrogel matrix as * Corresponding author. E-mail:
[email protected].
temperature decreases is also expected to result in increased oxygen affinity as a consequence of reduced resistance to oxygen diffusion through the matrix. The oxygen affinity of a hemoglobin-based oxygen carrier is quantified by the P50, which is the partial pressure of oxygen at which half of the oxygen carrier’s oxygen binding sites are saturated with oxygen. The Hill coefficient (n) is typically determined along with the P50, and it reflects cooperative binding of oxygen to the hemoglobin-based oxygen carrier when the value is >1. Hypothermia is a physiological condition that occurs when the body’s temperature is abnormally low (i.e., 15 g/dL of hemoglobin, whereas LEHbs have been able to encapsulate up to 10 g/dL of hemoglobin and nanocapsules have encapsulated over 14 g/dL.7 This encapsulation efficiency meets the design criteria to encapsulate a minimum of the same amount of hemoglobin as present in human RBCs (15 g/dL). It has been suggested that an artificial blood substitute should have
