331 Ion concentration effects on bone streaming potentials and zeta potentials W.R. Walsh Department of Orthopaedics, Brown University School of Medicine, Division of Engineering, Brown University, Biomechanics Laboratory, Rhode /s/and Hospital, 593 Eddy Street, Providence, RI 02903, USA N. Guzelsu University of Medicine and Dentistry of New Jersey-School of Osteopathic Medicine and Rutgers University Biomedical Engineering, Biomechanics Laboratory, 675 Hoes lane, Piscataway, NJ 08854-5635, USA Electrical potentials are dependent on the properties of the solid and fluid phases of bone. The solid phase in bone is composed of an organic matrix and inorganic bone mineral fibre, while the fluid phase is separated into compartments associated with the vascular channel system and mineralized matrix. Recently, a piezoelectric and electrokinetic response following mechanical deformation was demonstrated in fully hydrated bone. However, alterations in the fluid phase and the effects on streaming potentials where flow through the sample due to pressure on the fluid phase without prior solid matrix mechanical deformation have not been examined. Streaming potentials in high ionic strength solutions reveal a flow-dependent streaming potential in the absence of mechanical deformation not previously observed in stress-generated potentials. Streaming potentials in high ionic strength sodium chloride solutions (0.75 M) of control and deproteinized samples suggest that organic molecules and ions in the electrical double layer may be susceptible to flow-induced alterations which can modify the streaming potentials generated. Alterations in properties of the fluid phase can modify the streaming and zeta potentials and may play a role in the biofeedback response of bone tissue. Keywords: Bone, electromechanical properties Received 14 May 1992; revised 14 August 1992; accepted 6 September 1992 The electrokinetic phenomenon of streaming potential has generally been accepted as the dominating mechanism for the electrical potentials observed in wet bone upon deformation’-6. A piezoelectric contribution in stress- generated potentials produced upon mechanical defor- mation has recently been reported7. Both mechanisms were found to be present at full hydration, with the piezoelectric effect leading streaming potential in the time domain7. However, streaming potentials dominate the signal upon ion motion due to fluid flow. Bioelectrical potentials have been hypothesized to be involved in the generation, repair and remodelling of bone tissue81g. Streaming potentials arise due to fluid flow and displace- ment of the fluid phase and mobile ions in the electrical double layer with respect to the charged solid surfacelo. Streaming potential measurements can be used to characterize the solid surface-fluid phase electrical double layer through zeta potential calculations. Streaming potentials and the electrical double layer are dependent on the properties of the solid as well as the liquid phases. In bone, the solid phase is composed of the mineralized organic matrix of type I collagenous matrix Correspondence to Dr W.R. Walsh. and non-collagenous proteins embedded with bone mineral (hydroxyapatite analogue (Ca,o(PO,),(OH),)l’. Bone is organized into fluid compartments within the mineralized matrix (canaliculi) and vascular channels (Haversian and Volkman’s canals)“*” where flow may occur. Organic material in the Haversian canals separates bone fluid spaces into compartments of the vascular channel system and extracellular fluid of the mineralized matrix13. The organic linings include fenestrated blood vessels lined with endothelial cells14, basement membrane and bone lining cells [resting osteoblasts)15. The magnitude of streaming potentials has been demonstrated to be inversely related to ionic concentration and conductivity of the equilibrating solution in a four- point bending stress-generated potential set-up3, as well as in a piston-driven cylindrical streaming potential apparatus where fluid is forced through the samplezs4. An alteration in the fluid-phase ionic concentration, pH, viscosity or dielectric constant will modify the electro- kinetic potential. Such a modification in the electrokinetic (zeta) potential may play a role in the biofeedback system in repair and remodelling of bony tissue. Little is known, however, about the fluid flow characteristics or associated pressure levels during bending that give rise to stress- 0 1993 Butterworth-Heinemann Ltd Biomaterials 1993, Vol. 14 No. 5 0142-9612/93/050331-06