Streaming potentials in cartilage have been studied by several investigators. Direct permeation forces a fluid to flow across a cartilage disk results in a measurable electrical potential accross the disk. Compression of cartilage also induces interstitial fluid flow and generates electric fields and potentials in the tissue. A converse phenomena has also been observed where forcing a current to flow through cartilage results in measurable surface stress.
Interest in these techniques has increased due to their practical uses. For example, enzymatic degradation of cartilage showed that streaming potentials were a sensitive indication of matrix degradation, more sensitive than purely mechanical measures. This high sensitivty to functional matrix properties was also apparent in a model of matrix development, chondrocytes cultured in agarose gel. Our study builds upon these previous results by adding the aspect of spatial resolution to streaming potential measurements. In addition to use as an indicator of matrix quality, this technique provides us with detailed information on spatially specific physical events. We can use this data to investigate mathematical models of physical behavior and to further understand what phenomena are responsible for biological responses to tissue loading.
Independent of the particular experimental configuration of streaming potential generation, the same underlying molecular phenomena are at work. A molecule bearing a net fixed charge must be immobilzed to the macromolecular network. In the case of cartilage this molecule is aggrecan entrapped in the collagen network. Due to the negative charge of aggrecan, there is an excess of mobile, non-fixed, positive charge in the fluid.
Under equilibrium conditions, with no load or fluid flow, these opposite charges are symetrically arranged so that no net macroscopic electric field exists. During load, fluid flow entrains a displacement of counterions relative to the fixed charge, resulting in an electrical field and streaming potential.
To measure these potentials at several points along the articular surface of loaded cartilage we fabricated an array of microelectrodes by weaving platinum wires through a nylon mesh, casting the mesh in an epoxy cylinder and then glueing that cylnder into a hole drilled in the bottom of a testing chamber.
The regular spacing of the nylon mesh allowed a regular distance between adjacent electrodes 380µm across a total length of 1.14 mm for the linear array. The platinum surfaces were further platinized to reduce their electrical contact impedance. 1
The test chamber was one for unconfined compression between two smooth surfaces. Electrical signals were conditioned by a unit containing high input impedance voltage followers followed by high and low pass filters and amplifiers.
The test chamber was placed on the electromechanical actuator of a mechanical testing device. The cartilage disk is placed articular surface down in contact with the electrodes.
I will now briefly describe two applications of this apparatus using example data for each application. We have obtained the streaming potential radial profile under dynamic sinusoidal compression and during stress relaxation. For all of these example applications we have positioned the linear electrode array across the 1.5mm radius of a cartilage disk and have imposed an initial static compression offset of between 100 and 250 microns.
Here is an example of the raw data obtained during a dynamic sinusoidal test with a 5µm amplitude sinusoidal displacement at 1 Hz. The resulting sinusoidal load and streaming potentials between adjacent electrodes is shown. The spatial dependence of the streaming potential is evident since the potential amplitude is much larger at the periphery than at the center. 2
The radial profile of streaming potential was constructed by considering the potential at the most peripheral electrode to be ground. Lowering the frequency lowers the amplitudes of the potentials and increases the phase advance between the potential and displacement signals. 3
In addition to being dependent on frequency, the radial potential profiles may be modulated by the amplitude of displacement. Potential amplitudes increase nearly in proportion to the applied displacement amplitude. 4
The streaming potential and radial potential profiles may be obtained during ramp-rise/stress-relaxation tests. A 10 µm step achieved in 5 seconds generates a typical stress relaxation profile. The microelectrode measurements provide further information on the physical events occuring with the disk during this relaxation process. 6
The radial profiles were constructed as a function of time during the ramp rise and stress relaxation periods. The radial profiles of electrical potential may be directly related to interstitial fluid pressure profiles so that these curves also approximately indicate the fluid pressure as a function of time and position in the disk. 7
In summary, a linear array of microelectrodes conctacting the surface of cartilage during compression can spatially resolve compression-induced streaming potentials. These potentials may be measured under dynamic sinusoidal compression and stress relaxation. We intend to exploit this information to investigate model predictions of spatially varying physical parameter due to load and to correlate these parameters with biological responses which are also known to be spatially varying. These measurements may also be used as a means to detect matrix degeneration and matrix development.
This work was funded by the Medical Researhc Council of Canada , the Arthritis Society of Canada and le Fonds de la Recherch en Santé du Québec.
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