Introduction
Designing the Optopatch has been by far the most challenging project that we have ever undertaken, but it has also been a tremendously interesting one. Patch clamping is itself a challenging technique, but since it is now so well established, those people who politely asked us during the design stage why we were bothering to get involved in it now, certainly had a point. To be honest, at first we rather agreed with them. And yet, the Optopatch came into being because people were specifically asking us for the improvements it incorporates. The design is all ours, and we’re very proud of it, but we are also very grateful to the people who pointed out to us the problems that needed to be solved. There is a great deal of unique electronic circuitry in the Optopatch, allowing us to offer the three innovations. The optical current-passing technique (from which of course the product takes its name), the automatic hardware-based capacitance measurement facility, and greatly improved current-clamping performance. We also threw in a few other useful improvements, such as a wide-range high-frequency filter, a wide-range frequency generator, and extensive use of electronically controlled ("analogue") switches in the signal pathway in order to keep the path lengths shorter and the front panel wiring simpler.
It all started at a Physiological Society meeting in Leicester in the spring of 1993, when someone asked me if I had any better ideas about headstage design, in particular with regard to current passing, as he felt there could be room for improvement over both the standard high-value resistor technique and the more recently introduced integrating (capacitive) technique. At the time we had no plans in that area, and I wouldn’t have given the matter any serious thought, but following the Society Dinner and its rather alcoholic aftermath, I was left with a more than usually severe hangover. To check that there had been no (further) lasting mental damage, I started to think about the problem, and at the time an optical technique seemed to be a most interesting alternative solution. It still seemed a good idea a few days later, and a few preliminary electronic experiments suggested that it could be made to work well in practice, but at the time we were particularly busy with other projects, so we had to wait for nearly eighteen months before we could proceed any further.
The problem was that a patch clamp headstage is literally the tip of the iceberg, so we knew that designing the rest of the electronics would be no small task. Although we weren’t ready to go public on the headstage idea, we nevertheless sounded out the possibility of there being a Cairn patch clamp one day, and asked people what facilities they might like to see on it. As a result of that, our attention was drawn to the interest in following vesicular secretion from cells by membrane capacitance measurement, and this is what determined our design approach for the main electronics. We are impressed by the EPC-9 computer-controlled patch clamp, which can do this and other functions in software, but we also liked the idea of the hardware approach using a lock-in amplifier, which can make fast and sensitive capacitance measurements directly but is not so easy to use. Faced with the choice of a "me-too" computerised approach, or the new challenge of an improved analogue system, we opted for the latter. What this means is that where the EPC-9 might implement a gain control using a digital-to-analogue converter ( DAC ) programmed by the computer, the Optopatch might implement it using an amplifier with a gain that is set by an analogue control voltage. This means that we can implement automatic control functions by deriving the appropriate control voltages internally, instead of digitising the relevant signals and then performing the control via programming of the DAC’ The output signal from the headstage is amplified by a factor of 100, and is single-pole filtered with a corner frequency of 10Hz to remove DC and low-frequency AC components. High-frequency components are removed by a three-pole Bessel filter with a corner frequency of 10 KHz. {The significance of these terms is explained in the section of the manual that describes the output filter, so please see there for further information.} This leaves signals in the 10Hz-10KHz frequency ranges by the computer software. Although the comparative performance of DAC’s and gain-controlled amplifiers is a subject in itself, the resolution of the amplifiers we use is at least as good as a 16-bit DAC, and we think that either component is capable of giving satisfactory results.
Our approach tries to offer the best of both worlds, so most functions are set by conventional front panel controls. A computer is not essential for the Optopatch! However, we are well aware of just how useful it is to be able to record additional parameters such as a telegraph voltage to indicate the gain setting, so outputs representing the important switch and control settings have been provided. Although we did not set out to design a computer-controlled product, it was only a small extra step to make these signal lines bi-directional, so that a computer can set them as well as read them. Whether or not a computer is used to control the Optopatch, the internal analogue signal processing allows outputs such as direct linear measurements of membrane capacitance to be generated in real time. These can sent directly to any general-purpose data acquisition system, and there is no need to process them any further by special software. This approach is particularly useful when other data (such as fluorescence signals!) are being captured at the same time. While of course we would love you to use our own computer interface and software, you remain free to make your own choice. From the design point of view, our approach left us a fair amount of extra electronics to implement, but the individual functions were all simple enough. However, the overall schematic is quite impressive....
We thought all this would be enough to be getting on with, but towards the end of the design process, our attention was drawn to the fact that the current-clamp performance of other patch clamps can be noticeably less than ideal. The problem and its solution are described in detail in a later section, but we were made aware of the problem in time to reconfigure the interconnections between the headstage and the main electronics, in order to allow the easy implementation of its solution.
We have given considerable thought to making the construction of the Optopatch as easy as possible. The product’s electronic complexity would justify us charging a premium price, but we wanted to price it competitively with all the other patch clamp amplifiers, in spite of its unique facilities. We have minimised the construction costs by placing most components on a single circuit board, although those specific to the optical headstage design are on a separate daughter board. This is to give us options such as to introduce headstages of more standard design, e.g. for passing higher currents in other applications, where the resistive method is as good as any other, and/or to compensate for even larger (i.e. nanofarad) membrane capacitances. All active devices on both boards are socketed, and all external connections are via pluggable connectors, allowing easy identification and repair of any fault. The use of analogue switches greatly simplifies the wiring to the front panel controls, as well as keeping the signal paths short. For example, the filter range switch is a single-pole rather than a four-pole type, and instead of being in the analogue signal pathway, the switch connections need to take only digital control voltages. This arrangement gives improved performance and reliability at lower overall cost. Furthermore, the actions of some switches depend on the settings of others, and this is easily handled when only control voltages are involved, since standard logic circuitry can be used to produce the desired effects.
