Patent Description:
One example process where such a heater is required is DNA amplification by the polymerase chain reaction (PCR), where the heater provides fast thermocycling to reduce the time for completion of PCR.

Prior art heaters are fabricated from electrically conductive tracks supported by an electrically insulating substrate. Prior art heaters are controlled using a separate temperature sensor to sense the heater temperature and a control algorithm and electronic drive circuit to modulate the electric drive to the heater.

In order to provide fast thermal response, the heater must have low heat capacity and the heater must be in close thermal contact with the reaction surface. In particular the thermal diffusion time from the heater element to the reaction surface must be less than the required temperature change response time, so the heater and reaction surface can only be separated by a thin layer.

Conventional heaters and temperature control systems have several disadvantages when trying to achieve fast response, precise temperature control and uniform temperature distribution.

For example, use of a temperature sensor separated from the heater introduces a delay in the heater control loop which can reduce the response speed or introduce temperature overshoot.

Additionally, temperature non-uniformity results from heat generation within spatially separated resistive heating tracks located close to the reaction surface, resulting in hotter regions directly above heater tracks and colder regions above the gaps between heater tracks. Temperature non-uniformity on the reaction surface is undesirable as it may reduce the efficiency and specificity of the PCR amplification. Therefore an objective of the invention is to increase the temperature uniformity and references to improved temperature uniformity and increased temperature uniformity are equivalent.

Temperature non-uniformity at the reaction surface can be reduced by using narrower tracks and gaps, but this complicates fabrication using standard printed circuit board techniques. Temperature non-uniformity at the reaction surface can also be reduced by increasing the distance between the heater tracks and the reaction surface, but this increases the thermal diffusion time from heater to reaction surface and slows the heater response.

Temperature non-uniformity also results from edge effects, where the temperature of the heater is reduced at the edges due to lateral heat flow. In prior art, edge effects are reduced by design of heater track patterns with increased heat output near the edge of the heater, for example by reducing the track and gap widths of the heater element in these areas. However this approach needs to be carefully designed for a particular operating temperature and reaction surface geometry and thermal load, and may be difficult to achieve if the heater track and gap widths are already close to the minimum values practical for standard fabrication processes. It is also desirable to minimise the heater track and gap widths in the central area of the heater to minimise temperature non-uniformity and therefore it is difficult to further reduce the heater track and gap widths near the edges of the heater.

In order to allow rapid cooling when the heater power is reduced, a heater may be connected to a heat sink via a controlled thermal resistance. However the heater temperature uniformity will depend on the uniformity of the thermal contact between the heater and heat sink. In particular, any air gaps between the heater and heat sink can introduce substantial thermal resistance and temperature non-uniformity.

<CIT> describes methods and devices for control of an integrated thin-film device with a plurality of microfluidic channels. In one embodiment, a microfluidic device is provided that includes a microfluidic chip having a plurality of microfluidic channels and a plurality of multiplexed heater electrodes, wherein the heater electrodes are part of a multiplex circuit including a common lead connecting the heater electrodes to a power supply, each of the heater electrodes being associated with one of the microfluidic channels. The microfluidic device also includes a control system configured to regulate power applied to each heater electrode by varying a duty cycle, the control system being further configured to determine the temperature of each heater electrode by determining the resistance of each heater electrode.

<CIT> describes methods and systems for thermal control of a device having (i) a heated zone including two or more resistive sensors and (ii) a common electrode connected to each of the two or more resistive sensors. The two or more resistive sensors may be driven with heater control signals having alternating polarities. One or more portions of a thermal boundary of the heated zone may be heated by one or more thermal guard heaters.

<CIT> describes methods and systems using thermal systems including heat spreading devices, including interconnection methods and materials developed to connect heat spreaders to microfluidic devices. Also described are methods and systems for controlling, measuring, and calibrating the thermal systems.

<CIT> describes a printed circuit structure containing a fluidic chamber configured to receive an aqueous solution containing a sample to be analyzed and fluorophore for polymerase chain reaction analysis. The printed circuit structure also contains a heating element that provides for temperature cycling of the fluidic chamber to support polymerase chain reaction analysis. In one example, a heat smoothing layer is placed between the heating element and the chamber.

<CIT> (re-published as <CIT>) describes temperature control in a rectangular array of reaction vessels such as a thermal cycler such as is used for PCR procedures which is achieved by use of a temperature block that is in contact with a combination of Peltier effect thermoelectric modules and wire heating elements embedded along the edges of the block. The elements can be energized in such a manner as to achieve a constant temperature throughout the array or a temperature gradient. Further control over the temperature and prevention of condensation in the individual reaction vessels is achieved by the use of a glass (or other transparent material) plate positioned above the vessels, with an electrically conductive coating on the upper surface of the glass plate to provide resistance heating.

In view of the above problems and objectives, the present invention provides a heater for thermocycling to carry out PCR amplification, as set out in claim <NUM>. The reaction surface heat spreader layer improves temperature uniformity at the reaction surface. In addition to improving temperature uniformity at the reaction surface, the back surface heat spreader layer improves thermal contact with any heat sink adjacent to the back surface.

Preferably, the main heater track comprises a central region comprising a plurality of substantially parallel track sections having widths Wtrack and separated by gaps of width Wgap, wherein the thickness HD of the thermal diffusion layer is less than a minimum width of the track sections Wtrack or less than a minimum gap width Wgap, where Wtrack or Wgap are evaluated in the central region of the main heater track. This means that the main heater track can be manufactured using PCB manufacturing techniques. This also means that the heater is thin enough for many applications requiring rapid temperature change.

Preferably, the gap width Wgap and/or the width of the track sections Wtrack is lower for a track section near an edge of the main heater track than for a track section in the central region of the main heater track. This increases temperature uniformity in the central region of the main heater track.

Preferably, the heater further comprises: a guard heater track between the heater track support layer and the thermal diffusion layer, the guard heater track substantially surrounding the main heater track; and two further electrical contacts to the guard heater track independent from the four-terminal electrical contacts to the main heater track. This inhibits lateral heat flow and increases temperature uniformity in the plane of the main heater track.

Preferably, the heater track support layer has a thermal resistance × area product in the range <NUM>×<NUM>-<NUM> to <NUM>×<NUM>-<NUM> K. m<NUM>/W and more preferably in the range 3x10-<NUM> to <NUM>×<NUM>-<NUM> K.

Preferably, the reaction surface heat spreader layer is more thermally conductive, has a greater lateral thermal conductivity and has a lower heat capacity than the one of the thermal diffusion layer or the heater track support layer.

Preferably, the reaction surface heat spreader layer is located within the heater track support layer at a distance Ls from the main heater track, wherein Ls is less than <NUM>% of the minimum of the heater track width Wtrack and heater gap width Wgap evaluated in the central region. This further improves temperature uniformity at the reaction surface.

Preferably, the heater further comprises a heat sink in contact with the back surface. This has the effect of reducing the temperature of the heater when the heater is not being driven.

In another aspect, the present invention provides a single use consumable comprising a heater and a reaction cell arranged in contact with the reaction surface.

In another aspect, the present invention provides a method of operating a heater or a variable temperature reactor according to the invention, comprising driving the main heater track, simultaneously sensing a resistance of the main heater track, and calculating a temperature of the main heater track based on the sensed resistance.

Preferably, the method comprises performing feedback-based driving of the main heater track according to a sequence of temperature set points for the main heater track to cycle the temperature of the reaction surface to carry out PCR amplification.

Preferably, the method further comprises driving the guard heater track to provide a higher heat output per unit area than the main heater track.

Preferably, the heater or single use consumable as previously described further comprises a control circuit configured to perform a method as previously described.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:.

We describe below an example heater suitable for carrying out thermal cycling for PCR amplification. It is desirable to carry out thermal cycling at a speed fast enough that the time required for temperature changes does not account for the majority of the total time for thermocycling. The total time for thermocycling is the sum of the time for temperature changes and the time for reactions, and the slowest part of the PCR reaction is the extension phase, which requires approximately <NUM> or more for a typical sequence length of <NUM> base pairs. Therefore we target a time of <<NUM> for temperature ramping. The target temperatures for PCR are typically between <NUM> and <NUM> so we need temperature ramp rates of <NUM>/s or more for heating and cooling to reduce the temperature change time to <NUM>. Much higher temperature ramp rates (<NUM>/s or higher) offer limited speed advantage as the total time required will be dominated by the reaction time and not the time required for temperature changes.

In one embodiment described below, a heater for carrying out fast thermal cycling has a temperature ramp rate of approximately <NUM>/s without the disadvantages of conventional heaters and temperature control systems.

The heater may, for example, be arranged together with a reaction cell in a single use consumable. The single use consumable may be supplied with the necessary reagents and power to perform a single reaction test and then disposed of.

The heater contains the following elements: main heater tracks configured to enable simultaneous heating and temperature sensing via the heater track's temperature-dependent resistance; guard heater tracks substantially surrounding the main heater tracks; a thermal diffusion layer located between the heater tracks and the reaction surface; and a heater support layer located between the heater tracks and the back surface of the heater. The heater may also be provided with a heat sink in thermal contact with the back surface to allow rapid cooling of the heater when the heater drive power is reduced.

<FIG> shows a schematic cross-section of an embodiment of the invention comprising a heater <NUM> and a heat sink <NUM>.

The heater <NUM> has a reaction surface <NUM> on one face and a back surface <NUM> on the opposite face. The reaction surface <NUM> is heated by the heater to provide a time-variable and substantially spatially-uniform temperature. The back surface <NUM> is in thermal contact with the heat sink <NUM> to allow cooling when the heater <NUM> is not driven.

In the following description, we define an axial direction to be perpendicular to the reaction surface and a lateral direction to be in the plane of the reaction surface.

The heater comprises a main heater track <NUM> for resistive heating of the reaction surface. However, it is desirable to limit lateral heat flow associated with temperature gradients and temperature non-uniformity across the reaction surface, which reduce the precision of temperature control.

In order to limit lateral heat flow within the area of the main heater track, the main heater track <NUM> is substantially surrounded by a guard heater track <NUM>. A guard heater track is an additional heater track located near the edge of a main heater track and driven to maintain a temperature close to or greater than a target temperature of the main heater track. The heat output per unit area of the guard heater track is higher than that of the main heater track to compensate for lateral heat loss. The guard heater track may be driven independently from the main heater. The main heater track <NUM> and the guard heater track <NUM> may, for example, be formed from a metal such as copper.

The main heater track <NUM> and the guard heater track <NUM> are located between a heater track support layer <NUM> and a thermal diffusion layer <NUM>. The heater track support layer <NUM> may, for example, comprise a printed circuit constructed from FR4 or polyimide or another electrically insulating support material.

A reaction surface heat spreader layer <NUM>, <NUM> is within or in contact with each of the heater track support layer <NUM> and thermal diffusion layer <NUM>. The reaction surface heat spreader layer <NUM>, <NUM> is a layer of a material with higher thermal conductivity than the thermal diffusion layer or heater track support layer. The function of the reaction surface heat spreader layers is to increase the temperature uniformity on a reaction surface <NUM>. Each of the reaction surface heat spreader layers has a thickness HS, thermal conductivity kS, density ρs, and specific heat capacity Cs, while the heater track support layer <NUM> and the thermal diffusion layer <NUM> each have respective thicknesses HB, HD thermal conductivities kB, kD, densities ρB, ρD and specific heat capacities CB, CD. In order to increase temperature uniformity while maintaining fast temperature response, the reaction surface heat spreader layer must have a greater lateral thermal conductivity and/or a lower heat capacity than the heater track support layer <NUM> / thermal diffusion layer <NUM>. For the heat spreader layer to have greater lateral thermal conductivity than the thermal diffusion layer, HS kS > HD kD. For the heat spreader layer to have a lower heat capacity than the thermal diffusion layer, HS ρS CS < HD ρD CD. For the heater support layer, these conditions are replaced by HS kS > HB kB and HS ρS CS < HB ρB CB respectively.

Each of the reaction surface heat spreader layers <NUM>, <NUM> is located near to the main heater track <NUM>. In this particular example, the reaction surface heat spreader layer <NUM> in the thermal diffusion layer <NUM> is provided at a distance of <NUM> from the upper surface of the main heater track and the reaction surface heat spreader layer <NUM> in the heater track support layer <NUM> is provided at a distance of <NUM> from the lower surface of the main heater track.

The back surface <NUM> is also provided with a back surface heat spreader <NUM> to increase temperature uniformity on the reaction surface <NUM>.

The heat sink <NUM> may take any form, including the solid block shown in <FIG> and the individual pillars shown in <FIG> and described below. The back surface heat spreader <NUM> is particularly useful when it cannot be guaranteed that the thermal contact between the heater track support layer and the heatsink is uniform.

To achieve a good thermal contact with the main heater track, a thermal resistance × area product between the main heater track and the back surface heat spreader or the heat sink should preferably be in the range <NUM>×<NUM>-<NUM> to <NUM>×<NUM>-<NUM> K. m<NUM>/W and more preferably in the range <NUM>×<NUM>-<NUM> to <NUM>×<NUM>-<NUM> K.

The heater and heat sink (if used) can have planar or curved forms. A planar form may be preferred for ease of construction and optical monitoring of a reaction for which the heater is to be used. However other forms such as part-spherical or cylindrical are possible and these may have benefits in allowing tensioned flexible reaction cell and heater layers to make good thermal contact with each other and with the heat sink which is typically a rigid metal part.

<FIG> shows two examples of schematic layouts of resistive heating tracks and electrical connections in the heater <NUM>, including the main heater track <NUM>, the guard heater track <NUM> and electrical connections to these heater tracks.

As illustrated in <FIG>, the main heater tracks <NUM> of these embodiments have serpentine configurations. Alternatively, the main heater tracks <NUM> may comprise a plurality of track sections both located and electrically connected in parallel. Similarly, as illustrated in <FIG>, the guard heater tracks <NUM> of these embodiments have a serpentine configuration. As can be seen from the examples of <FIG>, in some embodiments the guard heater track <NUM> does not completely surround the main heater track <NUM>, but substantially surrounds the main heater track <NUM> to an extent required to achieve the effect of limiting lateral heat flow within the area of the main heater track. In many embodiments, this requirement corresponds to the guard heater track <NUM> surrounding more than <NUM>% of a perimeter length of the main heater track <NUM>.

<FIG> shows a heater with uniform track and gap width in the main heater track <NUM>, while <FIG> shows main heater tracks <NUM> with larger track and gap width in the central region <NUM> and smaller heater track and gap widths near the edges of the heater <NUM>. The edge regions <NUM> provide increased heat output per unit area and also contain tracks oriented parallel to the edge of the heater in order to reduce thermal conductivity in the direction perpendicular to the edge of the heater and thereby reduce lateral heat flow and increase temperature uniformity in the central region <NUM>.

A spatially separated temperature sensor could cause a time lag between temperature changes at the main heater track and temperature changes at the temperature sensor. This time lag could cause problems such as overshoot or oscillation of the heater element temperature. To avoid these problems, the main heater track is configured as a temperature sensor where the resistance of the heater element is used to determine its temperature. A metallic heater element will usually have a positive temperature coefficient of resistance while a metal oxide or semiconductor heater element will have a negative temperature coefficient. It is desirable that the magnitude of the temperature coefficient of resistance (TCR) of the heater element is large, preferably greater than <NUM> ppm/K, and more preferably greater than <NUM>,<NUM> ppm/K.

The main heater track <NUM> has <NUM>-wire connections comprising electrical drive positive and negative connections <NUM> and <NUM>, and voltage sense Vsense positive and negative connections <NUM> and <NUM>. Measurements of Vsense can be used to accurately monitor of the track resistance using a circuit such as shown in <FIG>. In combination with a known temperature coefficient of resistance, TCR, or desired temperature setpoints, of the main heater track <NUM>, Vsense can be used to perform temperature sensing for the main heater track <NUM>. The use of <NUM>-wire connections with separate contacts for driving the main heater track and sensing a voltage across the main heater track, instead of using a conventional <NUM>-wire connection for both the driving and sensing, has the advantage of eliminating any voltage drop due to internal resistance of connections through which current is supplied to the main heater track.

The guard heater track <NUM> has positive and negative connections <NUM> and <NUM> to be driven independently from the main heater track <NUM>.

<FIG> schematically illustrates an electronic circuit driven by supply connections Vpos and Vneg, which can be used for driving the main heater track, simultaneously sensing a resistance of the main heater track, and calculating a temperature of the main heater track based on the sensed resistance. Such a control circuit may be included with the heater <NUM> or could be connected when the heater is in use. Referring to <FIG>, current flows through the heater track <NUM> via a positive drive connection <NUM> and a negative drive connection <NUM>. The heater track is provided with <NUM>-wire contacts to allow the voltage Vsense across the heater track to be measured using positive voltage sense contact <NUM> and negative voltage sense contact <NUM> and a voltage measuring circuit <NUM>. The current flowing through the heater track <NUM> is measured using a current sense resistor <NUM> with known resistance Risense and a voltage measuring circuit <NUM> for measuring the voltage Visense across the current sense resistor. The current through the heater is calculated as: Iheater = Visense / Risense. The resistance of the heater track <NUM> is then calculated as Rheater = Vsense / Iheater. Feedback-based driving of the main heater track may then be performed according to a sequence of temperature set points. Temperature control is implemented by determining setpoint values of Rheater corresponding to desired temperature setpoints and controlling the heater drive to meet the heater resistance setpoint values. Alternatively, temperature control may be performed continuously across a temperature range based on the known temperature coefficient of resistance, TCR. A switch <NUM>, which may be a transistor, is turned on to measure the heater resistance and is then turned off or left on for a predetermined time interval depending on whether Rheater is above or below a currently required setpoint resistance. Alternatively, the switch <NUM> may driven with a pulse width modulated waveform with duty cycle selected to drive the heater with the required power. In both approaches the switch <NUM> is used to modulate the electrical drive to the main heater track to cycle the temperature of the reaction surface to carry out PCR amplification.

The guard heater track may be operated with closed loop control with a temperature setpoint equal to or greater than the temperature setpoint of the main heater track or the guard heater track may be operated with the same controller or on/off timing as the main heater element but with a different drive voltage which can be adjusted to optimise the temperature uniformity at a specific temperature setpoint.

Referring back to <FIG>, sections A and B along longitudinal and transverse directions of this example configuration were simulated to determine temperature distributions as shown in <FIG>. The results of these simulations illustrate the increased temperature uniformity obtained by using guard heaters.

Turning to <FIG>, the results of simulations of the temperature distributions on the reaction surface were obtained from the centre of a rectangular heater area to the edge in a longitudinal direction (A) and a transverse direction (B). In each Figure, temperature is shown on the vertical axis and position along the longitudinal/transverse direction from the centre is shown on the horizontal axis. Temperature distributions are shown without guard heater (solid lines) and with guard heater (dashed lines), showing more uniform temperature distribution when guard heaters are used. The locations of the main heater and guard heater are indicated on each of <FIG>.

<FIG> shows another schematic cross-section through the heater <NUM> and the heatsink <NUM>. As illustrated in <FIG>, the main heater track <NUM> comprises a plurality of substantially parallel track sections having a width Wtrack spaced apart by gaps of width Wgap. The track sections need not be precisely parallel, so long as it is possible to define the gap width Wgap. The heat output from the main heater track <NUM> is non-uniform due to the finite width of the tracks and gaps. This is exacerbated by the need to make a thickness HD of the thermal diffusion layer small in order to achieve rapid temperature changes. In this embodiment, the thickness HD of the thermal diffusion layer is less than a minimum width of the track sections Wtrack or less than a minimum gap width Wgap. Narrower track and gap widths will increase temperature uniformity at the reaction surface, but this is limited by typical design rules, such as the requirements of PCB manufacturing techniques.

<FIG> also indicates a simulation region C for which the heater and heatsink were simulated. <FIG> show results of simulations of the temperature along the reaction surface within the simulation region C. The simulations assumed a heater with copper tracks where Wtrack and Wgap are constant at <NUM>, and further assumed that the heater track support layer <NUM> is made of FR4 and the thermal diffusion layer <NUM> is made of polypropylene. The effect of increasing a heat spreader layer thickness is shown for two cases where, in each Figure, temperature on the reaction surface is shown on the vertical axis and position along the reaction surface within simulation region C, starting from the centre of the heater track portion, is shown on the horizontal axis. <FIG> shows the results of simulations where a reaction surface heat spreader layer <NUM> made of aluminium is located within the thermal diffusion layer, between the heater tracks and the reaction surface <NUM>, at a distance of <NUM> from the heater tracks, and reaction surface heat spreader layer <NUM> is omitted (configuration A). <FIG> shows the results of simulations where a reaction surface heat spreader layer <NUM> made of aluminium is located within the heater track support layer, between the heater tracks and the back surface <NUM> of the heater, at a distance of <NUM> from the heater tracks, and reaction surface heat spreader layer <NUM> is omitted (configuration B). In both cases the heat spreader layer increases temperature uniformity, with thicker heat spreader layers being more effective, and configuration A is more effective than configuration B.

<FIG> show simulation results in which the location of a reaction surface heat spreader layer <NUM> is varied. In <FIG>, the reaction surface heat spreader is located in the thermal diffusion layer, and the distances shown in the legend of the graph in <FIG> show the separation of the upper surface of the heater tracks and the heat spreader layer. In <FIG>, the heat spreader is located in the heater support layer and the distances shown in the legend of the graph in <FIG> show the separation of the lower surface of the heater tracks and the heat spreader layer. In both cases, the heat spreader is made of aluminium and has thickness <NUM>. The reaction surface heat spreader position has little impact on temperature uniformity when the heat spreader is located within the thermal diffusion layer (<FIG>). However when the reaction surface heat spreader is located in the heater support layer, the reaction surface heat spreader is preferably located within <NUM> of the heater in order to provide a substantial improvement in temperature uniformity (<FIG>). This distance scales with the track and gap width and corresponds to <NUM>% of the minimum track and gap width as evaluated in the central region.

In <FIG>, the heater <NUM> includes a back surface heat spreader <NUM>. While this feature is not required in all embodiments of the invention, the back surface heat spreader <NUM> has the advantage of further improving temperature uniformity at the reaction surface <NUM> as demonstrated using simulations. <FIG> show simulation results comparing heaters without (<FIG>) and with (<FIG>) a back surface heat spreader <NUM>. In each Figure, the upper plot shows temperature contours from <NUM> to <NUM> on a simulated heater. The simulated heater includes heater tracks shown as a dashed line, where shorter dashes indicate the main heater track <NUM> and longer dashes indicate the guard heater track <NUM>. Above the heater tracks, a reaction cell <NUM> is surrounded by the thermal diffusion layer <NUM> having the reaction surface <NUM>, such that a temperature of the contents of the reaction cell can be controlled according to the temperature of the reaction surface. Additionally, in each Figure, the lower plot shows the temperature profile along the reaction surface (solid line, "A" in legend), in a plane cutting through the heater tracks ("B" in legend) and on the back surface of the heater ("C" in legend). The simulation of <FIG> assumes that the back surface heat spreader is constructed from a <NUM> thick layer of copper. In both simulations, a heat sink <NUM> with non-uniform thermal contact is represented by a set of three aluminium pillars, width <NUM> and height <NUM>. The geometry and results are shown for a 2D half-model, with a symmetry plane at position x=<NUM>. In both cases the heater set-point temperature is <NUM>.

<FIG> shows simulation of the transient response of a heater as described above with a back surface heat spreader, thermocycling with <NUM> cycle time using temperature setpoints of <NUM>, <NUM> and <NUM>. The temperature of the main heater track is shown in trace A (dashed line) and the temperature at the centre of the reaction surface is shown in trace B (solid line).

As an example, a heater according to the invention may be used for providing heat to a reaction. In such a usage, the reaction surface of the heater is located in contact with a reaction cell having a reaction volume containing a sample. In order to heat the reaction surface, the heater element is switched on, and heat generated by the heater element flows through the reaction surface into the reaction volume. If rapid cooling is required, the heater can contact a heat sink on its back surface so that when the heater element is switched off, heat flows from the reaction surface, through the heater and into the heat sink.

When the heater is applied for thermal cycling, such as for driving PCR reactions, it is advantageous for the thermal diffusion time between the heater and the sample to be small compared with the target cycle time. In general, thermal diffusion time t for a material sample is given by: <MAT> where L is a characteristic length scale of the material sample and D is the thermal diffusivity of the material. Table <NUM> below shows an example choice of materials for a heater according to the invention, in which the thermal diffusion time of the thermal diffusion layer is less than the reaction time for PCR, which we take as approximately <NUM> for amplification of a <NUM> base pair DNA sequence.

Additionally, the thermal resistance of the heater track support layer RT can be optimised to minimise the thermal cycling time for a given temperature profile and heatsink temperature TSink and heater power pHeat. The time required for thermal cycling between a TLOW and THIGH is minimised when the heating time is equal to the cooling time and this condition is satisfied when RT = RT,Opt as follows: <MAT>.

Table <NUM> shows example values for heater power, optimal thermal resistance and thermal cycle time. These are shown for the case of a reaction surface with area <NUM><NUM> and with heat capacity <NUM> J/K, cycling between <NUM> and <NUM> with a heat sink temperature of <NUM>.

<FIG> shows an alternative schematic layout of resistive heating tracks in a heater <NUM> according to the invention. In this embodiment, two main heater tracks <NUM> are arranged next to each other in order to heat individual respective areas of a reaction surface <NUM>. The main heater tracks are both surrounded and separated by a guard heater track <NUM>.

The embodiment of <FIG> illustrates how a heater according to the invention may be provided with a main heater track for each of a plurality of individually temperature-controlled areas of a reaction surface. The guard heater track <NUM> inhibits lateral heat flow and thereby increases the accuracy with which each of the individual areas of the reaction surface may be temperature-controlled.

As shown in <FIG>, the guard heater track <NUM> has three connections <NUM>, <NUM> and <NUM> such that the current and heat output per unit area may be different between and around the two main heater tracks <NUM>. Alternatively, each main heater track <NUM> may be provided with a separate guard heater track <NUM> with two connections.

In the above-described embodiments, the heater is provided in an assembly with a heat sink. However, the invention is also applicable in cases where uniform heating is required, but a heat sink is not required. For example, the heat sink may be omitted for applications where a cooling time is less important.

In the above-described embodiments, the heater is provided with a guard heater track <NUM>. However, in addition to or instead of providing the guard heater track <NUM>, the main heater track <NUM> may be designed to have higher heat output near its edges and to extend beyond the reaction volume. This higher heat output effect may be achieved by increasing the density of the heater track by reducing the gap width between two or more heater track portions near the edge of the main heater track as compared to heater track portions at the centre of the main heater track. Additionally or alternatively, this effect may be achieved by increasing the resistance of the main heater track by reducing the width or height of one or more heater track portions near the edge of the main heater track as compared to heater track portions at the centre of the main heater track. The higher heat output of the heater element near its edges can compensate for lateral heat flow and provide more uniform temperature conditions across the reaction volume. Furthermore, where a heater has a reaction surface that extends significantly beyond the required reaction volume, it is possible to omit both of the guard heater track and the modifications near the edge of the main heater track.

Additionally, in the above description, comparisons are evaluated between including and omitting each of the heat spreaders.

Claim 1:
A heater (<NUM>) for thermocycling to carry out PCR amplification, the heater comprising:
a thermal diffusion layer (<NUM>) having a reaction surface (<NUM>) for transferring heat to a reaction cell;
a heater track support layer (<NUM>) having a back surface (<NUM>) for cooling;
an electrically conductive main heater track (<NUM>) supported between the heater track support layer and the thermal diffusion layer; and
four-terminal electrical contacts (<NUM>, <NUM>, <NUM>, <NUM>) to the main heater track adapted to provide electrical connection for driving the main heater track and simultaneously sensing a resistance of the main heater track,
wherein the lateral dimensions of the reaction surface are greater than a thickness H of the heater, such that reaction surface area A > H<NUM>,
the heater further comprising a reaction surface heat spreader layer (<NUM>, <NUM>) located in contact with or within one of the thermal diffusion layer (<NUM>) or the heater track support layer (<NUM>),
wherein a back surface heat spreader layer is located on the back surface.