Nanotechnology in ophthalmology

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An increased level of sophistication would allow external control of the device by, for example, light-modulated control signal. This level of control could stop and start a device and alter the drug delivery rate. In a development of this communication function, it would also be possible to interrogate the device by activating a 'talkback' mode, where data could be read back from the device using optically coded techniques.

Encoded data could include, for example, details of the equivalent volume of the drug dispensed and the status of the power source. Modern electronics is certainly more than capable of providing this level of functionality. A key element of the design of such a system would be the resilience of the software.

Ideally, power sources should be able to power the application without the requirement to top-up or replace the source. Materials technology used for battery function require high energy/volume factors. Energy transfer into energy source using microwaves and electromagnetic induction are also possible. Such systems have already been developed for larger implant systems by Mussivand et al.7 High energy density rechargeable batteries such as IBM 08K8192 Battery, IBM 92P1101 Battery, IBM 92P1089 Battery, IBM 92P1087 Battery, IBM 08K8196 Battery, IBM 92P1102 Battery, IBM 92P1077 Battery, IBM 92P1073 Battery, IBM 08K8199 Battery, IBM 08K8198 Battery, integrated into silicon circuits for biomedical implantation applications, are also described by Notten et al.8 A range of technologies have been identified as having potential for miniature pump delivery systems. In particular, MEMS technology9 is identified as a viable approach. The core element of such a pump-driving system is indicated in Figure 2, where alternate voltage pulse (positive and negative) causes deformation of membrane in opposite senses.

On the inlet phase, fluid is drawn into the pump chamber with outlet value closed. On the outlet phase, fluid is driven into the outlet channel with the inlet channel closed. Such pump systems tend to have an optimum frequency for drug delivery rates. A key design element is to minimise the operating stress on the diaphragm material to prevent its failure.

Advanced design techniques of extensive numerical simulation are used to optimise design configurations. Electro wetting, electrochemical and ion conductive polymer film (ICPF) actuator micro pumps are further described by Nisar et al.10 Provided sufficient energy and also sufficient volume of drug is available, existing micro pump technology can deliver this function.

Drug reservoir options In association with the capacity of the power source, the issue of volume of drug reservoir is another key issue and, if the issue of energy transfer to micropumps can be achieved, is the key limiting factor of the technology.

In terms of Lucentis administration, the initial loading regime is 0.05ml in the first three months and then 0.05ml administrations at monthly intervals if required. In the first year a typical dose could be 0.6ml per year. It is also relevant to note that the mode of drug administration is intermittent and not continuous within the conventional administration pattern.

While further research on drug diffusion is required, it may only be required to deliver an initial loading dose of 0.05ml, followed by daily equivalent doses of around 0.0015ml over a year. In this mode, the pump device is essentially dormant and is activated for a short time for each daily administration. The dominant power requirements of the device may therefore be for a hibernation mode in the monitoring electronics.

The volume of the micro pump system, however, needs to be considered in relation to the total volume of the eye, which is typically in the range 7.00ml to 8.00ml. Also, the emptying of 1ml of fluid into the eye requires the filling with an equivalent volume of displaced fluid. Where would this come from? Also, the stability of the injected drug is another relevant issue. Lucentis has a short active half life and may be an unsuitable choice for micro pump delivery, since the drug would presumably have to retain efficacy over a period of at least a year.

The technology of micro pumps does offer, in their time line of development, future exciting capabilities. Drugs from separate reservoirs can be delivered as a 'cocktail' - with the proportion of drug mix as well as separate drug rates part of the selectable options.