For some years a number of researchers, such as Robert Freitas, have modeled and proposed designs for nanomechanical robots capable of interacting with and repairing cells or replacing some of the tasks of cells so as to conduct those tasks far more efficiently. This is early groundwork in a field that has yet to exist beyond concepts and models: the actual construction of such things still lies in the future. On this topic Frank Boehm, the author of a comparatively recent book on medical nanorobot design, pointed me in the direction of an interesting piece on the design of medical nanorobots to remove lipofuscin from cells, which is quoted below.
Lipofuscin is made up of metabolic wastes that cells cannot break down, and it clogs up the cellular recycling system, leading to a deterioration of cellular function as damage builds up. Where it happens in long-lived cells this process contributes to a range of age-related conditions. Liposfuscin removal will be a going concern in the years ahead if the SENS Research Foundation does well with its research programs and in persuading other organizations to join in, but the near future of this project won't involve nanorobotics. It will be a matter of adapting bacterial enzymes that are both safe to introduce to the body and highly effective at breaking down the various compounds that make up lipofuscin. Still, we should look farther ahead as well, as mechanical medical nanomachinery will almost certainly be plausible to manufacture and control effectively twenty to thirty years from now.
It is conceivable that the future development of nanomedical robotics [might] enable the capacity for the therapeutic removal of lipofuscin from individual cells in massively parallel fashion. Conceptual dedicated autonomous nanodevices (~200 nm in diameter - where one nanometer is a billionth of a meter) might penetrate the cell membranes of neurons and other cells and undertake the removal of lipofuscin through various means.
Advanced autonomous nanodevices might precisely locate lipofuscin granules by exploiting its strong fluorescence signatures [to] match with onboard reference spectral profiles. The prospective armamentarium at the disposal of these autonomous diamondoid "defuscin" class nanodevices [might] allow for the complete eradication of lipofuscin aggregates utilizing a feedthrough digestive strategy. These entities may be propelled by arrays of oscillating piezoelectric "fins" or via integrated magnetic nanoparticles, which might be activated and controlled externally. The conical inlet port of the nanodevice would be lined with molecules that possess high affinities for A2E [a primary lipofuscin constituent] and other lipofuscin elements.
Once a lipofuscin granule has been captured it would proceed to be drawn into the core, where it would be digested by potent encapsulated enzymes or nanomechanically minced into a liquid state and subsequently purged from the outlet port. This functionality would be similar to Freitas's microbivore artificial mechanical phagocytes, which operate under a "digest and discharge" protocol"