Sinclair's microdrive -- was it based on compact cassette technologies?

The tape was actually the same quality as used for VCR tape - according to the QL Service manual "high-quality video tape" was used.

Other than the rotating heads in VCRs, however, Sinclair Microdrives use standard, two-track heads, apparently from some dictaphones.

It's unlikely they deeply looked into compact cassette technology, given all these differences, like Continuous loop tape, which first showed up in the Exatron Stringy Floppy and a tape using different coating and tape width.

(No definitive answer, just some thoughts)

Tape size isn't a major indicator as custom size manufacturing isn't a big deal. Already the Exaton Stringy Floppy of 1978, which can be seen as predecessor, did use a 1.6 mm tape. Similar BSR's wafer drive, used in the Rotronics Wafadrive or the Quick Data Drive for Commodore computers (*1), which used, IIRC, as a 1.8 mm tape

I wouldn't wonder if Sinclair simply had ordered it's own tape type, as the cassettes had to be manufactured special to type anyway, but used an existing head. So I guess the best lead would be to follow the head used to see what it was originally intended for.

Such integrated heads (erase and write) were common for voice recorders were the lower quality could be tolerated. These devices often used mini/micro cassettes (standard tape width). An attempt to use a smaller tape was the Dictaphone Picocassette which came about the same time as Sinclair's microdrives and uses, IIRC, a 1.9 mm wide tape. Sony tried it again (way later, ca. 1990) with the NTC using a 2.5 mm and digital recording.

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*1 - A quite advanced beast as it connected to the cassette port and booted its Kernal extension like loading from cassette. Seriously thoughtful made, worth to spend some time to understand its details.