Flats in close succession where caused by a small glass shard that had penetrated the threads. I removed the shard and got a second flat, 9kms later. No shard this time, but perhaps gains of sand. I've glued a boot inside the tire. Got a third flat 4kms later. I was close to a Decathlon by then, and purchased a new tire and butyl tubes. No flat 2500kms later.

The tire was a 3-year old (10 000 kms) Almotion. Plenty of threads left, but the rubber looked past its prime with several tiny cuts.

BUT the flat on the rear wheel did it for me. Here again, the tire is not new, but the flat occurred after I had to remove the wheel - the tire is too wide to slide between the brake pads unless deflated. Post inflation there was a slow leak. Replaced with butyl. No issue since.

So... I'd think that inside a fresh, undamaged, tire, TPU will work. Unfortunately, glass shards and other detritus will eventually penetrate the skin. Butyl, being thicker, is more tolerant. TPU is not.

I remember 6 flats with TPU tubes over the past 3 years. None with butyl.
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