This is the second hardness sensor i made. The two reasons that pushed me to do an HS2 is that the first one was too expensive, even if better, and it was too big. As the first version, this sensor has the capability of an internal wire connection, but, useless to say, you have to choose IR transmitter and receiver small enough. You could use SFH 3100 F as photodiode and GL4800E0000F as emitter phototransistor. The material should be plastic, as flexible as it could be. WSF could be ok, i'll now when it will arrive.