Syringes can double as hydraulic cylinders! This is a mockup using cardboard as a proof of concept.
One potential problem I’m hoping to keep under control is the weight of the arm. Having multiple motors, especially if they are made of metal, will make this a very heavy device, which means it will struggle to move naturally. Instead, I’m hoping to use syringes to replicate a hydraulic cylinder that can use tubing to have them controlled remotely by motors fastened to the base.
As an additional benefit, the water has a small amount of compressibility which offers an easing in and out of motion that can help in making the arm move more naturally. I found that the best syringes are 3ml ones which provide the least amount of resistance.