I assume because the prosthetic limbs require a good deal more function, mobility, and maintenance than the average doll. Not to mention having them designed to be "aesthetic" would likely up the cost
More or less. The joints and structure of a sex doll don’t have to be able to handle the routine forces a human body sees all day every day. What’s more is maintenance. Sex dolls require careful and regular upkeep with use. The artificial skin used on such reploids will tear and crease with constant use. A sex doll might take months or years to see the same number of repetitive movements as a single human limb.
While more durable materials can be used- weight is a factor in the comfort and performance of synthetic limbs as well- and many are designed with advantages a normal human limb doesn’t have for things like running or climbing. The fact that you don’t have fine motor control or the direct ability to manipulate the joints of a prosthetic limb also mean that designing it exactly like a human limb in mechanics and function would leave you with a non practical replacement.
Consider that if a perfect reproduction of a human leg were designed and used as a prosthetic- it would be no different than having a natural leg which was paralyzed as far as being useful to walk and move or support you standing.
A prosthetic limb has to take into account budget of course, what you plan to use it for, how rugged it must be- and what the Precise nature of your injury and remaining muscles and function are. In a below the knee amputee- how much of the knee mechanism is left and functional? What is the damage or degeneration of muscles in the upper leg and pelvis? Etc.
A person who will be constantly wading through salt water for instance would quickly ruin prosthetic skin that is “real” style. A rock climber or mechanic would scratch up any pseudo realistic finish. Unlike real skin any friction from even clothes rubbing will wear in marks that won’t just be regenerated. It’s also quite a feat to make a functional appendage that can pass as natural- and many amputees don’t want to “hide” their status. There’s also an aesthetic component like “body modification” similar to tattoos and piercings and the like. If a person has to replace their limb anyway- many would rather have something different or to a certain aesthetic they find interesting.
That's something I forgot to talk about. As you said guest_, it becomes much more difficult to find a material suitable in most conditions. Sex dolls will mostly be kept at room temperature in a relatively benign environment. Robots it may very (such as with the Mars rover), but they can also endure conditions that having a human body attached would make impossible.
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Metal, for example, is more durable, but heavier, and can cause any number of issues from frostbite to burns if in direct contact with skin. Not to mention it could very well just freeze or rust, which would limit functionality for the person using them. Other materials might fair better, but be more expensive and less durable, and come with issues of their own.
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I imagine in winter if a limb gets frozen or such it would be much more difficult to deal with the issue if you have to peel open a skin casing. And then presumably you have to repair the casing somehow or possibly buy an entirely new one
Very true. “Fake skin” tends to need special care like dusting with powder and it doesn’t do well in cold at all. Most prosthetics have some type of collar or harness to not only secure it, but help cushion the body part it attaches to from the movement and for e of the hard prosthetic as well as to help prevent irritation. But things like allergies and sensitivities to various materials also factor in, and honestly bones are quite strong. It takes about 30x human weight to break a femur. Bone is stronger than steel- and unlike most materials it heals wear and damage over time. So a functional load bearing prosthetic must be very durable for all purpose use.
Upkeep is really important too. It’s one of the things that made machines like cars and computers beyond the average person even when they became affordable- and what stops flying cars. It’s hard enough to get people to do basic maintenance on a car they use daily and rely on for mobility. But most people just don’t want to or have the personality to do regular upkeep on machines. One of the keys to design of consumer products is in making upkeep as minimal as possible or simply designing things that are replaced instead of serviced.
I was going to make a joke about being depressed and already having that, but decided not to, so I talked about the joke, thus making it while acting like I didn't
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Metal, for example, is more durable, but heavier, and can cause any number of issues from frostbite to burns if in direct contact with skin. Not to mention it could very well just freeze or rust, which would limit functionality for the person using them. Other materials might fair better, but be more expensive and less durable, and come with issues of their own.
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I imagine in winter if a limb gets frozen or such it would be much more difficult to deal with the issue if you have to peel open a skin casing. And then presumably you have to repair the casing somehow or possibly buy an entirely new one