A NAKED sex doll rests on the dining room table, its limbs cold and rigid, as a man thrusts a finger deep into its mouth, testing how it responds.

As he stares into the moaning doll's glassy blue eyes, his wife looks down at the floor.

But he knows what he's doing; this is Sergei Santos, the sex robot designer who bills himself as the "Robin Hood of sex" because of his efforts to bring affordable, AI sex dolls to everyone.

Together with his wife, Maritza, he creates lifelike, bespoke sex robots in his Barcelona home, with Sergei focused on coding the bot's mind and his wife taking care of the intricate sensors which bring the droid to life.

The pair are the team behind Samantha, an amorous android designed to look and feel like a real woman - priced at £2,500 each.

Each week, hundreds of bots are produced all around the world, including one or two Samantha bots, joining the legions of amorous androids already walking (well, lying) among us.

The bots are the subject of a new BBC3 documentary, Sex Robots and Us, which looks at how these in-demand love droids are leaving their mark on the world.

James Young, a "cyborg" who has been obsessed with technology ever since he replaced a lost limb with a bionic arm, presents the show, delving deep into the world of sex robots to find out where the futuristic tech is heading.

He says: "The development of the sex robot industry seems unstoppable."

And some people have started getting seriously twitchy about where the controversial new tech is taking us.

The sinister side of sex bots

In the doc, Noel Sharkey, from the Foundation for Responsible Robotics, says: "We're just doing all this stuff with machines because we can, and not really thinking how this could change humanity completely.

"Some people have suggested that sex robots create an attitude of 'too-easy' sex which is always available.

"This could take meaning out of our lives and turn us into zombies," he says - a sentiment realised in sex doll brothels where paying customers are free to do whatever they like to passive dolls without even needing to ask.

Noel has also has previously warned that child sex robots are the inevitable consequence of this sexual revolution, calling for a government ban on the sale of sick dolls modelled after kids.

From brothels to care homes

As sex bot production ramps up, sellers are looking for increasingly innovative ways to make money from the droids.

Arran Squire, Sergei's Liverpool-based business partner, has started stocking Samantha bots in his sex shop, and he has a bold plan to tap into a potential new market: old people's homes.

"Residents could have one in their room," he suggests.

"It's a possibility that they could share the dolls. They're completely hygienic - everything is removable so it can all be cleaned.

"But I'm hoping that they could have their own Samantha. If people had a companion and a sex aid in a Samantha, that would take massive pressure off carers and nurses.

"I think people's attitudes will improve towards it. I'm hoping so anyway."

Away from the pastel walls of British care homes, the brothel business has already integrated sex dolls seamlessly into their offering.

Sex doll brothels around the world (including one in London), where many of Sergei's creations have ended up, are where the experts' warnings about "too-easy" sex are most relevant.

The documentary visits one of these brothels in Barcelona, where stripped-back versions of the dolls, without their AI components, are available alongside actual human women.

But as realistic as the dolls are designed to be, without the circuitry which brings them to life, they are more like rubbery shells than believable beings.

And the wacky dimensions some are made with (back-breakingly huge breasts and unfathomably long legs) only add to the strangeness.

For those going in for a session with a doll, there are two rules: pay up a deposit beforehand and don't let the doll touch the floor - for its own "protection".

Unlike the real women working in the brothel, "the dolls can't say no," the brothel owner says, so nothing is off-limits.

Robot wars

Back at Sergei and Maritza's Barcelona home, the couple are getting ready to dispatch one of their dolls, this time for an Australian client.

This Samantha is destined to end up in a brothel, where Sergei is a little concerned that rough overuse could end up breaking his creation.

But with demand for sex robots sky high, business is booming for the technological gods creating artificially-intelligent life.

Computer whizz Sergei gets so many requests from prospective customers that he can't possibly keep pace, and is currently in the process of expanding his business.

"I'm outsourcing the manufacturing and the selling," he says. "In a factory I could easily do 200 a month."

Sergei also has his own model which he keeps at home, and which he has programmed so it knows how long he lasts.

And he insists his droids are so realistic that even his mother isn't bothered by them.

"I'm dynamic. I move a lot, I go to the gym. I do sport," he says, explaining why he relies on his products as well as his wife to fulfil his needs.

"I could have sex three or four times a day."

Sergei finishes his final tests by talking dirty to the naked robot, making sure its responses fit the situation.

"It's just part of the job," Maritza says . "If anything I'm just hoping the sensors work, because I put them there."

But a bizarre technological malfunction crops up at the last minute, threatening to delay the doll's delivery.

Samantha's vagina sensor has become jammed, even without anything to stimulate it, so she's not responding properly to Sergei's checks.

As she tinkers with the body sensors, Maritza explains why she's not fazed by criticism of her sex robots.

She says: "We've had a lot of feminists contacting Sergei who want to interview him from a feminist point of view.

"What I've found is that most of them avoid talking to me.

"They don't want to talk to me because I'm a woman working on the project. And that goes against their argument that this is something by men and for men."

Sex Robots and Us will be available on BBC3 from 10am on 8 April.