Samsung promised us a foldable phone -- rumored to be called the Galaxy X, Galaxy F, Galaxy Flex or Galaxy Fold -- in the first half of 2019. Turning the concept into a real, working device that people actually want to use presents a host of challenges, from design to getting top app-makers on board.

With the Galaxy S10's unveiling set for Feb. 20, Samsung could show off a fully functioning foldable phone, the Wall Street Journal reports. Samsung's foldable phone is said to have a 4.5-inch screen on the outside and a 7.3-inch screen inside when you open it up like a book. It'll run Android 9 Pie OS with Samsung's new One UI on top, and you'll be able to multitask in up to three panels on the larger screen.

Foldable phone designs that can open into a tablet to become a true two-in-one device have the potential to shake up a stale industry that's seeing a slowdown in sales and innovation. As the largest phone-maker on the planet, Samsung wants to shine in this space, although failure means suffering a financial hit and the very public loss of stature for a brand that's losing its footing as an industry innovator.

Samsung would do well to learn from recent mistakes in foldable phone design while setting its plan in motion. Getting a foldable phone design right may prove more important than getting it first. Here are the biggest challenges that Samsung -- and other device-makers -- face when making phones that fold and bend.

Samsung did not respond to a request for comment.

1. Portable enough to carry around

We only got a glimpse of the prototype foldable phone back in November, so it isn't clear how thick the device will be. Even if we had seen it up close, Samsung warned at the time that the external casing was basically a husk to house the internal parts -- the final design will look much different. 

But a foldable phone will be thicker than your average handset by definition. It has to be, considering that you're doubling up two screens when it's in "closed" mode.

The trick will be to make the device thin enough over all so that the Galaxy X/Galaxy F isn't busting through your pocket, or will require its own satchel just to carry it around. If a foldable phone is too big for people to use naturally, it'll quickly lose its value. Having used the foldable Royole FlexPai and the hinged, dual-screen ZTE Axon M, we've seen the pitfalls.

"In many ways, Royole has done Samsung a favour by setting the bar so low," said Ben Wood, research chief at analyst firm CSS Insight. "How Samsung communicates the benefits of such a product will be key to its future success."

2. Make it fold flat

Part of the portability challenge is working out exactly which shape a foldable phone will take when it's closed. Don't expect it to fold flat.

Just look at the FlexPai and Microsoft Surface Book 2. Both have a flexible central hinge or seam that bends the flat sides closed. The result is a loop you can fit a pen into. 

The only reason that the ZTE Axon M closes flat is because it's made up of two completely separate screens that can stack on top of each other with a hinge. A truly foldable phone, however, aims to be one uninterrupted panel that bends inward or outward on itself.

There may be no way today to overcome the huge engineering challenge of building a foldable device that folds flat, not until materials are perfected and the manufacturing process go through several iterations to shrink that air gap more each time.

3. A premium, not toy-like, feel

Nobody wants to buy a device that feels slower, older or less potent than a standard flagship phone that costs less. The foldable phone's camera and processor have to be just as good as the standard Galaxy S10's to take great photos and control amped-up multitasking on the phone's two screens. This was a major problem with the ZTE Axon M, which used older components to keep costs in check.

There's also the question of how nice a plastic screen can feel. Samsung's "Infinity Flex Display" for the Galaxy X/Galaxy F's 7.3-inch interior screen will be made of a plastic material, as is the FlexPai's. The plastic screens of yore felt awful. Here's hoping that Samsung's for the foldable phone is still pleasant to use.

Corning is developing a bendable glass thin enough to one day cover foldable devices, CNET was first to report. Until then, Samsung will have to rely on detailed build quality and lush finishes to make its foldable phone feel upscale.

"Consumers want more real estate in a smaller package and have expressed that through their buying patterns," said Stephen Baker, VP of industry analysis at NPD Group. "This will be the best way to deliver on that consumer demand but it is likely a multiyear process before pricing, software, apps and the product itself have the kinks worked out." 

4. It's got to be balanced

I can't stress this one enough. The ZTE Axon M lodged the battery on one side, which made the phone feel freakishly off-balanced and hard to use. It's the same problem that tablets have when you use them in keyboard mode -- unlike a laptop, with its electronics under the keyboard, tablets store everything behind the screen. That makes them feel like they're always about to tip over.

Samsung could add counterweights or multiple batteries, perhaps, but it should take care not to make the phone too heavy to comfortably carry (see No. 1 in this list). I think buyers will accept a uniformly heavy device, but they'll hate an imbalanced one.

5. A strong, subtle seam

A foldable phone's flexion point is the "seam" that runs down the middle of the display. This is where the bending happens, and it's really important that it's strong. Royole says the FlexPai's seam will last over 200,000 flexes in the phone's lifetime -- that's the equivalent of opening or closing the phone 548 times a day for a year or 274 times a day for two years.

The thickness of that seam is also important, since a band down the middle of the display is going to keep that 7.3-inch screen from being one uninterrupted panel like you have on a true tablet. The ZTE Axon M's fat bezels got in the way when watching video and trying to play even casual games in full-screen mode.

Will Samsung's seam be smooth and easy to open every time? Will it ever stick? These and other durability question will undergo some of the most intense scrutiny, especially later on in the phone's lifetime.

6. No awkward cameras, please

One of the more awkward Axon M features to use, the single camera took all photos on the phone. But to shoot as you would a rear-facing camera, you had to flip it over. This configuration kept selfie quality high, but meant you missed a lot of shots when taking a photo of anything else.

We know from the Pixel 3 that you don't have to have more than one camera to take excellent low light and portrait photos, but retaining those popular features will be necessary for the foldable phone to compete.

7. Killer apps that work every time

It helps that Google has committed to supporting Android on foldable phones, paving the way for apps to smoothly transition across screens as you open and close the phone.

Getting app and game developers to modify their apps to work on a foldable phone is another beast entirely. It's one thing to make sure that the apps transition well. It's another to take advantage of the three active areas that Google says it will support on the larger screen. (Here's an example of the Flipboard app.)

"Unless the foldable device is supported by solid operating system and software support, a foldable phone risks going into history as a gimmick," said Werner Goertz, a senior director at analyst firm Gartner.

The complication here is that it's entirely up to the developer how many active areas any given app will use. A game could use just one window, for example, and a productivity app could turn a second pane into a full-size virtual keyboard. Developers with the most resources are likely to get their apps ready first, while smaller outfits may fall behind.

Read also: For Samsung's foldable phone, killer apps would seal the deal

8. All-day battery life is a must

The bigger the screen, the more power you suck down in use. The foldable Galaxy X/Galaxy F will need significant battery reserves if it's going to last all day with both those screens going. But batteries don't bend, and one large battery is more efficient than two smaller ones.

Samsung will have to come up with some creative solutions to keep the device from running out of steam by the middle of the day. And early adopters may have to accept that the foldable phone needs more top-ups than a typical phone.

9. Keep that price in check

Samsung's foldable phone won't be cheap. My guess is that it'll start somewhere in the $1,300 to $1,500 range. Samsung will want to recoup the costs associated with a higher bill of materials, research and development, a new manufacturing process and marketing campaigns.

It will also expect to sell far fewer units while the technology is still new, counting on more refined future iterations to build its fan base, as Samsung did with the stylus-equipped Galaxy Note. In the meantime, that means Samsung will make far less money on what is likely a more expensive phone than its flagship Galaxy S10.

But if the sky-high price isn't backed by a thoughtful design, robust hardware, strong battery life and smooth user experience, Samsung's Galaxy bendable Galaxy experiment could fold before it ever has a chance to take off.

Originally published Jan. 23 at 4 a.m. PT.
Update: 8:41 a.m. PT with analyst quotes.

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