Gadgets that connect to Apple’s iPhone already include TV remotes, audiophile speakers, and financial payment systems. The next wave may be medical devices. Think iPhone breathalyzers, thermometers, or stethoscopes.

For any innovator with a clever medical hack, a place in the Apple retail store would mean success. But how to get there? Technology Review interviewed several companies that already have health gadgets in Apple stores, or are in negotiations with the Cupertino, California, computer giant to see what it takes to get on Apple’s shelves.

Early on in the iPhone’s life, Apple touted the possibility of turning the gadget into a medical instrument in its iPhone videos. But applications have been slow to develop, even with Apple chairman Steve Jobs promoting the idea. One reason is that most such medical devices need approval from the U.S. Food and Drug Administration.

Apple stores now sell simple devices such as the Withings body scale, which transmits your weight to an iPhone or other iOS devices. Also on sale are blood-pressure cuffs that plug into an iPad dock.

Soon consumers can expect to see a new generation of more sophisticated medical devices that are integrated even more closely with the iPhone. In Europe, for instance, Sanofi Aventis sells a thumb-sized glucose meter that clips into the iPhone port. It is made by a U.S. company, Agamatrix, which is seeking approval for U.S. sales. The device should appear soon in Apple stores, say people familiar with the product.

The first companies to meld their products with Apple devices have a competitive advantage. Normally, glucose meters are given away free to diabetics. Companies instead make money in the multibillion-dollar market for disposable test strips. Sanofi, however, has been selling its iPhone glucose meter for 60 euros. The device locks onto the phone so diabetics will be less likely to forget their meter at home.

But how to get a medical device on Apple’s shelves? Here is what we learned from innovators who’ve interacted with the company.

First stop, FDA: A medical device is a medical device. If you aren’t cleared by the FDA, or working on it, Apple won’t talk to you.

Apply: To sell an app in the iTunes store, submit it to Apple’s website. 

Developing devices, as the steps below show, is more involved. 

First step: Apply to Apple’s MFi program (Made for iPhone) for hardware developers.

Prepare for feedback: Apple’s health-care developer team is led by Liang Hoe, who has an MBA and a biology background. He didn’t return our calls, but we hear he loves wild ideas and helping developers refine them.

Integrate: The tighter the connection with the iPhone, the better. Slip-on cases or cordless plug-ins are a plus.

Stay small: Space in Apple stores is at a premium. Small devices have a better chance of getting on shelves.

Design at will: Surprise—design-centric Apple doesn’t dictate what medical devices should look like.

Submit your device: You’ll have to send Apple plenty of information about your product, including prototypes.

Keep mum: Apple makes device makers sign strict nondisclosure agreements. Apple’s tough legal language “gets personal,” developers say.

Secret sauce: Now you’re ready for the secret stuff. Developers who get this far will obtain the “iPod Accessory protocol,” the secret technical specs needed to communicate with the iPod, iPhone, and iPad.

Connect: The iPhone is famously locked down. Apple provides special hardware and other components needed to connect through Apple’s 30-pin dock connector.

Get approved: All that’s left is final approval by Apple. Developers say that’s harder than getting a green light from the FDA. Unlike the government, Apple can reject you without telling you why.