Not content with dominating the market for digital music, smartphones and tablets, Apple is about to outline a future in which wearable devices monitor everything from our heart rate and caffeine intake to saturated fats, blood sugar and oxygen levels.

At its biggest launch event yet, on 9 September in Cupertino, California, Apple is widely expected to announce a health-based wearable device that will help capture biometric data about activity, setting its sights on the $3tn (£1.8tn) US healthcare industry.

Its first partnerships with hospitals and health organisations were announced in June 2014 with the launch of the Healthkit software for developers. The software will allow developers to build bespoke health apps for a broad range of medical needs. Crucially, Apple claims, the user will retain control of the data but can share it with a doctor if he or she wishes.

Dr Mohammed Al-Ubaydli runs Patients Know Best, a startup that helps patients and doctors share medical records securely. "Healthcare is fundamentally an information industry," he says. "It's about knowing the right thing about the right patient at the right time. The information tools currently available are poor, and there's a large opportunity for tech companies to fix that."

The healthcare industry is a potentially lucrative market, worth about 10% of the economy of developed nations. In Britain, more than £100bn a year is spent on the NHS, according to the Department of Health.

Ubaydli says: "The only way to provide more healthcare for the people living longer and growing older, without more money, is to use technology. There's is a big demand for technology in cash-crunched healthcare and it's a very interesting time."

The 9 September event will also unveil Apple's highly anticipated larger iPhone, with a 4.7in screen to compete with bigger smartphones from its rivals Samsung, LG, HTC and Sony, as well as Microsoft. With 48% of US smartphone users expected to upgrade their handsets in the next 12 months, Apple – under the leadership of its chief executive, Tim Cook – hopes the launch will trigger its biggest sales period yet.

It will mark the biggest change to the dominant smartphone since its inception in 2007, a response to pressure from competitors' smartphones with 5in-plus screens that have proved popular. A 5.5in iPhone is also expected, although it may not appear immediately.

The new iPhone is expected to have a hardened, more scratch-resistant screen. Apple has patents and expertise in sapphire crystal – the third-hardest mineral – suggesting that the iPhone could have a sapphire screen similar to those used on high-end watches.

Apple is also expected to introduce a mobile payments service that would use the iPhone like a credit card to pay for goods. A Financial Times report indicated that Apple would build contactless, short-range secure communication technology into the iPhone that probably links in with its Touch ID fingerprint sensor for authorising payments.

The company has long resisted near-field communication (NFC) technology (short-range communications such as contactless payments), which has been available in many Android smartphones for years, despite holding patents for mobile payments systems using NFC.

The company has one of the largest databases of payment details in the world, with about 800m credit cards on record through its existing iTunes store. It could use them to ignite the mobile payments market, which has struggled to take off, despite efforts from PayPal, Google, mobile phone providers and banks.

For Apple, it is more than gadgets; it is about data and what can be done with that data, and it has partnered with Cambridge University Hospitals NHS in the UK, and Mount Sinai, UCLA Health and Stanford Hospitals in the US, among other healthcare institutions.

But the company told developers this week that it would be strict on data privacy and that they must not sell user data to third parties. They will be able to share data for medical research purposes, but only with explicit consent from the user.

Companies such as Nike will be able to collect health data on users through Apple's new Health app, which will launch for new and existing iPhones in September, providing information for both the individual and the healthcare industry.

Fitness trackers and smartwatches are an expanding market, with 9.7m wearable devices sold worldwide in 2013 and 22.3m forecast to be sold by the end of 2014, according to the research firm CCS Insight. Samsung, LG, Sony and Motorola each have smartwatches that connect to smartphones, while hundreds of manufacturers, Jawbone, Misfit and Fitbit among them, sell fitness trackers that count steps, calories and sleep.

What to expect on 9 September

• A larger 4.7in iPhone with a scratch-resistant screen produced in the US using a new process to build a sapphire-glass laminate. Some expect Apple to also launch a larger phone with a 5.5in screen, further exploiting the consumer appetite for "phablets" - or very large tablet-phones

• A smart, wearable device that can monitor and collect health data and connect wirelessly to an iPhone. The device could track steps, calories and activity worn all day and sleep through the night

• More details on Apple's Health data service, which collects information from wearable and medical devices and makes it easy to understand, and in a format patients can share with their doctors

• Sources in the chip-supply industry indicate Apple could be about to introduce an NFC-based contactless payments system, which would be built into the iPhone and coordinate with Apple's Touch ID fingerprint reader - so that paying for items would be confirmed via fingerprint or tapping in a code on the phone.

• Partnerships, and launch information for HomeKit, a software framework that will allow users to control smart devices in the home through a central app on the iPhone or, perhaps, the Apple TV set-top box

• An emphasis on privacy around user data: Apple is trying to emphasise that it doesn't make money from advertising, and so doesn't need to collect user information to relay to advertisers

• More details about Apple's Continuity system, which allows people to make and take phone calls and send text messages via an iPhone on an iPad or Mac computer, even when the phone is in another room.