Science publishing today is much like the pre-iTunes days of music sales, when customers who wanted just one song from an artist had to buy a whole album.

University libraries and companies have to buy yearlong subscriptions, called site licenses, to give ­researchers access to a handful of articles. But at several thousand dollars or more per subscription, even the richest libraries can’t ­afford to buy every scientific journal that’s published. And most researchers can’t justify the $30 to $50 single-article fee or the wait of weeks or months for an interlibrary loan.