Since it will be a new year, the second Historical Sew Fortnightly challenge of 2014 (due Sat Feb 1) will be all about new things: Innovations.
Innovation is one of the biggest drivers in new fashions, today, and in the past. New inventions, the introduction of new materials or new styles from abroad all create new trends. Some of these fashions faded quickly, others had a lasting impact on what we wore. To celebrate the way inventions, introductions and discoveries have impacted fashion, make an item that reflects the newest innovations in your era. Because the Historical Sew Fortnightly is about learning as well as creation, I’d really encourage you to share the research you did into your innovation when you present the finished item.
There have been innovations in fabric that have changed fashion, like the introduction of Chinese silks into ancient Rome (much to the distress of Roman officials, who worried about the economic impact of all that Roman gold going East to pay for the silk coming West – to the point where they even hired notable authors and playwrights to write anti-silk propaganda).

Maenad in silk dress, Roman
Eastern silks created fashion waves again in the Middle Ages, as Chinese & Ottoman brocades & velvets made their way to Northern Europe, revolutionising elite fashions in the late Middle Ages & early Renaissance.

Mary Magdalene from the Braque Family Triptych (right panel), ca. 1450, by Rogier van der Weyden (early Flemish, 1399:1400-1464)
In the 18th century, it was cotton and not silk that revolutionised fashions, helping to bring in the softer, more relaxed look of Neoclassical fashions. Cotton made garments like Marie Antoinettes infamous chemise a la reine possible.

Marie Antoinette in a muslin dress, 1785, Vigee Le Brun
More recently, the development of cellulose-based rayons provided a cheap alternative to silks, and paved the way for the first full-synthetic fabrics, like nylon. While we mostly associate early rayons with the ’30s and ’40s, it was actually available for sale in New Zealand (and the rest of the Western world) from 1912.

Pyjamas and lingere in rayon, 1934
Innovations in dye have also drastically changed fashions. While indigo dye was known in Europe from ancient times, and woad, which produced indigo dye (but in much smaller quantities than from the indigofera plants) was grown commercially as a dyestuff throughout the Middle Ages, the introduction of large quantities of indigo dye from India by Portuguese explorers in the early 16th century revolutionised the amount and depth of the blue colours that could be achieved with dye. Other Western powers, wanting their own share of indigo, soon set up plantations in the West Indies and the American South. Despite bans on indigo imposed to protect the woad dyers, everyone wanted the new dye, especially as it combined with the new cotton fabric to create the classic blue and white toile de jouy and calico prints. Toile de jouy also owes its characteristic delicate details to another new innovation: engraved printing on fabric.

Pocket, printed cotton & linen, 18th c, American, MFA Boston
Indigo, combined with logwood which was imported from Central America & the Caribbean from the 1570s dyed fabric a rich, dark black which did not fade or destroy the fabric it was dyed into like earlier black dyes had done (you can see how the black threads of this jacket, dyed during an English ban on logwood black, are literally being eaten away). The fashion for the newly effective, and considerable cheaper, though still very expensive, black fabric is reflected in early 17th century fashions (and again at the end of the 17th century, when bans against it in England and other countries were lifted), and in the dress of the Puritans (who would think that the dress of a group who have come to epitomise severity actually dressed based on a fashion for the newest dye?). Without the addition of indigo, logwood dye faded to orange in a few short weeks, and without logwood, indigo could not dye black.

Jan Frans van Douven, Anna Maria Luisa de’ Medici dancing with her husband Johann Wilhelm, Elector Palatine (1695)
Most famously, of dye innovations, aniline dyes completely revolutionised the dye industry in the 1860s, ushering in a range of glaring brights, and eventually almost completely replacing natural dyes.

Afternoon Gown with Jacket, aniline green dyed silk, 1860s Antique Textile
Not just colour, but an absence of it, (or, technically speaking, a combination of all of it) was also an innovation. Before the early 1800s bleaching was a time consuming, and rather nasty, process which involved stale urine and fabric spread out over ‘bleaching fields’ and left to sun-whiten for month. The work of scientists and inventors such as Carl Wilhelm Scheele, Claude Berthollete, and Charles Tennant led to the development of chlorine and its combination with line, resulting in a ‘bleaching powder’ that bleached fabric white in a matter of hours or days. All of this made the Neoclassical fashion for white frocks accessible to a wide range of social levels, and made elaborate Victorian patterned fabrics possible.

Anne Isabella Milbanke, later Lady Byron, 1800, by John Hoppner
Bleach is an innovation that still affects fashion today, but other innovations have become less relevant with the change in fashions. Corsets now sit on the periphery of fashion, but the introduction of front-fastening slot & stud busks in the mid-19th century (there isn’t an exact date, but they were definitely in use by 1850) made it possible for every woman, even a factory worker who lived alone, to lace herself into a corset everyday, and corset-wearing became nearly universal as a result.

Corset with embroidery of oak leaves and wheat sheaves, 1876, Royal Worcester Corset Company, Metropolitan Museum of Art
And, as long as inventors were making it possible for every woman to wear a corset, they also made it possible for every woman to really cinch herself in: metal eyelets were invented in the mid-1820s, and made it possible to lace corsets more tightly, as well as speeding up the manufacture of corsets.
The invention of steel corset bones is perhaps more relevant today. They were originally created to replace more expensive whalebone, but with the over-hunting of whales and subsequent bans on whaling, they have replaced them for reasons of ethics rather than cost.
These are just a few of the innovations that changed fashion that I can think of off the top of my head: I’m sure there are literally thousands more. Go forth, research, sew, create, and I can’t wait to see what you make, and the information you find to share!