are typically too small and fragile to be played with sticks, and are played with fingers. Dedicated drum pads such as the Roland Octapad or the DrumKAT are playable with the hands or with sticks. There are also percussion controllers such as the vibraphone-style MalletKAT, and Marimba Lumina. Pads that can trigger a MIDI device can be homemade from a piezoelectric sensor and a practice pad or other piece of foam rubber. Stringed instrument controllers A guitar can be fitted with special pickups that digitize the instrument's output and allow it to play a synthesizer's sounds. These assign a separate MIDI channel for each string, and may give the player the choice of triggering the same sound from all six strings or playing a different sound from each. Some models, such as Yamaha's G10, dispense with the traditional guitar body and replace it with electronics. Other systems, such as Roland's MIDI pickups, are included with or can be retrofitted to a standard instrument. Max Mathews designed a MIDI violin for Laurie Anderson in the mid-1980s, and MIDI-equipped violas, cellos, contrabasses, and mandolins also exist. Specialized and experimental controllers DJ digital controllers may be standalone units or may be integrated with a specific piece of software. These typically respond to MIDI clock sync and provide control over mixing, looping, effects, and sample playback. MIDI triggers attached to shoes or clothing are sometimes used by stage performers. The Kroonde Gamma wireless sensor can capture physical motion as MIDI signals. Sensors built into a dance floor at the University of Texas at Austin convert dancers' movements into MIDI messages, and David Rokeby's Very Nervous System art installation created music from the movements of passers-through. Software applications exist which enable the use of iOS devices as gesture controllers. Numerous experimental controllers exist which abandon traditional musical interfaces entirely. These include the gesture-controlled Buchla Thunder, sonomes such as the C-Thru Music Axis, which rearrange the scale tones into an isometric layout, and Haken Audio's keyless, touch-sensitive Continuum playing surface. Experimental MIDI controllers may be created from unusual objects, such as an ironing board with heat sensors installed, or a sofa equipped with pressure sensors. GRIDI is a large scale physical MIDI sequencer with embedded LEDs developed by Yuvi Gerstein in 2015, which uses balls as inputs. The Eigenharp controller is a combination of a breath controller, a configurable series of multi-dimensional control keys, and ribbon