Variable-focus liquid microlenses and microlens arrays actuated by thermoresponsive hydrogels

Traditional optical systems utilize mechanical parts (e.g., gears, motors, and drivers) to allow adjustable focusing and magnification. Emerging variable-focus microlenses have exhibited the potential to miniaturize and advance optical systems without the need for mechanical parts, impacting significantly on a multitude of fields, such as cameras, biomedical instruments, and lab-on-a-chip systems. A variety of variable-focus liquid microlenses have been demonstrated based on different mechanisms, including reorientation of liquid crystals, electrowetting of a liquid droplet, and mechanical actuation of polymeric materials. These microlens technologies rely on external controls and power supplies; for example, additional electric fields are necessary to drive both electrowetting liquid microlenses and liquid-crystal microlenses, and external actuation devices are required to control the pressure in flexible polymer microlenses. The requirement for additional discrete components increases the complexity and makes integration, especially in lab-on-a-chip applications, challenging. We are interested in taking advantage of smart materials to realize variable-focus liquid microlenses without requiring external controls and power supplies. Responsive hydrogels are smart materials that undergo significant and reversible volume change in response to environmental stimuli by absorbing and releasing water through the network interstitials of the hydrogels. The ability to convert chemical energy directly into mechanical work means that the hydrogels can function as sensors, actuators, and power sources in specific conditions (e.g., the human body) where most external controls and power supplies are limited or difficult to obtain. By taking advantage of responsive hydrogels, we previously reported an innovative approach for realizing smart-liquid microlenses responsive to environmental changes. The liquid microlens extends the concept of pinned liquid/liquid interfaces to apertures to form a liquid microlens. A hydrogel ring located within a microfluidic channel responds to an environmental change by expanding and contracting, which regulates the shape of a water/oil interface (i.e., a liquid microlens), thus, tuning the focal length of the microlens. In this paper, we demonstrate thermoresponsive variable-focus cylindrical microlenses and spherical microlens arrays formed using liquid/liquid interfaces. Thermoresponsive reversible N-isopropylacrylamide (NIPAAm) hydrogels were employed; unconstrained NIPAAm hydrogels swell by as much as a factor of ten when the temperature decreases from above to below the hydrogel’s lower critical solution temperature (LCST). First, arbitrary shapes of microlenses (e.g., cylindrical and spherical) can be realized by patterning apertures that have corresponding shapes (e.g., rectangular and circular, respectively). This benefits the implementation of various microlenses with different shapes, among which variable-focus cylindrical lenses are of strong interest for a multitude of optical applications such as stretching an image, focusing light into a slit, and correcting low-order aberration. Second, we extended single variable-focus spherical microlenses to microlens arrays, allowing the devices not only to have wide-angle observation and parallel optical signal acquisition, but also to implement combinatorial interrogation of complicated environments in microfluidics by individually regulating the focal lengths of the microlens elements. In a typical variable-focus cylindrical microlens (Fig. 1a), a rectangular window was photopatterned in a 250 lm thick polymer slip to form an aperture. The sidewall of the aperture was treated to be hydrophilic (water contact angle of 29°) through an oxygen plasma treatment; the top-side surface of the aperture was rendered hydrophobic (water contact angle of 118°) by coating it with an octadecyltrichlorosilane solution. Thus, a hydrophobic/hydrophilic contact line coincided at the boundary between the sidewall and top-side surface of the aperture. A thermoresponsive NIPAAm hydrogel (LCST = 32 °C) ring was photopatterned in a 750 lm deep microfluidic channel. When the hydrogel ring was filled with deionized water through the aperture, a water meniscus emerged from the rectangular aperture, and the peripheral boundary of the meniscus was pinned stably at the hydrophobic/hydrophilic contact line. Mineral oil was stored within a C O M M U N IC A IO N