Controllable liquid colour-changing lenses with microfluidic channels for vision protection, camouflage and optical filtering based on soft lithography fabrication
© The Author(s). 2016
Received: 31 January 2016
Accepted: 26 April 2016
Published: 10 May 2016
In this work, liquid colour-changing lenses for vision protection, camouflage and optical filtering are developed by circulating colour liquids through microfluidic channels on the lenses manually. Soft lithography technology is applied to fabricate the silicone liquid colour-changing layers with microfluidic channels on the lenses instead of mechanical machining. To increase the hardness and abrasion resistance of the silicone colour-changing layers on the lenses, proper fabrication parameters such as 6:1 (mass ration) mixing proportion and 100 °C curing temperature for 2 h are approved for better soft lithography process of the lenses. Meanwhile, a new surface treatment for the irreversible bonding of silicone colour-changing layer with optical resin (CR39) substrate lens by using 5 % (volume ratio) 3-Aminopropyltriethoxysilane solution is proposed. Vision protection, camouflage and optical filtering functions of the lenses are investigated with different designs of the channels and multi-layer structures. Each application can not only well achieve their functional demands, but also shows the advantages of functional flexibility, rapid prototyping and good controllability compared with traditional ways. Besides optometry, some other designs and applications of the lenses are proposed for potential utility in the future.
Microfluidics technology has been the focus of intense research and development as it promises a multitude of advantages in a number of markets including chemical and biological analysis (Shih et al. 2015; Liberale et al. 2013), drug delivery (Majedi et al. 2013) and medical diagnose (Lee et al. 2014; Ng Alphonsus et al. 2010), such as small sizes, high throughput and low cost of microfluidic systems (Paul et al. 2006). Microfluidic has also revolutionized some aspects of optical area (Tseng et al. 2009; Liu et al. 2012). Lim et al. (2014) reported a microfluidic optical fiber devices composed of microfluidic channels which can be used for sensitive refractive index sensing and biosensing applications. Fuentes-Fernandez et al. (2013) proposed an electrowetting-based variable focus liquid lens used for curvature sensors, which can reduce the overall size of the system without the need of extra moving parts. In recent years, a few examples of surface property control (shape, pressure etc.) of materials through microfluidic combined with optics were reported (Iimura et al. 2015). Roy and Ghatak (2014) designed an adaptable optofluidic aspherical lenses by using elastocapillary instability induced by surface tension of a soft rubbery layer with microfluidic channels. Chang et al. (2009) presented a flexible material of controlled shape and stiffness embedded with microchannel networks. When the channels were filled with photoresist, deformed and exposed to UV light, the photoresist inside the channels was solidified, locking in the programmed shape of the materials. However, the reports on the applications of colour control in optometry by using microfluidic are few.
In our daily life, wearing colour-changing sunglasses has become popular way for vision protection and aesthetic increasing. The traditional colour-changing glasses are made of solid photochromic glass containing silver halides (Armistead and Stookey 1964; Tian and Zhang 2012) inside, which can change their molecular construction for colour changing under different light conditions. But these solid photochromic glasses have shortcomings, such as single colour, poor controllability on colouration process and high price.
Camouflage glasses are essential equipments for soldiers or hunters in the wild to blend with the surroundings for self-camouflage. Compared with common used camouflage nets, they have higher transparency and more flexibility for faces concealing. Camouflage technology in the animal field has been extensively studied and increasingly used by human in previous literatures (Surmacki et al. 2013; Watson et al. 2014; Dimitrova and Merilaita 2014). Kang et al. (2015) presented experiments and discussions about the concealing mechanisms of moths during behavioral choice of a resting position, which told us that some species reinforce their crypticity in terms of both background matching and disruptive colouration to improve camouflage against natural predators. Yu et al. (2014) conducted an adaptive optoelectronic camouflage systems with designs inspired by cephalopod skins, which provided critical capabilities in distributed sensing and actuation for mimicking biological colour tuning. These camouflage ways are fine for body-concealing, but impossible for optometry, because optical transparency is not considered in these systems. Moreover, complex principle and structure hard to mimic are mostly involved for achieving near perfect camouflage result in their systems. Morin et al. (2012) reported a soft machine with microfluidic networks which could realize camouflage/display of the body surface by pumping different colour liquids into the channels. But little attention was paid to the characteristics and applications of microfluidic in optometry. Meantime, the fabrication process of this soft machine was based on conventional soft lithography way (McDonald et al. 2000; Becker and Gaertner 2008), of which the key fabrication parameters and bonding method are weak for the manufacturing of camouflage glasses based on the optical resin material (CR39).
Optical filters have been widely used for photo taking to obtain different photography effects. Traditional optical filters in the market are made of coated glass or plastic. The main drawbacks of these coated filters are singleness of filter function and complexity of making process (Taichung and Hsinchu 2004; Moon et al. 2008). Furthermore, the uniformity of the coating, especially on the edges, remains a critical technical issue for traditional manufacture techniques (Yoon and Lee 2010; Yang et al. 2015).
Here, we fabricate liquid colour-changing lenses with microfluidic channels based on soft lithography with proper fabrication parameters. For irreversible bonding of silicone colour-changing layer with optical resin lens (CR39), a new surface treatment way by using 5 % (volume ratio) 3-Aminopropyltriethoxysilane (APTES) solution is investigated and validated. By carefully designing of the channels and controllably circulating proper colour liquids through the channels, the liquid colour-changing lenses can be used for vision protection, camouflage and optical filtering. Compare with conventional ways, these applications of the lenses can not only well achieve the functional demands, but they also show the advantages of simple principle, flexible function and good controllability. Meantime, the liquids filled in a cavity structure usually used in previously published or commercialized adjustable spectacles (Ren and Wu 2005; Santiago-Alvarado et al. 2013; Zhao et al. 2015) may be easily affected by gravity, which will result in incompletely replacing from the cavity, the liquid colour-changing lenses with the design of microfluidic channels adopted in this paper are the key to this problem, which can realize reliable colour liquids circulation.
In this paper, the liquid colour-changing lenses with microfluidic channels are presented and fabricated based on soft lithography. Proper fabrication parameters and surface treatment way are investigated. Different shapes and dimensions of the channels are designed and applied for vision protection, camouflage and optical filtering.
Working principle and fabrication
The shape and size of the channels can be diversified designed for different functions. In this paper, when the channels are carefully designed and proper liquids are selected for vision protection, diversity of colours, high transparency and effective ultraviolet resistance can be realized without bringing any side effect to human vision. When disruptive channels are designed and appropriate colour liquids are filled to match the background, good camouflage effects of the lens can be achieved. If the liquid colour-changing lens is used as optical filter, different wavelengths of monochromatic light can be absorbed by the colour liquids selected and various photography effects can be obtained. Moreover, in order to realize gradient overlay effect of colours and satisfy personalized requirements of wearers, two or more liquid colour-changing layers filled with different colour liquids are designed and fabricated, as shown in Fig. 1c.
In this research, the liquid colour-changing layer of the lens is made of polydimethylsiloxane (Duffy et al. 1998; McDonald and Whitesides 2002) (PDMS) silicone which is less fragile, less expensive and has good optical transparency. More importantly, it is very convenient to make microfluidic devices with PDMS silicone by using soft lithography technology for fast prototyping and without involving any mechanical manufacturing.
The bonding process of the PDMS film to the substrate lens can be reversible and irreversible. The reversible bonding of a microfluidic system is not tight enough, but the dismantled microfluidic system can be reused after cleaning. The irreversible bonding is sufficiently strong to withstand a high supply pressure, but the internal cleaning of the channels would be more difficult. In this paper, the latter is adopted for the bonding of PDMS film to optical resin lens (CR39). Plasma treatment has been validated and used extensively for irreversible bonding of PDMS to PDMS/glass substrate in the literatures before (Thuillier and Malek 2005; Eddings Mark et al. 2008; Hemmil et al. 2012). But for CR39 substrate lens, only plasma treatment is not strong enough to form irreversible bonding. In this paper, the irreversibly bonding of PDMS with CR39 substrate lens is successfully realized by surface modification with 5 % (volume ratio) 3-Aminopropyltriethoxysilane (APTES) solution, as shown in Fig. 2b. A 1 mm thick PDMS film and a commercial 2 mm thick CR39 optical lens (Mingyue Optical Limited Company, China) are rinsed with deionized water and dried in nitrogen stream (Yuanye Biotechnology Limited Company, China). For better silylation effect, the latter lens is activated in a plasma cleaner (Mingheng Technology Limited Company, China) for 40 s with the power of 80 W and air flow of 400 mL/min. The CR39 optical lens is then submerged in 5 % APTES solution (Yuanye Biotechnology Limited Company, China) at 80 °C for 30 min for surface silylation, which can introduce Si-containing groups ( ) on the lens surface. Then both the PDMS and silylated CR39 substrate lens are subjected to another plasma treatment (80 W, 400 mL/min) for 40 s for the formation of hydrophilic groups (Si–OH) on their surfaces. Immediately after the treatment, bond the two treated surfaces together and strong Si–O–Si covalent bonds will be formed between the bonding surfaces. Baking the bonding structure in a vacuum oven at 60 °C for 12 h to make sure of that the bonding is strong enough. In order to validate the effectiveness of the surface treatment, the bonding strength between PDMS and CR39 substrate lens is examined by tensile tester (Handpi Instrument Limited Company, China). For APTES-assisted sealing, the bonding strength of the structure is 1060 kPa, which is much higher than 650 kPa obtained based on plasma treatment alone.
Applications and discussion
With different shapes and designs of microfluidic channels, the liquid colour-changing lenses can be applied for vision protection, camouflage and optical filtering respectively.
Colour-changing glasses for vision protection
In order to understand the effect of thickness and temperature on the transmittance of PDMS film, different sizes of PDMS films (10 × 5×1 cm, 10 × 5×3 cm and 10 × 5×5 cm) are fabricated and tested. The transmittance of all the PDMS films is over 80 % in different wavelengths of light, especially in visible light, more than 85 % transmittance is achieved, as shown in Fig. 4b (top). Meanwhile, it also indicate that the transmittance of the PDMS film gradually decreases with the increasing of its thickness, and increases with the rising of environment temperature. Generally, PDMS films with different thickness at different temperature can all meet the optometry demands with the satisfying transmittance.
Capable of absorbing the ultraviolet radiation in the sunlight, different concentrations of Fe2(SO4)3 solutions (Yuanye Biotechnology Limited Company, China) are filled in the microfluidic channels on the lens for experimental tests. The transmittance decreases significantly in ultraviolet region but keep above 80 % in visible light region, as shown in Fig. 4b (bottom). At a wavelength of 250 nm, only 4.29 % ultraviolet light can pass through when the lens is filled with 0.1 mol/L Fe2(SO4)3 solution, and down to 1.84 % when 0.2 mol/L Fe2(SO4)3 is applied. Therefore, this liquid colour-changing lenses demonstrate good functions of anti-ultraviolet rays and transmission of light. Meantime, in order to realize various colour-changing effects, the Fe2(SO4)3 solutions dyed red and blue are used to change the exterior colour of the lens. Little impact on transmittance of the lens is found after dyeing (Fig. 4b).
Some other personal designs of microfluidic channels and multilayer channels can also be easily achieved to meet different fashion desires of the wearers. Moreover, different optometry functions can be implemented when the lenses are filled with different solutions.
By changing the designs of the microfluidic channels on the lens, glasses with various camouflage effects can be easily realized to adapt to other different backgrounds. Compared with complex camouflage technology (Kang et al. 2015; Yu et al. 2014), this liquid colour-changing camouflage technology by microfluidic channels shows the advantages of simple principle, easy realization and good performances. The camouflage PDMS film can also be applied for other surfaces of human body or machines.
More photography effects can be obtained by this liquid colour-changing optical filter through changing the colours liquids filled with. Three or more layers of channels can also be easily fabricated for overlap effect. Compared with the conventional coated optical filter, this liquid colour-changing optical filter shows more flexible function, easier making and better colour uniformity.
In addition to above applications, some other designs of the channels and applications of the liquid colour-changing lenses can also be implemented. For example, when applied on the rearview mirror of cars, they can decrease the intensity of the reflected light, reduce glare effects and comfort driver’s eyes by the colour liquids filled with. When applied on windows of cars, airplanes or rooms, they can reduce the transmitting intensity of light and prevent human privacy automatically or under one’s control.
From the designs and fabrication of the liquid colour-changing lenses with microfluidic channels above, it can be seen that the microfluidic liquid colour-changing lenses with different designs can be easily and quickly fabricated by soft lithography technology with proper fabrication parameters. New surface treatment method for the irreversible bonding of PDMS with CR39 was investigated and proved to be effective. Carefully designed liquid colour-changing lenses can provide vision protection, camouflage and optical filtering functions for human. In addition to the successful implementation of their fundamental functions, the distinct advantages of the lenses for these applications over previous ways are the simplicity in manufacturing, easy operating and good controllability. Although we have focused on optometry field, other designs and applications of these microfluidic liquid colour-changing systems interfaced with other devices are proposed for presenting new opportunities for modifying their appearance.
SL conceived and designed the research. MZ and SL wrote the main manuscript text. MZ did all the experiments and prepared all the figures. All the authors contributed to the discussion of the results. All authors read and approved the final manuscript.
The authors would like to give their acknowledgement to the National Natural Science Foundation of China for the support (No. 51175101) on this paper.
The authors declare that they have no competing financial interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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