Templating or replication-based fabrication

A replication-based method generally involves three steps: template making (insert, stamper, or inlay) where a template having suitable structures is fabricated; a molding step where polymer materials are replicated on the template surface based on the specific processing techniques; and a demolding step where the structured polymer surface is very carefully detached from the template surface in a manner to avoid damaging the micronanostructures.

An insert is an exchangeable cavity that can extend the longevity of the main mold, provide the possibility of changing and testing various microstructural designs, and reduce the cost of tool making [28, 98]. A heatproof epoxy adhesive can be used to glue the insert onto the mold plate. This fixing is critical, especially for horizontal injection molding processes [91, 99-101]. A wide range of materials can be used for making inserts through various methods that include LIGA-based technologies, micro-electrical discharge machining (µEDM), micro-mechanical milling, electrochemical machining (ECM), deep reaction ion etching (DRIE), plasma treatment, wet-chemical etching, and laser ablation .

One of the most prevalent techniques for fabricating templates is laser ablation. Ultrashort lasers— lasers with a pulse duration of several femtoseconds or picoseconds—can create microstructured surfaces having minimal debris contamination, a good reproducibility, and a very limited heat-affected zone [77]. Nanosecond lasers produce heat-affected zones that are considerably larger than those of ultrashort lasers. Comparing femtosecond and picosecond ultrashort lasers, the femtosecond laser reduces the size of the heataffected zone due to its shorter pulse duration; however, the femtosecond laser is disadvantaged by a low average power, low repetition rate, a higher probability of breakdown, and higher cost. As such, the use of femtosecond lasers in industrial applications is more difficult [32]. The picosecond laser is therefore more widely used in the production of microstructured surfaces [32, 106]. Compared to picosecond- or femtosecondpulsed lasers used for micropatterning, the nanosecond laser has a lower quality of ablation due to the creation of rough edges or burrs. Moreover, nanosecond lasers cannot create laser-induced periodic surface structure (LIPSS) or ripples, which are necessary for fabricating hierarchical micro-nanostructures [32, 77]. Ripple structures can be controlled by altering the laser parameters. Nevertheless, the nanosecond pulsed laser is often used in industry for material processing [107]. Laser ablation parameters, such as the number of pulses, pulse width, wavelength, fluence, and laser Gaussian intensity distribution at the laser focus, markedly influence the resulting surface roughness [106]; for example, the greater the laser irradiation fluence, the higher the resulting WCA upon the replication of a polymer surface on a titanium (Ti) template due to the creation of more nanostructures on single spikes .

The molding process begins with the flow of polymer into the cavity, driven by the applied pressure. There is low flow resistance until the molten polymer reaches the microfeatures. At the edge of the microfeatures, resistance increases significantly, and the polymer hesitates to fill the microstructures. Higher pressure is required to overcome this resistance and force the polymer down into the microfeatures [108]. During the filling stage, due to the very fast heat loss of the molten polymer once in contact with the cavity wall, a solidified premature layer can quickly form. As soon as this solid layer is formed, there is not enough back pressure to push this layer into the structures and, consequently, the polymer is prevented from easily filling the micro-nanostructures. To avoid this problem, a number of solutions have been proposed including manipulating the mold temperature as well as increasing the molding pressure [109]. In addition to the high molding pressure and desirable mold temperature, an even distribution of cavity pressure is also required to achieve the highest level of uniformity and uniform height for microfeatures .

A means of ensuring the efficient filling and more economic process design, it is suggested that the micro injection could be first accomplished by using the conventional processing parameters, and then by using elevated temperature and pressure to ensure a high filling ratio [111]. However, this technique is highly questionable in the case of rubber injection molding. The mold temperature in the latter process is much lower than that for plastic injection molding. As such, in the rubber injection molding process, increasing the temperature after the filling stage leads to an increase in crosslink density and crosslinking reaction rate. This is highly undesirable especially in terms of micro-nanostructure filling.

Consequently, to achieve a complete and preferable replication, a combination of different processing parameters is necessary. A DoE method has been used in a number of studies [108, 110, 112-114]. Through a DoE method, experimental data is acquired in a controlled way; the significant and non-significant factors affecting a process are determined, and, finally, the behavior of molding of micro-nanostructures can be carefully monitored. To better understanding the effect of various processing parameters on the replication quality and to help finding the optimum conditions, a process window could be provided [115].

Both processing parameters and demolding conditions affect the final heights of the replicated micronanopatterns. In addition to a complete mold filling, high quality replication requires a flawless demolding where all micro-nanostructures withstand demolding forces. Inappropriate design of demolding forces used to remove the manufactured piece from the cavity can lead to irreparable structural deformation or even failure of the molded features on the polymer surface, and also can affect the lifetime of the mold .

Accurate design is needed considering many factors from polymer selection and mold conditions to processing parameters and part design [119]. Applied forces to the polymer surface, difference in the thermal conduction coefficient between the polymer material and mold metal, generated forces due the shrinkage of the polymer during the solidification stage, process parameters, cavity shape and material, molding material and geometry, features shape and aspect ratio-to be name but a few [117].

There are two different main demolding methods. Either one or both methods can be used to release the final product from the mold. The first uses demolding chemical surface agents named antistiction coatings. The second is a mechanical ejector such as pins, blades, rings, sleeves, and stripper blades [112, 117, 120]. Both methods have advantages and disadvantages. Use of chemical demolding agents is restricted in the medical or some microfluidic applications due to probable harmful effects on human health [121]. Mechanical ejectors can lead to permanent deformation especially when the part geometry is complicated or the distribution of the ejector pins is not appropriate [114, 117]. Therefore, a proper design of produced tool and the location of ejector pins is necessary to avoid damage and failure to the polymeric parts [112]. To circumvent the disadvantages of both chemical and mechanical demolding approaches, novel methods such as ultrasonic vibration [121] and reduction the surface roughness to decrease the coefficient of friction of the mold surface [122] require further investigation. In general, demolding is comprised of two different forces : adhesion and friction. Adhesion is the force at the intersection of the microfeature bottom and the top surface of polymer material. Friction is the force produced by the movement of the molded polymer inside the feature along the walls of the microfeature.