Xinchuan Liu

Fibroblast/fibrocyte: surface interaction dictates tissue reactions to micropillar implants.

Micropillar technology has shown great promise for medical implants or sensors in recent years. To study the influence of surface topography on cellular responses, polydimethylsiloxane (PDMS) micropillar arrays with pillar spacing (20-70 μm) and height (14-25 μm) have been fabricated. The influence of micropillar arrays on cellular behavior was tested both in vitro and in vivo. Interestingly, in vitro, we observe a distinct response for 3T3 fibroblasts and RAW 264.7 macrophages to the topographical cues tested. Attachment and proliferation of fibroblasts was substantially enhanced by increasing pillar height, whereas macrophage adherence is significantly diminished by reduced pillar spacing. When implanted in the subcutaneous cavity of BALB/c mice for 14 days, we find a prevailing trend with capsule cell density and capsule thickness increasing, as both pillar height and spacing rise. Collagen deposition and neoangiogenesis, two pivotal factors in granulation tissue maturation, are also observed to have a stronger response to the increase in both pillar height and spacing. In contradiction to our original hypothesis, we observed that fibroblasts rather than macrophages are a key contributor to the in vivo outcome of micropillar arrays. Investigation into fibroblast activation, however, revealed that recruited fibrocytes, rather than resident fibroblasts, correspond to the in vivo outcome. The results from this work support the critical and often overlooked role of fibrocytes in tissue response to biomaterial implants with varying topography.

Propulsion of microboats using isopropyl alcohol as a propellant

In this work, we explored the possibility to develop a microboat for potentially transporting desired targets in microfluidic systems. We studied its design, fabrication, actuation and motions. In three types of tests conducted on water surfaces of different heights in a 30-cm-long channel, the microboat had a speed in the order of 0.1 m s−1, and propulsive forces ranged from 159 to 250 µN. It took 1–2 s for the microboat to go through the channel, and resistance coefficients varied from 0.144 to 0.324. Also, when the resistance coefficient was 0.324, the microboat was still able to travel a distance of 91.4 cm within 5.33 s in a 94.5-cm-long channel.

Fabrication of super-hydrophobic channels

A new approach was developed in this work to create channels which had not only super-hydrophobic bottom surfaces but also super-hydrophobic sidewalls. Researchers have demonstrated that a flow experienced less drag forces and thus required smaller driving pressure in a channel of micro/nanostructure-formed top and bottom surfaces. The drag forces should be further reduced in a channel which has not only patterned top and bottom surfaces but also patterned sidewall surfaces. However, due to the limitation of the existing lithographic approaches, sidewalls could not be properly patterned. Therefore, a new approach was developed in this work to overcome this obstacle. Polydimethylsiloxane (PDMS) micropillars of aspect ratios 1.4, 2.0 and 2.7 were first generated on PDMS films using a molding method, and then transferred to the sidewalls and bottom surfaces of three 1 mm wide and 1 mm deep channels, respectively, applying a hot-embossing process. The corresponding deformation mechanism was considered. The widths of the PDMS films had a critical effect on the cross-section profiles of the generated channels. The radii of the channel corners and inclined degrees of the sidewalls increased with the film widths. Contact angles on the PDMS films before and after the deformations were measured and compared. The contact angles in the middle portions of the sidewalls, as well as at the bottoms of the generated channels, were little difference from those on the original PDMS films due to the small changes in the distances between the micropillars. However, the contact angles were increased and decreased, respectively, at the bottom and top corners of the generated channels since the PDMS films were compressed and stretched at these corners during the fabrication. The variation of the contact angle in each channel was further analyzed according to two existing theoretical formulas. These variations increased with the increasing aspect ratios of the PDMS micropillars. The super-hydrophobic channels fabricated could be potentially employed to reduce drag forces in microfluidic applications.

Thermal ablation of PMMA for water release using a microheater

A new approach was developed in this work to ablate a micropore through a polymethylmethacrylate (PMMA) layer using a microheater for the on-demand release of water stored in a microreservoir. The size of the ablated micropore in the PMMA capping layer is mainly governed by the heater temperature profile and ablation time. Furthermore, the molten PMMA resulting from the heating energy tends to retract away from the micropore location, taking away the gold heater lines from the pore area. This prevents the possibility of any gold traces blocking the flow of water released from within the microreservoir. Simulation was conducted to find temperature profiles on the surface of the microheater, and the results were used to interpret the phenomena observed in the ablation process. Simulation was also conducted for the case when the microreservoir was filled with water. In addition, two alternative ablation materials, unexposed SU-8 and polydimethylsiloxane (PDMS), were examined as possible microreservoir capping layers. The approach developed has potential applications in microfluidic systems to release encapsulated fluids in a controllable manner.

Fabrication of micropatterns on the sidewalls of a thermal shape memory polystyrene block

Substrate sidewalls in the current microsystems have not been well used in building devices. On the other hand, the patterns generated on these sidewalls could serve as vertical interconnects or electronic components in 3D circuits. Since existing lithographic approaches could only generate patterns of limited shapes on the sidewalls, a new approach was developed in this work to fabricate various sidewall patterns via strain-recovery deformations of a polystyrene film. The fabrication procedure consisted of three basic steps: first strain recovery, pressing and second strain recovery. The deformations that the polystyrene experienced during the fabrication were first investigated with the help of mm-scaled lines marked on the samples. We found that pressing, instead of lateral stretching, transferred part of side surface of a polystyrene block into part of the top surface, and that the heating after the pressing was capable of transferring the same part of the top surface to the sidewall. The effects of temperature and recovery times on the deformations of the polystyrene were then investigated. We determined how the dimensions of a polystyrene block changed with the temperature. We also examined how many pressing-recovery cycles a polystyrene block could properly experience at 120 °C and 160 °C, respectively. Finally, the developed approach has been applied to generate Ag 50 × 50 µm2 dots, 150 µm wide lines and serpentine-shaped microresistors on both top and side surfaces of polystyrene blocks, followed by the characterization of these products, including shrinking and pressing deformations, surface roughness, wrinkling deformations and resistance changes. We found: (i) the shrinking deformations could be roughly considered as the recovery of the pressing deformations, although the two types of deformations were not identical in magnitudes due to different configurations (i.e. the shrinking deformations were affected by surface patterns, while the pressing deformations were not), and (ii) wrinkles might arise in surface patterns when the polystyrene blocks had large shrinking deformations, which might affect the functionality of the surface patterns and cause, for example, the decrease in their resistances.

Reinforcement of a PDMS master using an oxide-coated silicon plate

In this work, a new method was developed to increase the stiffness of Polydimethylsiloxane (PDMS) masters using oxide-coated silicon plates, aimed at reducing the residual and deflecting deformations of the PDMS masters for proper pattern transfer. Using this method, these two types of deformations in the reinforced PDMS master have been reduced.

Innovative approach for replicating micropatterns in a conducting polymer

An intermediate layer lithography (ILL) method is developed in this work to replicate micropatterns in a conducting polymer. In the ILL, an intermediate layer of a nonconducting polymer material is introduced between a silicon substrate and a conducting polymer layer. Subsequently, the conducting polymer film is patterned through a mold insertion. We illustrate this method by patterning polypyrrole. The ILL is simple, free of aggressive chemistry, and straightforward to use. In particular, unlike existing lithographic methods, it is suited at creating well-resolved micropatterns of a conducting polymer without degrading material properties of the conducting polymer. In this sense, we believe that the ILL is a promising approach to create conducting polymer patterns.

Intermediate-layer lithography method for producing metal micropatterns

Motivated by a macrocutting process often used in manufacturing industry to pattern sheet metals, an innovative intermediate-layer lithography (ILL) approach is developed in this work to generate micropatterns in a thin metal film. In the ILL method, an intermediate layer of polymethyl methacrylate PMMA is introduced between a silicon substrate and a thin metal film. Subsequently, the metal is printed through the insertion of a silicon mold using a hot-embossing technique. Al films of thickness ranging from 100to500nm have been patterned by silicon molds of depths of 10–100μm. Various Al patterns of lateral dimensions ranging from 10to300μm have been produced, including channels, lines, square dots, square holes, and truss structures. The effects of printing temperatures, thicknesses of Al films, and geometries of silicon mold structures on the patterning results were experimentally investigated, followed by numerical exploration. As a new patterning method, the ILL approach has advantages of simplicity a...

Fabrication of micropatterns on channel sidewalls using strain-recovery property of a shape-memory polymer

In this work we have demonstrated a new approach to generate micropatterns on the sidewalls of polystyrene (PS) microchannels. The PS used is a thermal shape-memory polymer (SMP). The sidewall patterns were produced based on the strain-recovery phenomenon of a PS film, which results in the formation of high-aspect-ratio microstructures. Using this method, we have fabricated 50 × 50 µm2 square dots and 100-µm-wide, 500-µm-wide straight lines and 150-µm-wide serpentine lines on the sidewalls of PS channels.

Determination of compressive residual stress in a doubly clamped microbeam according to its buckled shape

In this work, an analytical relationship is derived for a doubly-clamped microbeam when it buckles after release from the substrate. In terms of the relationship, compressive residual stress in the doubly-clamped microbeam can be determined according to its buckled shape, allowing one to find the compressive residual stress directly without much experimental effort. This relationship has been used to determine compressive residual stresses in four types of doubly-clamped SiO2 microbeams. In addition, four methods have been applied to find the elongations of these SiO2 microbeams, and the corresponding results are compared. Finally, the residual stresses in doubly-clamped SiO2 microbeams predicted according to the derived relationship are compared with those found in SiO2 microcantilevers, and the results have a good match.