Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast, microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in terms of efficiency, ease of use, and number of traps simultaneously exploitable.