Superscripts in forest plots R

Question

I have tried several ways of creating superscript labels for the units in my forestplot but have been unsuccessful. Anyone have tips on how to make it work? Thanks

tabletext <-list(
  list("",expression("Hippocampal volume mL^3 (SD)"),expression("Ventricles mL^3 (SD)"),expression("Whole brain volume mL^3 (SD)"),"AD signature cortical thickness mm (SD)"),
                              list("Aβ+","7697.7 (174.9)","35504.4 (42.9)","1093523  (2974.2)","2.79839 (0.0071277)"),
                              list("Aβ-","7811.3 (178.9)","36138.1 (44.1)","1097632 (2950.8)","2.80512 (0.0070715)"),
                              list("P value","0.071","0.591","0.332","0.507"))


f <-forestplot(labeltext = tabletext, graph.pos=4, 
            mean = c(NA,dt$Beta),
            lower = c(NA,dt$Lci), 
            upper = c(NA,dt$UCI),
            xticks=xticks,
            ref_line = 0,
            xlim = c(-1, 1),
            boxsize=0.16, colgap=unit(5,"mm"), col=fpColors(box="black", lines="black", zero = "gray50"),
            zero=0, cex=1, lineheight = "auto",
            txt_gp=fpTxtGp(label=gpar(cex=0.75),
                           ticks=gpar(cex=0.75),
                           xlab=gpar(cex = 0.75),
                           title=gpar(cex = 0.75)))